2 Life on coastlines with tides.
1.1 To the rhythm of the tides
On most coasts, at the limits between the marine and terrestrial environments, there are systems that receive energy from the Sun, the waves, and the sea currents, but also absorb the energy of tidal movements. The part of the coast subject to the rising and falling movements of the tides is called the intertidal zone.
Surviving in a changing environment
The most notable characteristic of these ecosystems is the continuous and periodic fluctuation of the water level. The periodicity of the oscillation basically depends on the relative position of the heavenly bodies governing it, but the range of movement is dependent on local factors, sometimes including human modifications. This means that ecosystems differ greatly from coast to coast in the different oceans. Even so, advances in knowledge have made it possible to establish comparability between the different communities in each intertidal environment on the world's coasts with tides.
The intertidal environment is characterized by complex interactions and stress, as well as great variability, with locally predictable and extremely harsh effects caused by physical factors. At the higher levels of the zone, the organisms are regularly exposed to the air; while at lower levels, species that are almost always submerged are exposed to the air during the lowest spring tides, over a period more than long enough for the direct action of atmospheric variables to have catastrophic effects.
Life is conditioned by the frequency of disturbances and the severity of the effects of environmental variables. Unpredictable disturbances have dramatic effects on all the organisms. Cyclones, an excessively prolonged lowering of sea level, the dispersal of pollutants, etc., all worsen the effects of physical and chemical environmental variables and do not allow stable communities to develop. The intertidal environment is always subject to stress--the persistence of fluctuating conditions is the norm for the intertidal environment.
These changes are predictable, at least from an anthropocentric point of view. Organisms at lower levels are subject to limited stress, as they are always submerged (except for a few extreme oscillations), but organisms at higher levels have to withstand very harsh, but predictable, conditions. To be predictable a change must, in principle, be regular and more frequent than the life-span of the species. If this is not so, adaptation is impossible. Only organisms with high genetic variability and very short generations can withstand unpredictable systems, even if they are not very severe.
Although they are subject to drastic changes, rocky intertidal systems usually show very high species diversity. This is mainly due to the great diversity of habitats, such as channels, flowing water, pools, fissures, rock ledges, the upper and lower surfaces of stones, bare rocks, and rocks covered in silt or sand with considerable variations in the thickness of the sediments and their heterogeneity. In the second place, this diversity is due to the great spatial variability of tides, shown in the different possibilities of predicting the rhythm of exposure and submersion in each stretch of the intertidal space; species have been able to adapt even to this severe fluctuating environment, but only if they internalized the rhythm of the tide and the drastic amphibious alternation of environmental conditions to which they are subjected, along with the implacable selection pressure it implies.
In intertidal systems what is remarkable is the heavy competition for space and the rapid alternation of dominant species, the rapidity of phenological changes, the short lifespan of many species, the clear vertical organization, and the speed with which high production takes place. This system is a battleground between opportunistic but highly specialized species, all of which survive in this continuously changing system by taking advantage of any favorable change in resources.
The intertidal environment is ancient. The Earth has always had tides, or at least since before the Precambrian period. Since the moon has been present, the tides have varied in intensity depending on sea level, the relative position of the earth, the moon, and the sun, as well as the length of the day and the year (these, like the lunar month, have not always been constant). Paleochronological data show that the average sea level on coastlines with tides has often risen and fallen in the past; moreover, it is still changing as the sea level is also changing.
The intertidal environment has always been unstable and fluctuating. Thus, it is to be expected that the species found will show high rates of renewal to allow the presence of adaptive mechanisms of great genetic variability, and this is what allows some organisms to survive despite the drastic selection caused by this instability, at times, generation after generation. The fossil record of the most characteristic seaweeds of the intertidal zone, the Fucales, especially those of the family Fucaceae, begins in the Triassic period.
Intertidal levels and horizons
There are two physical conditions in the intertidal space, where the presence of water alternates with its absence. Biological responses are at least as divided as the physical conditions, and species have to survive both within the water and without it. An intertidal system showing a semi-diurnal oscillation is exposed twice a day and covered by water twice a day.
The lowest calculable tidal level on a given coast is taken as its base level. This separates the sublittoral zone, which is always covered by water, from the littoral zone, which continues up to the supralittoral zone, which begins at the level the water never reaches. The differences between the level of sea water at high and low tide correspond to the maximum tidal range, so the littoral system or zone is a synonym for the intertidal zone. Normally this level is divided into three stretches: the upper, middle, and lower. The limits of the middle intertidal zone are set by the upper and lower levels of the neap tides. An organism located at three quarters of the distance from the minimum tide level in an intertidal zone with a maximum range of almost 13 ft (4 m)--such as the coasts of Galicia, Spain--is outside the water for 70% of its life and submerged in water for 30% of its life, and this is exacerbated by the fact that exposure may last for a week or even 10 days. Organisms living halfway between the two levels spend half the time within the water and the other half outside it. Those living 20 in (50 cm) above the minimum tide level are within the water almost all the time, and may pass months with being exposed. In this case, periods of exposure constitute a special danger to their survival, even if they are infrequent and brief. Adaptation to each level has to be different and morphologically, physiologically, and genetically different forms have been selected, even within the not-always-clear limits of each species.
To the topographical variations there have to be added those due to local climate. Thus some intertidal zones are frozen, as in Canada, while others have hot, dry summers, as in the southwest of the Iberian peninsula. On rainy coasts, organisms suffer the effects of desalinization and acidification, as fresh rainwater is less alkaline than seawater. These circumstances may speed up metabolic processes or slow them down, but, above all, they increase diversification.
The most clearly differentiated intertidal communities occur in bands called horizons. The horizons are named after the dominant species, although each one shows a high level of organization. Each horizon usually contains many species in addition to the dominant one, with a tendency to form overlapping patches. This overlapping pattern is subject to change, giving the impression that it is never definitive: The presence of cracks, small depressions, rocky outcrops, mussel colonies, and holes covered by mud or containing sand derived from shells all contribute to the spatial heterogeneity. The intertidal system's complex structure might be described as the tesseras of a mosaic. Another appropriate simile might be a fabric printed with patches that gradually elongate or form circular patterns, but never form a definitive design.
Production and feeding strategies
The intertidal environment shows high external dependency as well as the ability to make use of neighboring systems. Light energy is obviously external, as is the additional energy bringing new nutrients on the waves and tides. The system receives nutrients from both sediments and rivers. Intertidal systems are highly productive and contain a considerable biomass that is not consumed internally. This high production of materials implies high losses, such as the loss of biomass that disperses into the pelagic system, or accumulates at the bottoms of the basins or at areas of confluence with terrestrial systems. These losses are almost always sudden and are often large. The behavior of the intertidal system is consistent with tidal oscillations. It is a forced system, showing constantly renewed production, and with major fluctuations in its exploitation.
The most conspicuous animals of the intertidal system are filter-feeders and grazers, although this does not mean herbivores and detritivores are not important. Filter-feeders acquire their food from the pelagic environment, while carnivorous or drilling species are more specialized and highly adapted to their typical food sources, although the detritivores and bacteria that digest seaweed tissues may be even more specialized. Trophic exploitation occurs in a way that leads to materials entering the intertidal environment; the larvae of intertidal benthic animals, for example, feed within the pelagic environment during the first stages of their life cycle and then return to their own environment. The material produced in the intertidal zone, however, is degraded far from it and returns in the form of nutrients or particles. The structure of the intertidal system is, to sum up, the strange result of random cycles that do not work together but always give rise to a very particular solution. Detached seaweeds or fragments do not die, but can live as drifting pleuston by making use of the nutrients of the pelagic environment. This is yet another way of exploiting neighboring systems.
Filter-feeders are important because they feed on the suspended material in the tidal water, making a wide range of particle types available to them. Mussels and barnacles feed in this way, and when exposed at low tide they remain immersed for long periods in their own intervalvular water, exhausting its oxygen and food. The response that has been selected is anaerobic respiration. Some filter-feeders, such as sponges, some hydrozoans, and bryozoans are less conspicuous than mussels and barnacles. Filter-feeders play a fundamental role in intertidal ecosystems. These sessile organisms can occupy space rapidly, ingesting all the suspended matter reaching their filtration apparatus, such as sestonic particles, algal propagules and animal larvae, even their own larvae. They are the system's street cleaners, yet at the same time they are the most important force shaping it. The filtration capacity of mussels has been used for rapid and large-scale production of flesh in permanently submerged floating systems.
Other animals are grazers. Some gastropods feed by scraping the substrate with their radula, such as the limpet (Patella) and other patellar gastropods, and the littorinids, which are microphagous or have beak-like mouthparts they can use to tear plants to pieces or to graze on small plants and diatoms, in a way similar to sea urchins. The regeneration of small seaweed plants is controlled by sea urchins and patellids, which repeatedly scour the smooth rock surfaces, especially in wave-beaten environments where no sediments are deposited on the stone.
Herbivores, such as sea hares (Aplysia and others anaspideans) or other gastropods, are less abundant and do not manage to consume all that is produced in the intertidal level. Some are highly specialized, possessing enzymes to digest the complex molecules of the seaweed cell-wall polymers.
Detritivores are a world apart. They eat everything that falls and is trapped at the base of the seaweed, in mussel colonies, or within cracks. Others, including sand fleas (Talitrus saltator) and other amphipods, harpacticoid copepods, many nematodes, and some gastropods, scavenge the wastes of filter-feeders by jumping from organism to organism. Detritivores are highly diversified, since the intertidal space shows great micro-environmental diversity and everything is miniaturized and contracted. Within a very small space there are many distinct environments.
1.2 The variability of the intertidal system
The variables that condition the interaction and survival of certain species are called key variables. They are the causal agents responsible for the regularities intertidal systems show in space along the light-depth axis, and along the axis going from greatest exposure to the beating of the waves to the least. Small-scale variability that arises from the slightest environmental differences is also important, and this is superimposed onto the variability following the two axes mentioned above. This interactive process gives rise to a mosaic of patches of vegetation, sometimes overlapping and sometimes segregating, that is very typical of the intertidal level.
Vertical zonation factors
Water movements, tides and waves, are the main factors that determine the biotic groupings of the intertidal level. Yet their effects are no more than the consequence of the simultaneous interaction of simple physical, chemical, and biological variables, such as air and water temperature, light, nutrients, as well as each organism's own metabolic characteristics and feeding and reproductive strategy.
It is generally considered that physical variables are decisive in determining a species' upper level, while its lower level is determined by competition. As a consequence, many organisms appear to move up and down, since they are found in different places at different times of year. The propagules of organisms living at some depth may reach the upper horizons of the intertidal level. If the tide is neap and the propagule is not out of water for long, it may develop into a small plant, but somewhat different in appearance from its ancestors living at greater depth. It may even reach maturity and become fertile, in which case its propagules will be dispersed again, although only those most protected from exposure or those that receive enough light to develop, will survive. In some locations this will lead to the appearance of different morphologies quite distinct from the original ancestor. As an example, the organisms of a given species at the limits of its vertical distribution are small, and those growing at the center are larger; this is because those in the deep zones receive little light, while those in the highest horizons not only suffer the effects of exposure to air, but also spend less time within the water, meaning they assimilate for a shorter period.
The high temperatures of the air may lead to excess evaporation of body water from intertidal organisms. Some intertidal organisms, such as some cyanobacteria and the lichens on the higher levels, are very resistant to water loss, but others, such as seaweeds and animals, are much more sensitive to desiccation.
Water temperature has been considered to be responsible for certain irregularities in the distribution of some species. Some Laminariales (kelp) are missing from the base of the Bay of Biscay that are common to the north and reappear to the west, halfway along the Asturian coast, and are present as far as the Portuguese coast. This might be caused by the fact that the summer temperature at the bottom of the Bay of Biscay is a couple of degrees warmer than the summer temperature off Galicia, where there is continuous upwelling of cold, deep water.
In almost all seas, the water is colder than the air in summer, and it is warmer in winter. This increases the stressful effects of exposure, especially in species that are almost always underwater. It has been shown that desiccation is the most damaging effect that exposure to air has on intertidal organisms. Wave splashes and air humidity are variables that may alleviate this stress and help organisms to survive prolonged periods out of water.
Light is very important for photosynthetic organisms, but it also conditions some animal responses. Seaweeds and the other photosynthetic organisms use light as energy for life. Water attenuates the quantity of light they receive, because the water and the substances it contains in solution absorb light. In the intertidal level organisms suffer a change from a situation of excessive light when exposed to one of insufficient light when submerged. Water shows selective absorbtion of the different wavelengths of the visible spectrum, with the result that green light reaches greater depth than blue light, and blue light in turn reaches deeper than the red light that is most useful for direct photosynthesis. Tidal waters are not clear, as they drag along the coast and contain many dissolved substances and suspended particles. Without going into details, the waters at high tide are clearly different from those at low tide; their continuous movement means they are always showing different levels of turbidity, and so light penetration and quality vary continuously.
Plants and other photosynthetic organisms shade each other; there is unremitting competition for light. In the intertidal zone, growing large is very dangerous. Large organisms dominating populations are exposed to desiccation and photoinhibition. However, small organisms that are underneath other organisms only receive the light the larger organisms let pass. Under the protective seaweed "forests" desiccation is less likely, although light is scarcer.
The light environment underneath a complex intertidal seaweed community is like that in a forest of fir trees. The leaf-area index (in reality, in the case of seaweeds it would be better to talk of the lamina-area index) is also similar in both cases, but at a depth of a few meters below the top layers of seaweed, the filtering of the light by the water has a greater effect than the shade cast by the seaweed lamina. The seaweeds that can tolerate excess light are rich in carotenes, while those living in filtered light have pigments suitable for catching it with the greatest efficiency, phycobilins. The change from excess light to twilight passes through all the intervening intensities. The stratification of the communities, favoring the development of organisms with distinct adaptations to intercept different types of light, also leads to increased species diversity.
Seaweeds and other photosynthetic organisms assimilate substances dissolved in water as the raw ingredients for synthesizing their materials. So organisms spending longer in the water can assimilate, and therefore grow, more.
Yet the nutrients in water are highly diluted, and the seaweeds of the intertidal zone follow two distinct assimilation strategies. Some seaweeds with little affinity for nutrients take them very quickly from the water and grow rapidly. Others do not use the nutrients immediately after incorporation but store them within their cells, and when there are no nutrients in the water, these seaweeds mobilize them from vacuoles or plastids. This is what happens in the rhodophytes and cyanobacteria, which use the nitrogen of their phycobilin pigments for cell growth if there are no external sources of nitrogen compounds. In this case, growth is truly a process of self-digestion and reconstruction.
Nitrogen is normally scarce, and only occurs in short pulses, after which there is growth of the photosynthetic organisms. While nitrogen is a relative limiting factor, phosphorus is a more general limiting factor. Inorganic phosphorus is only absorbed in the form of the relatively insoluble orthophosphate ion, which precipitates easily and is often totally depleted or unavailable. Seaweeds continue growing because they obtain orthophosphate from their organic molecules by using phosphatases, enzymes for this specific purpose.
Carbon can also be limiting. Carbon is present in seawater as a dissolved gas, but the most common form is the bicarbonate ion. This ion is incorporated by the cells but cannot be reduced by ribulose biphosphate carboxylase. Exposed seaweeds fix carbon dioxide well because air contains over 400 times more than water. Seaweeds use carbonic anhydrase to transform bicarbonate into assimilable carbon. The most common mechanisms are to assimilate bicarbonate into the cell (which is more acid than seawater), where it turns into assimilable carbon dioxide. If there is sufficient humidity, the photosynthesis of the seaweeds in the upper levels is more efficient in air than in water, which is what one would expect in aquatic organisms living outside the water.
Factors affecting horizontal diversification
Waves, currents, and strong salinity gradients are the variables that shape the horizontal distribution of intertidal organisms.
The height of the waves is very important because it extends the intertidal level. On coasts lacking active waves, this level is narrower than on shores beaten by vigorous waves. The effects of the waves and the tides are additive. The degree of exposure to the pounding of the waves defines particular communities. On the most battered shores there are organisms able to withstand the beating of the waves, especially mussels, cirripedes, and limpets. Their filtering and grazing activities and the force of the waves reduce the presence of seaweeds, which seek shelter at lower levels, where the waves do not break and where exposure is less frequent. Sheltered coasts have many seaweeds, and on semi-exposed coasts the situation is intermediate. The morphology of the organisms varies greatly and there is a great variety of different morphotypes among them. Thus the brown algae of the Fucaceae found in exposed sites are different from the forms found in calmer waters and, depending on exposure, there is a morphological cline of forms typically modeled by selection pressure. Sometimes, these forms have been considered different species, although they are only phenotypes of a single species determined by the environmental conditions in which they live.
The dispersal of intertidal organisms is random while selection is directional; most intertidal species are dispersed by the pelagic environment or, in other words, by planktonic propagules, spores, zygotes, or larvae. Dispersal is inevitably erratic and scattered in all directions, but the propagules always eventually return to the bottom and they may settle at the highest level of the intertidal zone, on the ocean bottom below the tides, or near where their immediate ancestors lived, and thus in an environment almost identical to the one they came from, in which case they will survive perfectly. In the first two cases, the pressures of environmental variables and competition with other organisms mean they run a clear risk of disappearance. Each morphotype represents the result of selection pressure.
Variability in time
Like the tides that affect them, intertidal systems are highly seasonal. Furthermore, the organisms' reproduction is conditioned by different stimuli, some of which also vary in time. Some of these, for example temperature or chemical stimulation by dissolved substances, directly influence the reproduction of many organisms. Light must also be taken into account, as it appears to be the variable of most importance as a stimulus.
The photoperiod in fact conditions the response of many organisms that are well adapted to it. Thus, a stimulus conditioning a given response in an individual living in Norway is not the same as the stimulus causing the same response in Mexico, because adaptations to photoperiodism are different at the two latitudes. The quality of the light is a further type of stimulus; for example, blue light stimulates the reproduction of some seaweeds, while red light stimulates their growth. It seems rather artificial to think that in nature there is selection by monochromatic light; in reality, the stimulus is transmitted in the form of changes in the proportions of blue and red light, which vary between dawn and at sunset. The fact that the proportions of the different wavelengths remain very constant during the day appears to suggest that the distinctive nature of the beginning and end of the day act as switches, one serving to turn processes on and the other to turn them off.
Many experiments have been performed involving the destruction of intertidal communities and the observation of their recovery. In the European Atlantic, for example, the first organisms to settle on the bare rock are bacteria, the spores and mycelia of fungi, as well as many types of algae, especially planktonic algae. It might be said that the organisms fall onto the rock by their own weight, thus forming more of an agglomeration than a true community.
The system is precompetitive and shows exceptionally high diversity. This phase does not last long and the species that reproduce fastest form a greenish film over the surface. At this stage the species present are almost entirely diatoms, but now mainly benthic forms. As time passes, this green organic sludge serves as a substrate for the settlement of the plantlets of large seaweeds that will start to grow vertically, while the sludge also acts as a refuge for the animals that eat the seaweeds. The first seaweeds that stratify the community are ulvaceous (Ulva, Enteromorpha), and they act as a refuge for the propagules and embryos of other seaweeds. One community is replaced by another, and after successive invasions, disappearances, and competition, they diversify and acquire a structure similar to that of the destroyed community after one or two years.
Precise patterns of succession have been defined, which can be related to an increasingly efficient use of energy. For example, the presence of upright forms allows better use of the energy of the water turbulence; stratification optimizes the use of the filtered light; and the settling of mud allows better use of detritus. The system's biomass increases and the flow of energy shows dissipative curves that range from the displacement of the equilibrium to the attainment of new equilibria with the passing of time.
1.3 The lichen, seaweed, and animal population
Intertidal organisms show great diversity. A large number of plant and animal groups are present, but only the most relevant will be discussed here.
Lichens and cyanobacteria
Lichens and cyanobacteria are the most representative organisms of the part of the rocky coasts called the supralittoral. It is possible to classify these organisms into zones running from the strictly terrestrial to the truly marine environment, depending on the effect of the salt spray from the waves.
Halophobic (in other words, salt intolerant) lichens mark the upper limit of rocky shores, where terrestrial conditions predominate over marine ones. Continuing towards the sea, the second horizon contains halophilic lichens, which may be encrusting lichens, such as some yellow or reddish Caloplaca; foliose lichens, such as some Parmelia; or even fruticose lichens, such as some species of the genus Ramalina. Below this there are supralittoral xerophytic lichens; this is the Ramalina sufruticosa horizon, with many species adapted to dryness. Xanthoria parietina grows in the horizon nearest the sea, which still contains foliose lichens and can tolerate salts well. Very close to the water is a horizon known as the "orange horizon," characterized by Caloplaca marina.
Truly intertidal lichens are black. Lichina pygmaea and L. confinis are very typical of the lower horizons, and Verrucaria of the higher ones. The photosynthetic component is usually a species of cyanobacterium, and these are often capable of living without forming the lichen association, as in the case of some cyanobacteria of the genus Calothrix, which form polymorphic groupings that vary depending on the abundance of nitrogen or phosphorus, while others form crusts in winter that turn into flattened or globose colonies formerly classified in the no-longer accepted genus Rivularia.
Small grazing snails and colonial crustaceans
Littorinid gastropods (periwinkles) are found in all the intertidal levels, but are very common in the upper levels. They are a masterpiece of adaptation that reaches its peak in the pseudovivipary of Melaraphe neritoides. The larvae of this species, unlike those of other gastropods, are not planktonic, but are highly developed when hatched (and similar to the adults in appearance), allowing them to avoid the dispersal that would be imposed by pelagic life. The littorinids on the higher levels feed on bacteria and black lichens of the genus Verrucaria, which they scrape with their radula.
Cirripedes, mainly the barnacles and acorn barnacles (of the genera Balanus, Chthamalus and Tetraclita) are found in dense continuous clumps in the mid- and upper-intertidal zones. Chthamalus is more abundant at higher levels and Balanus at lower ones. Both compete in the lower levels with other sessile forms, such as the mussel Mytilus. In this zone, which is exposed every day, there is strong competition. The cypris larvae of the cirripedes are planktonic, and when they fall on rocks they attach themselves by means of a cement-like substance secreted by glands in the first antennae. Once the animal has attached, its polarity reverses and it begins to filter actively. At low tide the animal hermetically seals itself, reopening when the tide rises.
The low zones are occupied by large seaweeds (Laminaria, Macrocystis, Nereocyctis, etc.) or cystoseiras (Cystoseira), considered as sublittoral-level elements that have risen to the intertidal level. The large algae organize space vertically because they filter light, excrete organic substances, and reduce hydrodynamic effects. The lower intertidal communities are dominated by coralline algae. Many seaweeds have a microscopic generation that lives in unfavorable periods and a separate macroscopic generation that lives in the favorable ones. Reduction of size is a very common evolutionary strategy. If there is alternation of macroscopic generations, one generation is more abundant in the summer and the other is more abundant in the winter. Another common adaptation is for the spores to be very dense and to disperse only a short distance, remaining close to the plant that produced them. Sexual reproduction is a risk, especially for seaweeds with easily scattered swimming gametes or spores, and also for seaweeds with small, widely scattered propagules. Normally intertidal forms have a high capacity to resprout after damage and to spread vegetatively, and in some algae, especially rhodophyceans, sexual reproduction is less important than vegetative reproduction.
1.4 The intertidal zone in the world's different seas
Within the general model of distribution of horizons in the intertidal zone, the distribution of its organisms clearly varies in the world's different seas, depending on each oceanic region's climatic characteristics.
We can consider zonation on the island of Mauritius in the Indian Ocean as an example of zonation in a tropical sea. Like everywhere else, in the supralittoral level there are cyanobacteria and littorinids. The most typical balanids belong to the genus Tetraclita, which dominate the upper levels of wave-beaten coasts, while Chthamalus dominates less wave-beaten coasts. Caespitose (tuft-forming) seaweeds are ubiquitous in the intertidal environment, and in the lower levels the most conspicuous seaweeds are rhodophyceans of the genus Laurencia, although there are also phaeophytes of the genera Ectocarpus and Chnoospora, in addition to highly-branched chlorophytes of the genera Cladophora and Chaetomorpha. In the levels below this, near the infralittoral, there is a typically tropical element, sargasso (Sargassum), which is replaced by Caulerpa on l ess wave-beaten shores. On tropical rocky coasts there is a wide range of crabs, polyplacophorans, gastropods, anemones, and zooanthids, as well as a mussel, Septifer bilocularis, the equivalent of the genus Mytilus in other seas. There are also many echinoids that browse on the filamentous seaweeds, leading to the establishment of large populations of coralline algae that can withstand this insistent attack, whereas fucaceans are absent.
Coral formations do not tolerate exposure well, although during spring tides the highest parts of the reefs may be almost out of the water. When this happens, the most conspicuous organisms are the sargassos mixed with calcareous algae, such as Corallina and Halimeda, and with corals of the genus Favia. There are also flexible phaeophyta such as Turbinaria and the spectacular Hydroclathrus, and colonies of some corals such as Montipora, Porites, and Psammocora. Encrusting calcareous algae of the genus Lithothamnium are present, but do not thrive as well as on rocky coasts. Flexuous seaweeds, such as chlorophytes of the genus Boodlea, phaeophytes of the genus Dictyota, and rhodophytes of the genera Galaxaura and Padina, form scattered populations. As regards to the fauna (apart from corals), the echinoids are found in holes and crevices, while the community's most spectacular feature is the huge shell of giant clams, the tridacna (Tridacna).
Temperate Atlantic European coasts
Coastal cliffs in the British Isles are the classic model for the description and study of zonation for the whole world's rocky intertidal zone, and even more so for the Atlantic coasts of Europe. The supralittoral system includes the typical black lichen formation described above. In the upper levels of the strictly intertidal zone there are populations of the rhodophytes Porphyra mixed with filamentous chlorophyceae of the genera Enteromorpha, Blidingia, Urospora, or Ulothrix, which show spatially heterogeneous distribution and seasonal abundance. Fucaceans are a typical feature at all levels, both with respect to their abundance and their wide distribution. On very exposed northern coasts Fucus distichus is present, while F. spiralis forma nanus is found everywhere, even reaching as far as the Mediterranean. The type form of F. spiralis is found on the least exposed sites within the intertidal environment, below Pelvetia canaliculata, another very resistant fucaceans that grows in the upper levels among the black lichens. With respect to the fauna, the level's most representative gastropods (littorinids) are Melaraphe neritoides and Littorina saxatilis. Melaraphe has a wider distribution than Littorina, which appears to be found preferentially in less exposed sites. This horizon's domination by littorinids has led to it being called the "Littorina horizon" or "periwinkle horizon" on some coasts. The isopod Ligia oceanica and the insect Petrobius maritimus, both highly mobile, are also present.
The middle layer of the intertidal system is called (depending on the dominant organisms) the "barnacle horizon," the "mussel horizon," or the various "wrack horizons." Seaweeds show the greatest biomass on less exposed coasts and the lower parts of exposed ones. Mussels are found in the most exposed sites, while barnacles are found on the coasts with intermediate exposure. On more wave-beaten sites barnacles are limited at their lower level by mussels, and on less wave-beaten sites they are limited by seaweeds. If the slope of the rocks happens to be very steep and mussels do not settle on them, balanids dominate completely. What is called the barnacle horizon is the mid-intertidal horizon dominated by these cirripedes (acorn barnacles). The most conspicuous species is Balanus balanoides. Above this is Chthamalus stellatus and below it, nearer where the seaweeds grow, is the larger species Balanus perforatus. Balanids and mussels are sucked by a drilling marine gastropod, the Atlantic dog whelk, Thais [= Nucella] lapillus, which causes catastrophic mortality, leaving clear spaces in the substrate that are rapidly recolonized. Several species of pagellids (limpets) are found on the stones without balanids. The most common, Patella vulgata, can be found throughout the horizon, while P. aspera is found in pools and the lower levels. Finally, on the southern coasts of western Europe, P. depressa is more abundant. Another gastropod, Gibbula umbilicalis, frequents the lower level of the balanid area. On balanid coasts seaweeds are not abundant--the activity of filter-feeders and the action of the pagellids control the attachment of propagules.
The lowest level of the intertidal level, together with the sublittoral includes populations of rhodophytes, such as Laurencia, Lomentaria articulata, Plumaria elegans (found more to the north), Pterosiphonia complanata (more to the south), Membranoptera alata, Mastocarpus stellatus, and Palmaria palmata. The coralline algae resist exposure and the action of animals well, forming a microcosm of great diversity. Some seaweeds from lower levels may form populations with a large biomass. The very typical long ribbons of the phaeophytes Himanthalia elongata can reach up to 10 ft (3 m) in length. They filter the light, which is used by other phaeophytes, such as Bifurcaria bifurcata, and in more southerly areas by chlorophytes such as Codium and many smaller species of rhodophytes and phaeophytes. The appearance of these lower levels is impressive in comparison with the exposed coasts, where there are only a few weak stalks of the bladderwrack (Fucus vesiculosus) that, despite its name, lacks bladders.
Mussels tolerate exposure well and form large, clean, shiny populations on exposed coasts, while on sites with intermediate exposure they are found below Fucus vesiculosus and are scarce. On inland coasts at the base of estuaries, where there is little agitation, they are again abundant. In these environments they grow to a large size but do not form large groups; they are covered in epibionts and are dirty from the sediment that precipitates on them. Growing on the mussels are species restricted to this kind of habitat, as well as Fucus vesiculosus and species of the genus Porphyra, which are more widespread. Fucus vesiculosus has wider fronds when exposure to the waves diminishes, and the less beaten the coast, the more vesicles it has.
This vigorous polymorphic species marks the halfway point of the rise and fall of the tide. Above the F. vesiculosus horizon is that formed by the phaeophyte Ascophyllum nodosum, and below the F. vesiculosus horizon is the one formed by F. serratus, overlain by specimens of the genus Himanthalia and seaweeds from the subtidal horizons that venture into the intertidal zone, an escapade that comes to an unfortunate end when the large equinoctial tides expose them to the air and desiccate them. These adventurers include rhodophytes, such as Laurencia pinnatifida and L. obtusa; Chondrus crispus and different species of Gelidium; chlorophytes such as Cladophora rupestris and some species of Codium; phaeophytes, such as Leathesia difformis; and many tiny seaweeds of different groups that form rapidly changing, highly diverse, species-rich cushions.
This environment is divided between the dominant species, but they only dominate for short periods, as if nothing could remain stable for long. The laminarias form "forests" with permanently submerged bases, but their characteristic blades may be exposed during spring tides. In the most exposed sites, the dominant species is Alaria esculenta, while mildly beaten shores are dominated by Laminaria hyperborea. L. ochroleuca is found in more southerly areas, as is Saccorhiza polyschides, which increases in abundance further south. Finally, L. digitata is found everywhere except in the most southerly coasts. Crevices and caves are populated by many subtidal species, including sponges such as Halicondria, anemones such as Sagarita and Tealia, hydroids such as Tubularia, and many compound sea squirts such as Botryllus and other related genera. Some animals such as the sea hare (Aplysia) lay their eggs among the seaweeds. Starfish are predators of mussel populations, and sea urchins perform massive migrations at certain times of year.
The Pacific coasts
The Pacific coastline of the Americas shows great variability in the composition of its fauna and flora: there are great differences between Alaska and Chile. The coasts of California can be described as an example of the different horizons, because they are perhaps the most varied and they have been studied in great detail over a long time.
Bygone authors were impressed by the Californian Pacific coast's violent seas, frequent fogs (due to upwellings), and the highly tortuous nature of its profile and coastal slopes. The zonation profiles, although they consist of elements equivalent to those found on all the world's coasts, include different genera and species. In some cases the species are vicarious, while in other cases this differentiation is at the level of the genus. At the supralittoral level, black lichens form patches rather than a continuous, distinctive zone. On some very beaten coasts they are almost absent. Rhodophytes such as Hildenbrandia are very common, together with other algae encrusted on the rocks and masses of cyanobacteria that fix atmospheric nitrogen. This very poor vegetation teems with periwinkles, such as Littorina scutulata, which is limited to the upper horizons, and L. planaxis, whose range extends deeper.
Balanids are present in the upper intertidal level and the dominant species is Balanus glandula; mixed within its dense, continuous populations are scattered patches of Chthamalus fissus. Immediately below is the balanid Tetraclita squamosa subsp. rubescens, different from the subspecies elegans that dominates the lower horizons on the edge of the infralittoral. The mobile species are mainly muricid gastropods, such as Thais emarginata, trochid gastropods, such as Tegula funebralis, or polyplacophorans of several species, and a range of pagellids of the genus Acmaea that play the same role as the genus Patella in European seas. The species that lives in the highest horizons is Acmaea digitalis, in the intermediate levels it is A. sabra, and below this, in order of increasing depth, A. pelta, A. limatula, A. scutum, and A. mitra. There are also some rhodophytes such as Gigartina papillata and Porphyra laminaris, and some fucaceans, including some clear examples of vicariance; the niche of the European species Pelvetia canaliculata, for example, is occupied by Pelvetiopsis limitata. The Fucus species present is F. distichus, although it is only occasional, as is another fucacean, Hesperophycus harveyanus.
On the most wave-beaten coasts the seaweeds form small, insignificant clumps, and in the calmer areas there are foliose rhodophytes of several species of the genus Iridaea. Nearer the infralittoral there are some very strange-looking palmiform seaweeds, Postelsia, which remain upright out of the water after the tide has gone out. They are strongly fixed, flexible, and resist the pounding of the waves very well. Within this vegetation are many mussels of the species Mytilus californianus, and the lower horizons of wave-beaten shores typically have populations of the ochre sea star Pisaster ochraceus, which eats mussels and is polychromatic, ranging from yellow to brown to purple. On coasts less exposed to the waves, this species is replaced by Patiria miniata.
In the low horizons there are also polychaete worms, such as Dodecaderia fistulicola, abundant pagellids such as Acmaea asmi, and many individuals of several species of snail of the genus Thais, polyplacophorans, colonial sea squirts, and limpets, such as those of the genus Crepidula. The sea urchins Strongylocentrotus purpuratus and S. franciscanus (which is in a lower horizon than S. purpuratus) are at the upper limit of their distribution. Some infralittoral seaweeds may be characteristic of the intertidal zone, such as the phaeophyte Egregia menziesii, similar to the European Himanthalia but reaching 11.5 ft (3.5 m) in length. There are also giant phaeophytes, such as Nereocystis luetkeana, which floats by a single, very large spherical air-bladder located between the fronds and the robust stipe, or the immense Macrocystis, several tens of meters long. Compared to this exuberance, the large laminarias of the Atlantic coasts, such as Leissoniopsis littoralis, Costaria costata, and Laminaria digitata, appear insignificant.
The polar coasts
In the far south and far north the coasts are very different. The permanent ice and the grinding of the ice blocks strikingly impoverish the intertidal environment. The supralittoral is almost a desert and only a few heroic lichens brave it, such as Caloplaca cirrochroides or Verrucaria centhocarpa. The same is true of the upper levels of the intertidal zone, while in the middle part there are abundant rhodophytes of the genus Porphyra or filamentous chlorophytes of the genera Ulothrix or Urospora, which form characteristic horizons that are sometimes the only vegetation present. The fauna is very poor. The most typical animal is the pagellid Patinigera polaris. The pools molded by the erosive effects of ice contain a surprising large range of organisms that breed quickly with short generations: rhodophytes such as Leptosomia simplex and species of the genus Iridaea, and phaeophytes such as Adenocystis utricularis and species of the genus Curdiea. Within these pools several rhodophyceae of the genus Plocamium live and die, as do some small delesseriaceans and phaeophytes, such as Ascoseira mirabilis and some species of Desmarestia from the infralittoral. Below this are the large phaeophytes Phyllogigas grandifolius, the equivalent of the populations of large laminarias in other seas.
2. Low coasts: beaches, mangrove swamps, and estuaries
2.1 On the edge of the ocean
The intertidal environment, that long, narrow frontier between the oceans and the continents, shows an intercalation of very different environments that are affected by what happens both seawards and landwards, varying in extent depending on the type of coast. The ocean has a generally greater impact than the dry land, and its effects are felt continuously, due to the oscillations of the tides, the movement of the waves, the salt spray of fine drops evaporating from the breaking waves, and the materials borne on it. When there are tempests or cyclones, the waves are much larger and break violently, and may cause great loss of life and goods along the coastline. The impact may sometimes be so great that it modifies the coastline.
Yet the impact of the terrestrial system cannot be ignored, especially in low-lying coasts. As well as freshwater, rivers also transport the many small particles produced by weathering of rocks and the erosion of surface materials. Large quantities of the world's soil and nutrients are lost every year through deforestation and inadequate agricultural practice, and most end up in the sea. After being transported by the currents and waves, at least a fraction of these materials eventually accumulates on beaches and in estuaries. Rivers not only bear sediments, but also transport many pollutants, such as fertilizers, pesticides, heavy metals, and industrial products, to coastal lagoons and the sea.
Fresh and salt waters
Salt water enters coastal ecosystems by means of the tide. Seawater enters an estuary at high tide through its mouth and circulates in the channels, creating a gradient from the highest salinity at the mouth to the lowest at the end of the estuary where the freshwater enters. In mangrove swamps on the edges of an estuary or a marsh, the trees closest to the mouth and those in direct contact with the open sea are subject to higher salinity than those rooted in dry land or at the base of the estuary. On the beaches, the tides affect the edge of the sandy shore more than the dunes, although the wedge of salt water that typically forms in coastal aquifers may sometimes affect the aquifers underlying the dunes.
The effects of rivers, temporary and permanent watercourses, and underground waters all counteract those of the tides. Moisture-laden clouds blown from sea to land collide with mountain ranges, leading to rainfall, part of which runs over the watershed's surface and into the rivers, and part of which permeates into the subsoil and into the aquifers. This water as a whole eventually accumulates at the lower part of the watershed near the sea, forming part of the rivers and springs supplying and maintaining freshwater lagoons, and in marshes and estuaries it interacts with the tides to create the salinity gradients mentioned above, as well as supplying coastal aquifers. Coastal community dynamics change when the flow of water reaching the coast diminishes in dry periods. Thus, for example, in estuaries and lagoons the mouths close because the influence of the marine system is greater than that of the land system, and the waves and tides cause sediment to accumulate on the sandbars, almost completely closing them. If the seasons are sharply different, the water in the lagoon may be insufficient to open the sandbar until the next rainy season.
Warm and cold waters
Coastal systems run along the continents, separating dry land from both cold and warm waters. Independently of climate and latitude, the contact between these two great environments always takes place in a narrow, delicate, and fragile strip. The same physical factors that determine each of the ecosystems are repeated: the influence of the tides, the movement of sand, flooding by freshwater, etc. Even so, different groups of animals, plants, and (possibly) microorganisms have adapted to the different latitudes and continents. Yet all have had to evolve over time to survive these environments dominated by physical factors.
The gradient of growth forms along the coastline from the temperate zones to the tropics reflects climatic conditions. Thus, marsh environments in temperate zones are subject to the influence of tides, and therefore suffer periods of flooding that alternate with others when the soil dries out. The dominant flora consists of the few species, mainly herbaceous forms and a few low shrubs, that can tolerate these conditions. Their equivalent in tropical zones are mangrove swamps, formed by tree species that have also had to adapt to these periods of flooding and changing salinity. The tropical humidity, rainfall, and year-round warmth allow these special growth forms to develop even though this unusual environment is very stressful.
Something similar happens on sand dunes. In temperate regions many annual species pass the unfavorable winter period as seeds. Herbaceous plants and grasses fix the dunes. As one moves towards the subtropics, the number of annual species diminishes, while shrubs that tolerate being buried appear in addition to the above-mentioned growth-forms. In the tropics, annual species almost completely disappear and the number of dune-fixing shrubs increases. They range from 1.6-10 ft (0.5-3 m) in height. From the sea one sees a forest that extends down to the beach. The climatic conditions also allow the plants to grow here in more exposed positions with less protection.
The colonization of coastal space
The low-energy coastal spaces where sediments are deposited are the most suitable (in tropical and subtropical areas) for colonization by mangroves, trees, and shrubs that can tolerate flooding and salinity. In temperate areas, the plants colonizing marshes are herbaceous and clump-formers that can also tolerate flooded, salty soils.
The difference lies mainly in the fact that heavier rainfall and higher temperatures allow the formation of species-rich tree communities accompanied by plants showing the most diverse growth forms (epiphytes, climbers, grasses, etc.). This does not happen in temperate regions because there is always an unfavorable season, due to either cold or dry conditions, that forces the plants to carry out most of their life cycle in the more favorable months, which means that these difficult coastal environments favor herbaceous plants and shrubs rather than trees. Mangrove swamps or marshes and beaches and coastal dunes are all produced by sea-borne sediments and stabilised by vegetation. While the former are formed by the very fine sediments transported by the water, the latter are formed by the coarser grains transported and shaped by the wind.
Why do coasts have characteristic plants and animals that are not usually found inland? This is because the land-ocean interaction creates special micro-environments that occur only in this zone and nowhere else. As an example, desert and coastal dunes share some features, such as sand mobility, soil nutrient scarcity, and limited water retention capacity. Yet coastal dunes are different because of their saline environment, their proximity to the water table, and the morning dew from the nearby sea, a major source of moisture for the organisms.
Each coastal community has factors that determine its dynamics. Physical factors control the environment, often creating difficult conditions for plants and animals. The species of this habitat have adapted over thousands of years to live and reproduce successfully in these conditions.
Coastal environments form a type of gradient of natural communities, governed by the degree of salinity and the degree of flooding, ranging from the estuaries to the dunes and forest communities that occupy them. They all depend basically on the equilibrium between the salt water supplied by the tides and the freshwater descending in the rivers and by drainage. To a lesser extent, they depend on the climatic conditions of the rainy season and the quantity of rainfall, or to put it another way, on the degree of flooding.
2.2 Beaches and coastal dunes
Beaches and coastal dunes are sand accumulations that form from sediments the sea deposits on the coast, which are then transported by the wind and deposited around an obstacle. Beaches, which are directly subject to the action of the tides, and dune systems, modeled by wind-blown sand, are normally inseparable environments.
The origin and nature of sandy beaches
The basic nature of a seashore or beach depends on the coast's geology and relief and the physical processes in operation, especially the action of rivers and waves, which generate, accumulate, and distribute sediments. When the slope is too gentle for the waves to bear materials away, a beach forms. The type and shape of the beach will depend on the coast's geomorphology, its waves, currents, and tides, and also on the size, class, and quantity of sediments. Among the restrictions imposed by the general topography, the width of the beach will depend on the quantity of material provided by rivers and other water courses, on coastal erosion, and on the movement of materials along the coast.
The type of sand forming beaches and dunes depends on the material
the sand is derived from, which may be limestone (calcium carbonate), feldspar (aluminium silicates), or quartz (silicon dioxide). This depends on sediments transported from the interior by rivers, but there are also variable quantities of calcareous biological materials (mollusk shells, the tests and spines of echinoderms, etc.). The sands of western Europe's coastline are mainly quartzite, while in other places the sand consists almost entirely of calcium carbonate particles (for example, the Yucatan peninsula in the Mexican Caribbean, where the dry land is karstic and the coasts are surrounded by coral reefs).
The beach is very dynamic. Its profile represents a balance between sediment accumulation and erosion. These occur on a time scale ranging from a few hours (strong storms) to weeks or months (changes in tidal range over the year). The main factor controlling beach dynamics is exposure to waves, which also determines the slope. The waves and slope together determine grain size, which in turn determines porosity and permeability.
The process of dune formation
For dunes to exist there must first of all be an abundant source of sediments--sand--with particles small enough for the wind to blow, and then the wind has to blow fast enough to move them. The lower limit of the zone of sand movement changes with the tidal regime and the interactions between the wind and the relative humidity of the atmosphere. The wind can only blow the sand particles when they are dry, so that to determine the size of the sand input it is important to know the extent of the tides and the time that elapses between high and low tide. Wave movement constantly supplies new sediments that replace those blown away by the wind. Some classic studies performed in the 1940s showed that grains of sand move by a process called saltation: a few grains of sand moved by the wind put other grains into motion when they fall on them at a certain angle. This process occurs whenever there is wind. The minimum wind speed for sand transport is 13-16 ft (4-5 m) per second.
When discussing sand movement, whether accumulation or erosion, the topography, a fundamental factor, must be considered. The wind reaches its maximum speed when it hits the windward side of a dune and reaches the top or the highest crest. Once there it rapidly loses speed, and the sandgrains it bears fall due to their own weight. On a much smaller scale, when the wind hits any object (a plant, a fence) it loses velocity and the wind-borne particles fall, accumulating around the object. This starts the formation of a young dune that joins to other similar ones, forming a row. With time, vegetation colonizes the dune and fixes the sand by preventing the wind from blowing it away. Plants play two roles in this process. They stabilize the sand surface by covering the bare sand, and they also encourage greater accumulation on the dune's surface by reducing the windspeed.
Stabilized dunes cease to move, while uncolonized ones continue to move and advance. This means that a single row of dunes may move at different speeds. The stabilized parts remain stationary, while the mobile zone advances in a crescent shape. Advances of 20-23 ft (6-7 m) per year have been recorded in the dunes of Newborough Warren on the Welsh island of Mon (Anglesey) in the Irish Sea; 16-20 ft (5-6 m) in Donana (in Andalusia on Spain's southwest, Atlantic coast); and from 3-7 ft (1-2 m) on the Gulf of Mexico. Very rapid advances of 98 ft (30 m) per year have been measured at Donana over short periods and for short stretches of dune. Repeated measurements taken at the same point of a mobile dune have shown that more than a meter of sand may accumulate in a year and erosion may reach similar values. This gives an idea of the ability to survive the shifting sands that is required of plants colonizing mobile dunes.
Soil structure and nutrient availability
Succession on dunes is accompanied by changes in the soil. Measurements on northern European coasts have shown that the first row of dunes covered by Ammophila and Elymus contained 0.4% humus. When the vegetation was formed by Hippophae, a genus of the family of Eleagnaceae with nodules containing nitrogen-fixing bacteria (as occurs in many Leguminosae), humus content increased to 0.8%. In grass meadows it reached 1.2%. In the birch woodlands behind the dunes, it reached 4.9% in dry conditions and 5.6% in wet depressions.
Another important factor in dunes is the temperature reached by the sand surface. At midday on a sunny day this may reach 149[degrees]F (65[degrees]C), dropping to 64-68[degrees]F (18-20[degrees]C) or less at night. For several hours a day, the plant parts closest to sand are thus subject not only to high midday temperatures, but also to large daily fluctuations. Moisture from morning dew and rain evaporates rapidly from the top few centimeters of soil. The orientation of the dune's slopes also leads to differences in temperature, and the sunnier slopes are much warmer and less moist. This heat effect disappears rapidly with depth. The sand only 3.9-7.9 in (10-20 cm) below the surface is moist and does not exceed 59-68[degrees]F (15-20[degrees]C), depending on the region, allowing the roots to continue absorbing water and nutrients. In some cold regions the dunes are covered by snow in the winter, and when the spring thaw arrives the many species that colonize dunes have to sprout and grow quickly in order to reproduce in the few months this favorable climate lasts.
The size of the sand grains means that there must be spaces of greater or lesser size between them, and these spaces determine the permeability and porosity. The lack of organic material means the grains do not show cohesion, and this allows water to drain rapidly. The sand of dunes does not form a developed soil with horizons, but it contains enough nutrients for plants to grow and for a soil community. For example, mycorrhiza play an essential role in dune colonization, and in temperate zones the species are different in mobile dunes and fixed ones.
The system has several important types of nutrient input. One type is the sea-borne material that decomposes on the beach, such as seaweed remains and wood. This source of nutrients is very diverse in both space and time, giving rise to local patterns in vegetation establishment. A second important source is the remains of the animals and plants that live in the system. A third source is nitrogen fixation by nitrifying bacteria associated with colonizing plants. Many of these plants, such as Hippophae and Chamaecrista, have nodules.
The first colonizers often include leguminous plants that contribute to increasing the quantity of nitrogen present, which in turn encourages the establishment of other species, and thus the growth of the plant cover. A final important input is the salty sea spray transported by the wind. Seawater contains a large variety of nutrients and even though the quantities are low, the spray that the wind blows landward is an important source of nutrients, especially nitrogen, but also phosphorus and potassium.
The accumulation of organic material in dunes during succession implies the unequal distribution of nutrients. Moist depressions liable to flooding are enriched by nutrients percolating from the higher parts of the dunes. This favors the vegetation and, together with the moist conditions, leads to greater development of the plant cover, as well as greater incorporation of organic material during decomposition. As succession progresses, the soils become more developed.
Strategies for the biological colonization of beaches
Beach dynamics are almost totally controlled by physical factors, and this makes it harder for organisms to colonize them and survive. The first to live on this mobile substrate, with its variable porosity and permeability, are diatoms, which are the primary producers in these environments. Two main physical factors limit their development--light and oxygen. Light is essential for photosynthesis but only penetrates a few millimeters into the soil. Oxygen is supplied by the ebb and flow of the tides. The mobility of the substrate prevents larger seaweeds from attaching, as happens on rocky shores or in deeper and less dynamic sandy areas.
In addition to these unicellular algae there are many other microorganisms (microbiota) that adhere strongly to the grains of sand. Among them are decomposers, responsible for degrading organic material and incorporating it into the flow of nutrients. Yet the system of spaces and channels between the grains of sand (porosity accounts for 32-40% of the total volume of sand on beaches and dunes) is an important habitat for the very specialized interstitial fauna (meiofauna) adapted to these conditions, which is dominated by nematodes, copepods, and turbellarians. The meiofauna may be found down to a depth of one meter, and it has been estimated that each square meter may contain a million individuals feeding on microorganisms and detritus.
However, the best-known inhabitants are the larger organisms (macrofauna) buried in the sand. These include many bivalves such as tellins (Tellina), clams (Donax), cockles (Mactra, Cardium), and razor clams (Ensis); polychaetes, such as sandworms (Ophelia, Nephthys), lugworms (Arenicola) or other species such as Nerine cirrulatus; amphipods such as those of the genera Urothoe, Haustorius, or Bathyporeia; isopods such as Eurydice pulchra; and some sea urchins, such as sand dollars (Mellita) in the Gulf of Mexico and other places on the Atlantic coasts of the Americas, or different species of the genera Echinocardium. Species diversity and biomass are high, although it depends on the degree of exposure to the action of the waves. On the most exposed beaches of Great Britain it is common to find up to 24 species and more than 8,000 individuals per square meter; on exposed beaches this variety may be reduced to a single species of isopod, a few species of amphipod, and one or two polychaetes. Other important organisms include crabs, some of which, such as the ghost crab (Ocypode quadrata) of the Atlantic coasts of America, can bury themselves up to 28 in (70 cm) in the sand.
These sand-dwellers show very irregular distribution. Apart from the changes caused by the supplies of freshwater, currents, local movements of sand, etc., there is zonation set by the flow of tides. The species are adapted to different degrees of exposure, so there is a gradient of distribution going from the zones permanently covered by the water to the upper part of the beaches, where the tide never covers the sand.
Permanently dry sandy zones (the interior of the beach and the rear of the sand dunes) also show dynamics governed by physical factors, the most important of which are wind movement of the substrate and the fluctuation of the water table. This part of the beach may be narrow or wide and the dunes separating it from dry land may vary in form and size. Dunes range from 3-7 ft (1-2 m) in height to 984 ft (300 m), such as some of those on the coast of Aquitaine, in southwestern France, which are covered in marram grass (Ammophila arenaria).
Strategies for the biological occupation of dunes
Few species have adapted to conditions on dunes. These environments are very unstable and are constantly disturbed by the movement of the sand, which modifies the level and the surface of the substrate. These nutrient-poor environments are often bare, meaning that they reach high temperatures. For thousands of years these factors have acted as selective forces, allowing only a few species to survive. On all the world's different coastlines the flora includes plants that grow more vigorously when some degree of burial in the sand occurs.
The most noteworthy dune species include the marram grasses Ammophila arenaria (on European coasts) and A. breviligulata (in North America). European marram grass has been used to stabilize dunes, and has even been successfully introduced into the United States and Canada. Both species regenerate easily from rhizome fragments. Two other important colonizer grasses are sand couch grass (Agropyron junceum) and lyme grass (Elymus arenarius). Species like Corema album and thrift (Armeria) grow on the Atlantic coasts, and thrift is also found on the shores of the Mediterranean. Tropical beach and small dune species include Ipomoea pes-caprae, the sandbinder (Uniola paniculata), the legume Canavalia rosea, and the euphorbiaceous Croton punctatus that colonize beaches. Different species grow on mobile dunes, such as the caesalpinoid Cassia [= Chamaecrista] chamaecristoides, or grasses like Trachypogon gouini, or species of the genus Schizachyrium, all of which tolerate different levels of sand accumulation.
When colonizing plants have fixed a zone, succession begins and new species replace those that started the process of fixation. This second group normally forms a turf covering the surface of the sand. The species vary in different parts of the world, but the processes and community structure are the same. In Japan, colonization of mobile dunes begins with the grass Elymus mollis, the sedge Carex kobomugi, and the composite Wedelia prostrata; the turf is invaded by thickets of Rosa rugosa, and later oaks (Quercus dentata and Q. mongolica) appear, as do conifers (Abies sachalinensis). In several zones of Japan, as in Europe, pine species have been planted on the dunes. In tropical areas, such as Mexico, fixation begins with the legume Cassia chamaecristoides; grasses then arrive, such as Schizachyrium littorale and Trachypogon gouini, which begin turf formation. This develops into a species-rich grassland with the rubiaceous Randia laetevirens, the legume Diphysa robinioides, or the cactus Opuntia dillenii, and then woodland with many species of trees, shrubs, and lianas. Some trees can reach 98 ft (30 m), such as the conacaste (Enterolobium cyclocarpum) and the incense tree (Bursera simaruba). Stabilization and succession thus begin with the initial colonization by the plants that fix the dunes, and once the substrate is less mobile due to the action of the vegetation, other species appear that form a grass community. Then the first clumps and shrubs appear, and finally a woodland forms that varies according to the climatic region in which the dune system occurs.
Dunes are very heterogeneous coastal systems. On the one hand, the species that have adapted to the different continents vary considerably. Some species are widespread, such as the pantropical species Ipomoea pes-caprae and Canavalia rosea, but this type of distribution is less common than more restricted kinds. There are large differences between the flora of the beaches and dunes of Great Britain and those of Japan, the Mediterranean, Canada, Mexico, and Australia, although there are groups of species that play similar roles within the community.
On the other hand, within a single dune system there are very different environments: mobile dunes; stabilized areas with turf, thicket, forest, or jungle; zones that are permanently or occasionally flooded, and so on. This is why a mosaic varying greatly in structure and species composition forms over a small space of a few hundred meters at most. Species-poor surfaces with little plant cover are interspersed with closed herbaceous communities with 10-12 species of grass and other herbaceous plants, and with jungles with many strata that may contain as many as 60 species. All these form part of beaches and coastal dunes, and the protection and conservation of these plants and environments must bear in mind their degree of stabilization and, thus, the long-term dynamics of the system.
The flora and the vegetation
Some species have already been mentioned that live on beaches and sand dunes and can tolerate sand accumulation or even benefit from it. These species belong to very different families and have different growth forms, such as creeping grasses, other herbaceous plants, creeping plants and shrubs, etc. In temperate zones grasses predominate, and towards the tropics creeping plants and shrubs dominate. There is a gradient of growth forms from the zones with harsher climates to those where temperature and humidity are generally high. In harsh climates, growth forms are more resistant to adverse conditions, and the buds giving rise to leaves are at soil level or slightly buried. On the other hand, creeping and thicket-forming plants and shrubs have exposed buds and cannot survive a harsh winter--the reason why they are mainly found on tropical costs, where temperature and humidity are high for most of the year.
The vegetation of sandy places
Several plants can survive burial in sand, and those that grow in temperate zones have been the most studied. They include several mosses that can tolerate a covering of up to 1 in (3 cm) of sand, which is a high value in comparison to their size. Many of the plants that tolerate burial are grasses; the most frequent are the marram grasses (Ammophila arenaria and A. breviligulata), some cord grasses (Spartina patens or S. alternifolia), sand couch (Agropyron), Uniola paniculata, Corynephorus canescens, Eragrostis pilosa, and Festuca rubra. All these species show different types of response such as larger rhizomes and greater leaf areas, as well as an increase in root number and density. Ammophila breviligulata can survive a covering of one meter of sand, while Eragrostis pilosa cannot withstand more than a few centimeters.
There is also a series of plants that can not only live under sand, like the grasses mentioned above, but also grow better under these conditions. These include the composites Artemisia stellaria and Solidago sempervirens, which can withstand more than 20 in (50 cm) of sand and the legume Lathyrus japonica, which withstands more than 16 in (40 cm). Other species such as the sea rocket (Cakile maritima), the chenopods Salsola kali and Atriplex laciniata, and the composite Erigeron canadensis only tolerate a few centimeters of sand. Shrubby species, including the figworts Scrophularia canina, S. frutescens, and Corema alba are common in Donana National Park on the Atlantic coast of Andalusia where they are found in the mobile dunes. Other species found in this region include the sea daffodil (Pancratium maritimum), thrifts such as Armeria gaditana or A. pungens, chufa (Cyperus schoe-noides), the borage Echium gaditanum, and the crucifer Malcolmia littorea.
In tropical areas there are also species that can withstand life under sand. There are several different growth forms: grasses (Uniola paniculata and Schizachyrium littorale); conspicuous herbaceous plants and several brassicaceous species of the genus Cakile; creeping plants like Ipomoea pes-caprae or the legume Canavalia rosea, with runners up to 98 ft (30 m) long; clump-formers like the euphorbiaceous Croton punctatus or the fabaceous Cassia [= Chamaecrista] chamaecristoides; and shrubs 7-10 ft (2-3 m) high, such as Suriana maritima, the only representative of the family Surianaceae; and the polygonaceous sea grape Coccoloba uvifera. Thus, mobile sands are colonized by plants with very different growth forms. These plants' growth forms, their rates of growth, and their survival all depend on their response to the conditions of burial, and this response in turn determines the size, form and stability of the dunes.
A dune starts to form when sand is deposited around a recently germinated plant--for example, a seedling of the poaceous grass Agropyron junceiforme. Its primary root quickly grows 4-6 in (10-15 cm) towards the greatest moisture. The first lateral roots grow horizontally near the sand surface, and a series of small buds appear along them, which then produce rhizomes that separate from the parent plant for about 12 in (30 cm) and produce new buds. This type of growth can continue for two seasons, but eventually longer, horizontal rhizomes are produced that increase the area occupied by a plant.
In the autumn the rhizome tips grow upwards and prepare to produce a new set of stems in the spring. This growth continues almost indefinitely as long as sand accumulation is not excessive, as the stems can sprout after being buried in up to 9 in (23 cm) of sand. When burial is deeper, the stems die and instead of spreading laterally, the rhizomes grow upwards until they emerge from the sand, and then they produce new buds at sand level. This growth form is called sympodial and can be sustained at depths of up to 5.9 ft (1.8 m).
It is not clearly understood why burial stimulates the growth of these plants. The accumulating sand provides moisture and nutrients, and maybe there are changes in soil aeration, pH, and the microbiota, but no mechanism has been clearly identified. The seedling stage is the most vulnerable to burial, one of the main causes of seedling mortality. Desiccation is another important cause, because the root has to grow quickly enough to cross the first few centimeters of the warmest and driest sand before it finds the moister and cooler conditions below.
Another interesting group of dune plants is found in wet depressions that may be inundated. The determining factor in these environments' dynamics is the fluctuating depth of the water table. When they are permanently flooded, aquatic plants become established. Temperate examples include Potamogeton (Potamogetonaceae), Zan-nichellia (Zannichelliaaceae), Juncus (Juncaceae), and Glaux (Primulaceae). The main tropical examples are Nymphaea (Nymphaeaceae), Cyperus and Eleocharis (Cyperaceae), and Hydrocotyle (Apia-ceae). Many algae also find these conditions suitable for growth and reproduction and form large populations. There is a moisture gradient between the deepest part of the depression, where the roots grow in a flooded area lacking oxygen, and the periphery, where they grow in moist sand.
The coastal forest margin
Further from the direct aggression of the spray that makes the coastal environment so salty, communities appear that are dominated by woody species that tend to be shrubby--the frontline of the inland forest vegetation.
On the coasts of southeast United States, for example, the first shrub to start colonization is the sea myrtle (Baccharis halimifolia), together with another shrubby composite, Iva imbricata. They are soon joined by other shrubs (often in reality small trees) such as yaupon (Ilex vomitoria), red cedar (Juniperus virginiana), and bayberries (Myrica).
Some subtropical species, such as palmetto palms (Sabal palmetto) or devilwood (Osmanthus americanus), reach the northernmost point of their range in areas as far from the tropics as North Carolina and Virginia. These shrub formations form a type of strip separating the evergreen or mixed forests of the coastal plains of the southeast United States from the areas of dunes and saline areas along the coast.
At the southern tip of Africa, the role of transition between dunes and fynbos (see volume 5) takes the form of a type of very dense scrub dominated by shrubs with small, leathery leaves and spiny branches. Some trees from the fynbos itself occur, such as the iron tree, Sideroxylon inerme (Sapotaceae), but it is dominated by shrubs, such as Olea exasperata (Oleaeceae), Euclea racemosa (Ebenaceae), and Rhus crenata (Anacardiaceae). In areas with yearly rainfall below 18 in (450 mm) these thickets are replaced by similar ones that also contain succulent plants (Euphorbia, Aloe, Crassula, Zygophyllum, etc.).
On the northeastern coast of Brazil, the shrub formation called restinga colonizes the dunes. There are two main types: heath restinga and myrtle restinga. Heath restinga is dominated by ericaceous species, such as Leucothoe revoluta or Gaylussacia brasiliensis, and other plants similar in form to the heaths; there is also the melostomataceous Marcetia glazioviana, the eriocaulaceous Paopalanthus polyanthus, and several cacti (Cereus pitaya, Melocactus violaceus, etc.). Myrtle restinga is dominated by myrtaceous plants, mainly of the genera Eugenia and Myrcia, which can form small trees. Other shrubs include the malpighiaceous Byrsonima sericea, the burseraceous plants Protium brasiliensis and P. icicariba, and the polygonaceous Coccoloba uvifera, as well as many terrestrial bromeliads, and in open areas, the small palm Diplothemium maritimum.
Another palm tree, the coconut palm (Cocos nucifera), plays an important role in beach vegetation along many of the shores of the tropical Pacific, although it has reached some places thanks to human transport. The coconut is a palm with a long, flexible, greyish, almost smooth stem, with a tuft of about thirty pinnatifid leaves 16-20 in (5-6 m) long. Unlike other palms, the coconut is monoecious and its flowers are unisexual--in other words, the male and female flowers are separate, but both are borne on the same plant, not even on separate inflorescences in this case. Its origin is unclear but everything appears to indicate it is from the coasts of Insulindia or Melanesia. From here, unaided by the navigators who populated the Pacific, it had colonized most beaches between 20[degrees]N and 20[degrees]S in the Pacific and Indian oceans, even colonizing coastal slopes up to the 2,953 ft (900 m) contour, before it was spread by cultivation to the Atlantic's shores at similar latitudes.
The fruit of the coconut palm is adapted to long distance dispersal by sea (it can germinate after floating 110 days in the sea, long enough to travel 2,485 mi [4,000 km]). It probably grew spontaneously on many islands in the Indo-Pacific region before humans arrived. It even appears that by the middle of the 16th century, the coconut palm (borne by currents or Polynesian navigators) was already under cultivation on the Pacific coast of the central American isthmus, from Mexico to Darien. Alphonse de Candolle pointed out that it could not have been cultivated there for long, or its cultivation would have spread to the Caribbean before Europeans reached Central America. (See pages 442-443.)
The fauna and animal communities
The fauna of the dunes is limited but may contain unique features. The dominant groups include insects (hymenopterans, coleopterans, and dipterans) and vertebrates.
Dune insects include those only found in coastal areas, as well as species that tolerate salinity but whose distribution is not exclusively restricted to dunes; other species that live in sandy soils but not saline ones; widely distributed species without defined preferences; and social ants. The vertebrate fauna of dunes is not very rich. In general, the front of a dune (facing the sea) is different from its more protected rear. The moister areas house more organisms, but they have not been thoroughly studied. The dune fauna is much poorer in arid climates than in wetter climates, where it increases considerably.
Beaches are very rich in invertebrates, including representatives of most phyla: Platyhelminthes, Nematoda, Annelida, Rotifera, Brachiopoda, Mollusca, Tardigrada, Arthropoda, Echinodermata, etc. Different dune systems have shown as many as 215 species of hymenopteran, 43 species of bee, 30 species of curculionid (weevil), 188 species of spiders, and 368 species of other arthropods.
All these animals have adapted to a highly unstable substrate and intense wave action. Many of their adaptations are responses to cycles of variation in environmental conditions. Waves, like their intensity, are cyclic phenomena. Tides respond to different cycles--diurnal, lunar, seasonal--and there are also cycles of erosion and deposition, sand accumulation cycles, cycles of water movement through the sand, etc. And, although it appears a contradiction, all these cycles occur in a changing, not easily predictable environment.
The aquatic animals of beaches are one of the few communities unable to modify their environment to make it more favorable. They have, in fact, had to adapt to it to survive. This is why these highly mobile animals can bury themselves quickly in the sand and orientate themselves to reoccupy their former space before they were displaced. Those that live buried in the ground are able to slide between the sand grains, and so they are often elongated in shape, although they usually only migrate vertically in the sand column. Even so, there are a large number of organisms adapted to these conditions, forming a large group of sand-dwelling species.
Birds of the beach
Their large populations and interaction with invertebrates and fish make birds one of the most important groups of animals found on beaches. The most common birds along beaches belong to the order of the Charadriiformes and include families such as the plovers (Charadiidae), oystercatchers (Haematopodidae), sandpipers, stints, curlews and snipes (Scolopacidae), and gulls and terns (Laridae). Ibises and spoonbills (Threskiornithidae) and herons and egrets (Ardeidea) are also sometimes found on beaches, while cormorants (Phalacrocoracidae) prefer to feed at sea where the waves break, rather than on the beach.
Many beach birds are migrants and are one of the most mobile components of the beach community, moving freely from one area to another in search of food. It has been estimated that birds can consume up to 44% of a beach's total annual production of invertebrates, although their catch is usually greater in temperate zones than in tropical ones. Birds also return a large amount of organic material to the beach in their excrement, feathers, and corpses, thus enriching the beach and the entire dune system.
Various species breed on beaches. For example, between April and August in Florida, eleven different nesting species have been found, each choosing a nesting site in accordance with the vegetation cover. The least tern (Sterna [=albifrons] antillarum), royal tern (S. maxima), Sandwich tern (S. sandvicensis), black skimmer (Rynchops nigra), and snowy plover (Charadrius alexandrinus) prefer bare sand with little or no vegetation cover. On the other hand, Wilson's plover (Charadrius wilsonia), American oystercatcher (Haematopus palliatus), and willet (Catoptrophorus semipalmatus) choose areas with scattered grasses and other plants. Three species hide their nests from neighboring nesters in areas with much denser plant or bush cover: the laughing gull (Larus atricilla), the gull-billed tern (Sterna nilotica), and the Caspian tern (S. caspia).
Few vertebrate species are endemic to or restricted to coastal dunes, mainly because they occupy such a small area. Coastal dune endemic species include a few micro-mammals, especially two rodents: the beach mouse Peromyscus pelionotus, found in dunes in the southeast US, and Microtus breweri on Muskeget Island, Massachusetts. The population of subspecies P. pelionotus trissyllepsis, found only in the area between Perdido and Pensacola bays in northwest Florida, was reduced to 26 individuals in 1929 by a hurricane. Most vertebrates are also found in other nearby communities, such as forests, jungles, or cultivated ground.
Little is known about the impact of mammals on beaches, partly because they are nocturnal animals. The main observations have been based on the predation of marine turtle eggs. The animals observed in studies on South African beaches include different species of mongoose, such as Herpestes [= Galerella] pulverulenta), the yellow mongoose (Cynictis penicillata), and the marsh mongoose (Atilax paludinosus), and occasionally chacma baboons (Papio ursinus).
One interesting trophic relation studied on dunes is that between herbivores and plants, especially rabbits and grasses. The system's low nutrient content does not allow the establishment of intense trophic chains. Most dune turfs show signs of herbivore activity, for which rabbits are mainly responsible. Their activity has important effects on the plant diversity of the meadow. When there is excessive or insufficient grazing, floristic diversity declines; in other words, the floristic diversity increases with moderate exploitation. When herbivores are absent, the dominant species are grasses, such as Festuca rubra, and sedges, such as Carex arenaria, which displace other herbaceous plants. Herbivore activity affects the system in several different ways. First, there is selection against the tastiest species, as they suffer most damage. Second, the production of droppings, especially around their warrens, enriches the soil. Third, excessive trampling may cause considerable damage, above all after livestock introduction, or even as result of the high number of individuals, as happened with the seagulls on Walney Island, on the English coast of the Irish Sea.
Few reptiles visit beaches. The most important are marine turtles, although their visits are brief. In fact, beaches are the only places they nest and thus play a very important role in the conservation of the few surviving marine turtle species. Their eggs are eagerly sought by mammals, by some birds, and above all by humans, which has led to their disappearance from many beaches. Together with some snakes and iguanas, marine turtles are the only reptiles adapted to the marine environment. Their distribution covers all the tropical and subtropical areas, and while some are found in all the seas, others, such as the western Pacific green turtle Chelonia agassizii, have more restricted distributions. They all depend on firm ground during the period of reproduction. They perform long migrations, leaving the ocean environment where they grow and feed in order to reach the beaches, where they excavate holes up to a meter deep at the upper limit of the high tide so they can lay their many eggs. If the nest is below the high tide line, all the eggs may be lost. It is unknown why they all head for a particular beach, totally ignoring an apparently identical neighboring one.
Generally, the females reach the beach at night, make a hole, lay their eggs, fill in the hole, and return to the sea. About 70% of the eggs hatch and the little turtles head straight for the sea. At this moment, they are easy prey for some birds, such as seagulls. Monitoring marked turtles has shown that a single female can lay eggs more than once in a season, and then not lay for two or three years. Curiously, the same females return again and again to the same beach.
Mangroves develop on the tropical and subtropical coasts, in sheltered areas with calm, shallow waters, such as bays, estuaries, coastal lakes and river deltas, where the salt water reaches. They vary in appearance, but they generally form forests 16-66 ft (5-20 m) in height that when well developed are dark, dense, impenetrable, full of aerial roots, and difficult to cross, except by using the intricate network of natural drainage channels.
The mangrove swamp is a true frontier between the terrestrial and the aquatic environments. It contains many different habitats, and it is estimated that the world's mangrove swamps may contain more than 3,000 species, mainly animals. The greatest species diversity is found in the Indo-Malaysian region and in Papua New Guinea.
In an environment that is so hostile to the land flora, the plants have to be adapted to constant changes in salinity (it may vary from 0-46 [per thousand]), high temperatures (68-86[degrees]F [20-30[degrees]C]), the scarcity of oxygen around the roots, the permanent flooding of part of the vegetative organs, and the problems of reproduction and dispersal. This is why they show a series of adaptations, such as the stilt roots of the red mangrove (Rhizophora mangle) that give the tree enough height to maintain its leaves above the water during high tide and also give it a better hold in silty soils.
One of the most surprising adaptations of the pioneer species found on the edge of the coastline, and thus most adapted to salinity, is that they show viviparity--the condition when seedlings begin to grow before falling from the tree. The red mangrove is a good example of this adaptation. This allows them to grow in a highly saline environment and advance seawards, supported by a junglelike tangle of stilt roots.
The mixture of freshwater and salt water, the rich supply of nutrients, and the unusually sheltered conditions make mangroves some of the world's most productive systems. The mangrove roots also form an intricate interconnected system providing shelter and support for many animals. This system's great richness is mainly due to its location at the interface between the land and the sea, meaning it receives a flow of energy and nutrients from both. The mangrove swamp not only receives nutrients, but also exports much of its production to adjacent terrestrial and marine ecosystems, especially in the form of fallen leaves that provide organic remains colonized by many organisms. This is the beginning of a trophic chain that ends with large predatory fish or fish-eating birds.
The physical factors determining the dynamics of the mangrove swamp are the degree of flooding, salinity, drainage, and soil composition.
The degree of flooding
The degree of flooding is a very important factor in mangrove seedling establishment. The extent of flooding, measured as the period when the community is not flooded, underlies the local distribution of species. The most flood-tolerant species is the red mangrove (Rhizophora mangle), and there are even specimens in the Caribbean and Florida that are underwater all year round. The black mangrove (Avicennia germinans, and other species of the same genus) needs periods of 10-110 consecutive days without flooding, while the white mangrove (Laguncularia racemosa and related species) needs almost half a year.
The composition of the substrate
Soil flooding leads to great changes in different aspects of the system. Oxygen content diminishes because its diffusion is slower and its solubility in water is low, and microbial activity is also reduced. At the same time, there appears to be an accumulation of toxic elements due to the reduction of iron and manganese. All this affects the tree's level of photosynthesis and its osmotic processes, and leads to less accumulation of biomass or lowered growth.
The acid sulphated soils that form in some mangrove swamps are called "cat-clays." Here, soil formation has led, is leading, or will lead to the production of enough sulphuric acid to affect many soil characteristics. Potential acid sulphated soils are immature muds with a pH of 7-8.
Where drainage is absent, sulphides may be produced, mainly iron pyrites (S2 Fe), due to the bacterial reduction of the sulphates in seawater to sulphur and sulphides. These reduction processes can only occur when sufficient organic material is present, and only in a reducing environment, such as that provided by the flooded conditions of the mangrove swamp.
The soil's drainage capacity depends on the proportions of the different fractions of sand, clay, and silt, which in turn depend on local soil conditions where the swamp is developing. As an example, look at two mangrove swamps on the island of Jamaica. Coarse sand ranged from 1.7-17.3% in the first and 30.5-93.7% in the second; clay varied from 9-54.7% in the first and from 0-1.4% in the second; silt varied from 5.1-44.9% in the first and from 0.03-1.85% in the second.
Salinity is another important factor. Several species have developed the ability to get rid of excess salt and grow without problems in different degrees of salinity, and sometimes even in direct contact with seawater. Tropical areas where mangroves grow show high levels of evaporation, so soil moisture reduces considerably during periods of continuous exposure, increasing salinity.
Mangrove swamps are communities typical of the tropics and sub-tropics, although some more tolerant species, such as Avicennia germinans, a black mangrove, live in areas with temperate climates, such as southern California. The greatest number of species are found in southeast Asia, India, and in the Pacific islands, while those in America are poorest in species.
Mangroves and associated plants
The term mangrove is applied to several species of woody plant (trees ranging from 13-20 ft [4-6 m] to 98-115 ft [30-35 m] in height or taller) belonging to very different families but sharing similar growth habits, adaptations to salinity and flooding, and the ability to establish in very geomorphologically dynamic areas.
So, although there are differences between the species of these communities in different continents, they are all called mangroves. They are common in coastal lagoons and estuaries, and sometimes spread upriver and penetrate a few kilometers inland. They are not usually found on beaches directly exposed to the sea.
Mangroves are adapted to flooding, and this is why they have developed structures that allow them to maintain physiological functions, such as nutrient uptake and respiration, in the very special environments where they grow. All mangroves have special roots modified for respiration in flooded conditions, though these roots need to be exposed to the atmosphere for some of the time.
Each type of mangrove has developed a different solution to this problem, and some have adventitious breathing roots that hang from the aerial parts, while others grow breathing roots called pneumatophores, sticking up from the submerged roots.
Pneumatophores were first described in 1660, but their function was not shown experimentally until 1955. They are typical of plants growing in flooded conditions and at their tips they show a swelling of the aerenchyma (spongy, cork tissue) that is responsible for gas exchange. The opening of the lenticels is so small that air can enter, but not water. The oxygen reaching the intercellular spaces is used up when the roots are totally submerged, and the CO2 generated quickly dissolves in the plant liquids, creating a negative pressure. This negative pressure causes oxygen to be absorbed rapidly when the roots are exposed at low tide. Thus, tides cause these structures to exchange oxygen. When the aerial root's lenticels are submerged, the oxygen concentration in the plant diminishes. If these conditions last, oxygen deficiency arises and the mangrove may die.
There are basically two types of pneumatophore. The first, corresponding to the model described, shows a determined longitudinal and radial growth form, like the simple, pencil-shaped roots of Avicennia. In the second type, the roots grow continuously acquiring a more-or-less conical shape, as in the species of Sonneratia, which can grow 7 ft (2 m) tall. A single root may often cross more than one environment, as happens when the root is partly in the air and partly in water.
All mangrove species are halophilic (flourishing in a salty environment), but they have different strategies to counteract salinity and flooding. The leaves of Avicennia have glands that efficiently remove salt from the sap, and the rain then washes the salt from the leaves. The solution excreted is saltier than seawater, and at 90% sodium chloride and up to 40% potassium chloride, it shows proportions quite similar to seawater. The species of the genus Rhizophora excrete salt from the sap by maintaining a negative hydrostatic pressure in the xylem, thanks to the maintenance of an unusually high osmotic pressure in the cells of the leaves. The roots function as a large filter permeable to water and almost impermeable to the salts of seawater. By means of this mechanism, they overcome the relatively high pressures generated by seawater penetration. Fresh water has occasionally been observed to flow from damaged plants submerged by the tide.
There are many species of mangrove. The most typical ones are the red mangrove (Rhizophora harrisonii, R. mucronata) and other rhizophoraceous plants such as the genera Bruguiera and Ceriops, and the species of the genus Lumnitzera (Combretaceae), whose highly conspicuous stilt roots give rise to adventitious respiratory roots and can colonize the most unstable substrates. Black mangroves (Avicennia germinans, A. marina and other verbenaceous plants, and Sonneratia alba and other sonneratacean plants), have typical pneumatophores and can live on substrates of medium stability. The white mangrove (Laguncu-laria racemosa, Conocarpus erectus and other combretaceous plants and Carapa [= Xylocarpus] obovata and other meliaceous plants) have purely underground fixation systems and so require silty substrates that are very stable. The buttress mangroves (Pelliciera rhizophorae and other theaceous plants, and Mora megistosperma, a caesalpinaceous plant), have robust trunks with large stem buttress roots and are found on very stable substrates.
The typical red mangrove (Rhizophora mangle) is distributed throughout the tropical coasts of the Americas, western Africa, and the Pacific islands. It grows in very variable conditions of flooding, salinity, and substrate, but even so it prefers silty soils sheltered from the direct attack of the sea (although it is often the species that establishes itself closest to the sea) with an abundant supply of fresh water and rainfall. In Mexico's Yucatan peninsula it can reach 164 ft (50 m) in height. It is a viviparous plant, as the seeds germinate on the plant and are dispersed as seedlings after 3-6 months of growth.
The adult tree has a system of aerial roots, known as stilt roots, that emerge at right angles to the trunk and then curve down into the ground. Their main functions are to grow into the soil surface and produce rootlets for nutrient uptake; to allow gaseous exchange, by means of their system of lenticels and spongy air storage tissue (aerenchyma), when the soil is flooded; and to anchor the tree in the unstable soil. This species has often been used as firewood and as a source of tannins for tanning leather.
The typical white mangrove (Laguncularia racemosa) is common in American and African mangrove swamps. It grows in different conditions and forms large populations, together with the black mangrove, on the less flooded edge of the mangrove swamp and in more protected inland areas on soils that may be sandy, clayish or silty, and in salty or brackish waters. It reaches 82 ft (25 m) in height.
It looks very similar to the black mangrove, and their leaves differ only in that the white mangrove has a pair of glands on the petiole. It can excrete salt from its leaves. The leaves often show white particles of salt that contrast with the shiny green leaves. The small, oval fruit rarely shows viviparity. The seed generally falls and sprouts a root within a few days. The seedlings float on the sea currents, in which they can only live for a few weeks.
They settle in not very flooded areas. They have a large surface root system, with far-reaching horizontal roots that give rise to very small pneumatophores. When a mangrove swamp is felled, this is one of the first species to recolonize.
The typical black mangrove (Avicennia germinans) is widely distributed along the coastlines of the American continents. It tolerates flooding, but is more frequent within the land areas inside the mangrove. Its distribution is affected by the microrelief, the changes this causes in salinity, and the height and duration of flooding. It tolerates sandier soils. It grows in association with the white mangrove or forms extensive pure stands. It can reach 115 ft (35 m) in height. The leaves can excrete salt, like those of the white mangrove. The small (2 cm) fruit is elliptical. It can also be considered viviparous. The seedlings float and after three weeks the first roots appear. They only establish in areas that are not flooded at low tide. The root system is superficial, with pneumatophores that develop from the horizontal roots. These structures can reach extremely high densities (up to 672 pneumatophores per square meter), although this may vary. Their size increases with the extent of flooding. In this species, the root system may represent up to 65% of the plant's tissue. The high level of root formation implied by these values allows the species to establish itself rapidly on the coasts where sedimentation is high. Its wood is used for fuel and to make signposts, fences, etc. It is often felled to plant rice and other flood-tolerant crops.
Other species considered to be mangroves (or at least trees of the mangrove swamp) include Crenea patentinervis (Lythraceae), Tabebuia palustris (Bignoniaceae), and Hibiscus tiliaceus (Malvaceae). There are even halophilic palms, such as Euterpe cuatrecasana found in American mangrove swamps, which can grow adventitious roots and pneumatophores, and the dwarf date palm (Phoenix reclinata) found in African mangrove swamps. The herbaceous plants include Acrostichum aureum, a large (almost 7 ft [2 m]) fern that is frequent in mangrove swamps throughout the world.
On the flat ground next to mangroves, other floodable forests form consisting of different species that either cannot tolerate salinity as well as mangroves, such as the manchineel (Hippomane mancinella), or that live only in areas flooded by fresh water, such as the custard apple (Annona glabra). They are called flooded forests because their complex structure includes a multi-species tree layer with very many climbers and lianas.
The roots of the mangrove are used as a substrate by many sedentary animals, especially filter-feeders, such as oysters (Crassostrea), sponges, tunicates (Ecteinascidia turbinata), etc. Several species use the lagoon, backwater, and channel habitats of the mangrove swamp to complete their life-cycle. Lobsters (Penaeus) and other crustaceans, mollusks, and several species of resident, migratory, or seasonal fish--including many predators such as the tarpon (Megalops atlantica), the gray snapper (Lutjanus griseus), and barracudas (Sphyraena)--all find stable conditions and protection favorable to their young in these waters. Not only do mangrove swamps have notable flora and fauna, but they are also of great importance to fisheries: many settlements make use of their fish, crustaceans, and mollusks.
The vegetation of mangrove swamps shows a typical zonation of different dominant species, depending on their salt tolerance, the soil type, and the depth of water. The fauna naturally follows a similar pattern. The most external zone, the lagoons and estuaries where the tide prevents stagnation and excessive increases in water temperature, is an important shelter for mollusk and crustacean larvae, and especially for the fry of fish (more than 400 species), mainly clupeids (Brevoortia), engraulids (Anchoa), sciaenids (Bairdiella and Cynoscion), ariids (Arius), centropomids (Centropomus), and others. It is also a temporary habitat for several migratory species that seek food in these productive waters, such as mullets (Mugil), sciaenids (Kuhlia), clupeids (Ethmalosa), serranids, belonids, and sparids. In the inner part, the physical and chemical conditions are more extreme and different species of small fish live in the marshes that form there. They are of freshwater and marine origin such as ciprinodontids, pecilids, tilapias, gobiids, blennioids, mudskippers (Periophthalmus and Boleophthalmus), and climbing anabantids (Anabas scandens). Crustaceans such as the fiddler crab (Uca) are also abundant in muddy areas.
The amphibious nature of the mangrove swamp extends to much of its fauna. Crabs (Grapsus, Aratus and others), snails (Littorina), and mudskippers (Periophthalmus) are present on the roots above the water level. Mudskippers can, surprisingly, climb up the roots and spend long periods of time out of the water. They can do this because of their adaptations to terrestrial life, such as eyes that stand out from the upper part of the head and possess a crystalline lens adapted to vision in air. They also have a cutaneous respiratory system and obtain part of their oxygen through their skin, giving them temporary independence from the aquatic environment.
Describing American mangrove swamps is a useful way to show what a mangrove swamp is like and how it functions. The species vary from continent to continent but their functional adaptations are similar, since they are all adapted to the special conditions of life in a mangrove.
The trees of the mangrove swamp
The best-known and most widely distributed American mangroves are the red mangrove (Rhizophora mangle), the black mangrove (Avicennia germinans), and the white mangrove (Laguncularia racemosa), but in spite of their dominance, the mangrove system is neither homogeneous nor floristically poor. For example, the mangroves on the central and South American coast of the tropical Pacific have other typical tree species, such as the malvaceous Pavonia rhizophorae, the rhizophoraceous Rhizophora racemosa, the caesalpinaceous Mora oleifera, and the verbenaceous Avicennia bicolor. These zones suffered less drastic changes than the Atlantic coasts at the same latitude, and they maintained their conditions of humidity during the Miocene-Pliocene period when the climate became colder and drier. The maintenance of favorable conditions and the formation of the Central American isthmus led to differences arising between the mangrove swamps on the two coasts, and some species were lost on the eastern Central American and Caribbean coasts that are still present on the Pacific coast.
Salinity and zonation
Soil salinity and rainfall govern the presence of some accompanying species. The amaryllidaceous Hymenocallis littoralis is found in soils with little salt, while the verbenaceous shrub Clerodendrum pittieri appears when salinity increases. The mangrove swamps in rainier areas are usually more species-rich than those in drier areas.
Mangrove swamps frequently show zonation from sea to land. Thus the red mangrove (Rhizophora mangle) establishes itself closer to the sea, where the conditions are more saline and flooding is greater, while the black mangrove (Avicennia germinans) establishes itself further inland. This phenomenon, which does not always occur, has been interpreted in a variety of ways. Succession is probably due to a series of different causes. On the one hand, as succession proceeds, a species or group of species displaces and occupies the space of its predecessor. On the other hand, the dynamics of the rivers, springs, and the seawater supplying the lagoons and estuaries create habitats that are more favorable for one or other species. Physiological requirements also lead to zonation, as does the dispersal of the propagule, which varies with their shape and size, and their resulting buoyancy and ability to be transported by the tides.
It is widely recognized that mangrove swamps are very important for the fauna, as they are areas where birds nest and where fish and mollusks reproduce. The fauna of the mangrove swamp itself is not very well known, nor are the differences between the different types of mangroves swamp. Even so, it is generally considered that mangrove swamps provide six main types of habitat for the fauna.
The first habitat, the tree canopy, is an essentially terrestrial environment occupied by animals that also live in the nearby forests, including many birds, monkeys, and insects, in addition to mosquitos (culicids) and gnats (ceratopogonids whose larvae live in the mud and whose adults form swarms at ground level). The second habitat is the holes and cracks in the tree trunks and branches, where water accumulates and the larvae of some insects grow. The third habitat is the soil surface inhabited by fish such as Periophthalmus, hermit crabs, and some mollusks. In the subsoil, there are snails, goose-necked barnacles, oysters, crabs (especially the genera Helice, Ilea, Sesarma), and others. Finally, there are the permanent and semi-permanent pools, where the larvae of mosquitos, gnats, and other insects grow, and the more-or-less permanent lagoons and backwaters that many animals need to complete their life cycles. Jellyfish and crocodiles (Caiman crocodilus, Crocodylus acutus) and manatees (Trichechus) are also present in these waters. In mangrove swamps, as in costal dunes, most of the fauna is not specific to the mangrove swamp, but also frequents adjacent terrestrial or aquatic environments. Yet these animals find both food and shelter in mangrove swamps, and this is why the fauna are abundant. It is worth mentioning that there is an enormous number of algae and invertebrates between the roots. The roots of the mangrove serve as a substrate for different sedentary animals, especially filter-feeders, such as bryozoans, mollusks, sponges, and crustaceans. Examples of these sedentary organisms living on the roots include the oyster (Crassostrea colombica and other species of the same genus or similar genera), the fire sponge (Tedania ignis), and several ascidians. These organisms are eaten by predatory mollusks, such as snails of the genera Mitra, Thais, and Chicoreus. Some mangrove animals are of economic importance. For example, every year on the Pacific coast of Costa Rica in the Gulf of Nicoya, eight million individuals of the bivalve piangua (Anadara tuberculosa) are extracted, while six million are caught on the coasts of El Salvador. Crustacean catches are also important and mainly consist of crabs, such as the blue crab (Callinectes toxotes), the juey (Cardisoma crassum), and several species of lobster and shrimp. Rearing them, especially the lobsters of the genus Penaeus, is receiving more and more attention in areas with mangrove swamps.
The mangrove swamp is also a growth and development area for the larvae that then form part of the fauna of the nearby continental shelf. The fish living here include species of commercial importance whose larvae find food and refuge. The main species include snappers (Lutjanus), mullets (Mugil curema), and sea bream (Eucinostomus gracilis).
The birds that nest and feed in these tropical American environments include the American white and brown pelicans (Pelecanus erythrorhynchos and P. occidentalis), the sooty and black-vented shearwaters (Puffinus griseus and P. opistomelas), various egrets such as the cattle egret (Bubulcus ibis) and night herons (Nycticorax), several ducks of the genus Anas, various plovers (Charadrius) and terns (Sterna), the roseate spoonbill (Platalea [= Ajaia] ajaja), the wood stork (Mycteria americana), and the white-faced ibis (Plegadis chihi).
One of the most conspicuous species, with large populations in north Yucatan, the Bahamas, and Florida, is the greater flamingo (Phoenicopterus ruber). Among the birds most consistently found in mangrove swamps are species of heron and egret. In arid climates, mangrove swamps and flood areas are essential habitats for resident, as well as migrant, birds. They are the only plant communities with green foliage all year round and thus provide the fauna with shade, humidity, and protection.
The most frequent visitors to these mangrove swamps include several mammals such as the opossum (Didelphis virginiana); several bats, such as species of the genera Pternotus and Glossophaga, and especially the fisherman bat Noctilio leporinus; ricerats Oryzomys melanotis and O. palustris; the grey fox (Urocyon cinereoargenteus); the coyote (Canis latrans); the red coati (Nasua nasua); the racoon (Procyon lotor); the striped skunk (Mephitis macroura); several species of feline of the genus Felis; the collared peccary (Tayassu tajacu); and the white-tailed deer (Odocoileus virginianus). Although monkeys, otters, and manatees also used to be frequent visitors to the mangrove swamps, they have now almost disappeared. The banks of channels are preferred sites for several species of crocodile and iguana (Iguana iguana, Basiliscus galleritus). Reptiles and amphibians are not very abundant, but there are some snakes, frogs, and turtles. More detailed studies may well show that these mangrove swamps contain a great diversity of fauna.
The types of mangrove swamp and their distribution
The distribution of mangrove swamps is determined by temperature--more precisely, by the occurrence of frosts. The frequency of these low temperatures is what sets the limits to mangrove swamp distribution. Species with stilt roots are less resistant to low temperatures than those with pneumatophores, and this is why the first group is confined to the tropics. Yet their distribution also depends on the substrate and the effect of the waves on both the seedlings and the adults, which is why they only grow on coasts with very low energy. They grow on many different types of sediment, including the calcareous sand derived from coral reefs. However, they grow best on fine sediments rich in organic material, such as the sediments in estuaries and river deltas.
Mangrove swamps change greatly, both in their structural and functional characteristics. They reach their greatest structural development in the riverside mangrove swamps, because of the flow of the water, the large supply of nutrients, and the low salinity (less than 10 [per thousand]). They produce the greatest quantity of leaf litter (13 tons per hectare [1 hectare = 2.47 acres] per year), and so more organic material falls into the adjacent waters. They are found mainly on the shores of estuaries, on river banks, and in coastal lagoons, whereas coastal mangrove swamps grow on protected seashores or on islands that are swept by the tides. They grow in highly saline environments, nearly as saline as seawater, that receive less nutrients than riverside areas. The tides are much more important than the rivers and other freshwater watercourses. Leaf litter production is approximately nine tons per hectare per year. They can settle in environments with little moisture, and occasionally there are hypersaline lagoons or xerophytic vegetation behind them. Basin mangrove swamps form in depressions that are further inland but that are still affected by the seawater. The usually seasonal drainage channels drain slowly. Only the highest tides flow as far as these basins. The mangrove swamps thus need a supply of rainwater or seepage to develop adequately. These mangrove swamps show the lowest production of leaf litter.
There are often considered to be two types of mangrove swamp, depending on the species found in the community. The first is called the New World mangrove, and is found on the Atlantic and Pacific coasts of the tropical and subtropical Americas and on the western coast of Africa. The second type is found on the eastern coast of Africa and the Indo-Pacific region and is called Old World mangrove. In both cases, these floodable forest communities thrive in low-lying areas, between the levels of the high and low tides, mainly in the regions where rainfall is high. When rainfall exceeds 79 in (2,000 mm) per year, the mangroves can reach heights of 115 ft (35 m). They are also found, but are not so well developed, in some arid areas of Asia and Africa.
Natural disturbances and constructive dynamics
Mangrove swamps are frequently altered by some dynamic environmental factors. When there is a water deficit, salinity is often high and the mangroves form narrow lines along the watercourses. In the Caribbean these hypersaline surfaces develop when annual rainfall is below 51 in (1,300 mm). The mangrove cover is unstable as the fluctuating area they cover shows periods of expansion (after storms or during a series of very wet years) and contraction (as a consequence of dry conditions lasting for several years).
Cyclones also cause major disturbances. Windspeeds can reach 75 mph (120 km/h). The exposed sites mangrove swamps occupy and their superficial root systems growing in soil that provides little support make them very vulnerable to these climatic facts of life. They may be eroded by intense water inputs and strong wave action. Strong winds can lead to defoliation, cause branches to fall, and uproot whole trees, as well as damaging their bark and foliage through the abrasive action of sand.
However, mangrove swamps have an immense capacity to reestablish themselves after alteration or disturbance. This is probably an adaptive response allowing quick and effective colonization of dynamic, changing environments that are resource-rich but ephemeral. This is why many mangrove species can tolerate a very wide range of environmental factors. It is also the reason why they grow quickly, reproduce prematurely, show almost continuous flower and fruit production, and why their propagules are adapted to both short- and long-distance dispersal.
There has long been controversy about their role extending dry land out to sea. Some ecologists consider that once mangroves have established themselves, their different root types cause gradual sediment retention and little by little the soil level rises; when the community is well established and is reproducing, it can advance seawards and thus gain ground. However, mangroves often grow in geomorphologically active areas with high sediment inputs--these opportunistic colonizers do not produce these situations, but settle where there is already sediment accumulation or obstruction. They respond and adapt to the geomorphological processes of a dynamic environment where external forces are continuously constructing, modifying, or eroding structures. Models for the establishment of mangrove swamp have been drawn up taking into account the conditions and rate of sediment deposition. On this basis, models can be described according to which different species of mangrove develop, depending on their individual adaptations, tolerances, and flooding requirements, as well as salinity conditions and soil texture. This makes it possible to understand the local distribution of mangrove species.
The word estuary usually implies the idea of the mouth of a river flowing into the sea, and this fundamental characteristic best defines it. Yet the dynamic interaction of the sea and the land makes the whole more complicated and variable than it appears. The most generally accepted definition is that an estuary is a partially-closed volume of coastal water that is open permanently or periodically to the sea, and in which there is a change in salinity due to the mixture of salt water and the fresh water supplied by rivers and other terrestrial sources. Sometimes it is difficult to distinguish an estuary from a coastal lagoon, which is defined as a depression on the coast located below the average level of the high tides and permanently or temporarily joined to the sea, but protected from its pounding by some type of barrier. Ecologically, estuaries and coastal lagoons form part of the same type of environment because they share several physical and chemical characteristics, their dynamics and biota, and this is why it is possible to talk of an estuarine lagoon environment.
The formative process
In general, estuaries are not marine in origin, but are river valleys that were flooded by changes in sea level. During the Pleistocene glaciations, more than a third of the world's land surface was covered by 0.6-1.2 mi (1-2 km) of ice, while the sea level along the world's coastlines was 328-492 ft (100-150 m) lower than now, coinciding more or less with the edge of the current continental platform. Since then the climate has become warmer and the ice has melted, causing the sea level to rise. Many valleys close to the coast were permanently flooded, and some now flow into submarine canyons at the edge of the continental platform, while others have filled in with sediments.
Most estuaries are shallow, although the channel's original bedrock may be buried under 230 ft (70 m) of sediments. So both the waves and the input of fresh water affect the substrate and produce turbulence and the mixing of waters. Sediment transport and settling patterns are not only the result of physical processes, but also chemical ones. Particles of suspended material flocculate easily, especially at the interface between fresh and salt water. Some authors consider that the sediments control biological processes.
On a geological time scale, estuaries are transient structures with a lifespan determined by their rate of sediment deposition. Many estuaries may be no more than 3,000 years old. The freshwater input bears materials in solution and in suspension. Sedimentation rates have recently increased, mainly as a result of inadequate agricultural practices (leading to erosion in the watersheds), and, to a lesser extent, because of the canalization and drainage of floodable areas.
The width and depth of the estuary's original basin are determined by geological history, the slope of the coastal plain, and the hardness of the rocks. Yet the current form of the estuaries is more closely linked to the rate of sediment deposition and the strength of the forces distributing the sediments. Estuaries are normally shallow; the deepest one recorded is 115 ft (35 m).
Types of estuaries
All the great variety of existing estuaries fit into three types, normal (or positive), inverse (or negative), and closed (or blind).
The most common estuaries are normal estuaries, which are also called positive estuaries. They typically show increases in salinity at the point where the river communicates with the sea. Over the tidal cycle there is a net flow towards the sea. There are several types of positive estuary, distinguished by their degree of stratification--in other words, the degree to which the fresh and sea water mix. Positive estuaries range from those where two layers of water form (fresh and salt water) and do not mix, those where some mixing occurs (meaning that they are highly stratified), those where there is partial mixing, to those where mixing is complete.
The second type of estuaries are inverse estuaries, also known as negative estuaries, in which salinity increases from the mouth inland, becoming hypersaline inland. Net flow is in the opposite direction to the case above, towards the land. This mainly occurs in very dry periods.
The third type are closed estuaries, also called blind estuaries because they are closed temporarily by the formation of a sandbank. In this situation there are neither tidal flows nor currents. Freshwater enters from the river and the circulation depends on the river's residual current and the force of the wind acting on the water's surface. The degree of salinity will be determined by the balance between evaporation, filtration through the sandbank, and the input of freshwater from the river and from rainfall. The estuary may become hypersaline, maintain average values, or become hyposaline.
The mouth of the estuary is an important point because it regulates the inputs of seawater and the outputs of freshwater. The form of this very dynamic location depends on the currents flowing in each direction, on the currents and waves in the coastal area, and on the transport of marine sediment. What is most important is the balance between the sediment input and the forces dispersing it, because this is what determines whether sediments are dispersed into the sea as soon as they arrive or whether they accumulate and temporarily close the mouth. Unlike river mouths, estuaries are dominated by tides. There are two types of tidal currents at the entrance of an estuary: rising tides and ebbing tides. Rising tides flow into the estuary, while ebbing tides flow from the estuary to the sea, transporting sediments from the river and depositing them in the sea. Tidal effects are most important in estuaries with mouths that are wide and deep in relation to the volume of their water mass; if the mouth is small, the tidal flow is insignificant. This zone of an estuary is subject to relatively fast geomorphological changes.
Current flow and salinity distribution patterns are interrelated, giving rise to a complex model of water circulation based on the relation between river flow and the volume of the water borne by the tidal currents flowing in each direction. Thus, an estuary may be dominated by seawater or by freshwater. The river's flow is basically determined by a series of processes related to its course, such as the watershed's area and annual rainfall, although seasonal distribution of rainfall or snow, temperature, the rate of evapotranspiration, soil permeability, plant cover, and the gradient all play a role.
Many temperate regions (such as the low-lying zones of Europe's Atlantic coast) have an evenly distributed annual rainfall of 26-29.5 in (650-750 mm), low evaporation, and a permeable soil with a good plant cover. The water percolates slowly and their rivers flow continuously throughout the year. The salinity regime in such estuaries is more-or-less constant throughout the year.
European and North American estuaries are usually wider at their mouth, where they are obstructed by the sandbanks that give them their typical wedge shape.
When an estuary in a calm, more-or-less enclosed sea has filled in, a delta forms, as is the case of the Mississippi and the Nile, while in seas with high energy, a sand bank and sandbar form in front of the mouth. In periods of low river flow or after a violent storm, the mouth may be completely closed.
On subtropical coasts, the annual rainfall of 39-49 in (1,000-1,250 mm) falls mainly in the summer. The often shallow soils are easily eroded, and the water draining into the rivers bears a lot of sediment. The estuaries of the longest rivers are short and have a high rate of sedimentation, and their salinity changes rapidly with the cycle of the seasons.
The speed of the river's flow is one factor influencing seawater penetration into the river's mouth. Sometimes so much water is discharged that no seawater penetrates, and the fresh and salt water mix out to sea. This is exemplified by the Amazon, whose water mixes with the sea many kilometers offshore. In rivers whose coefficient of flow varies between the dry and the rainy (or thaw) seasons, the river's mouth is estuarine for a time and then freshwater flows into the sea again.
Nutrients and productivity
Estuaries typically have abundant nutrient inputs, mainly from the freshwater entering them. The marshy zones of estuaries with low salinity retain pollutants as well as nutrients. In estuaries, phytoplankton photosynthesis is complemented by the meadows of marine flowering plants and the marsh and mangrove swamp vegetation. The abundant primary, secondary, and tertiary consumers, together with the producers and the decomposers, form a complex food web.
In fact, there are two different food webs. The first is based on the grazing or direct consumption of the living plants. The herbivores include many zooplanktonic organisms (copepods, small crustaceans and the larvae of many different groups), benthic filter-feeders (mollusks) and primary consumers. The second food web is based on detritus. Dead plants are colonized by several types of organism, including bacteria, fungi, protoctists, and small animals such as nematodes. Detritus has a high food value because its formation requires nutrients.
Estuaries show high nutrient availability almost all year round. They are very productive environments, and together with jungles, show the highest values of net primary production. They show strong exchange of both biological and non-biological materials with their neighboring ecosystems, such as mangrove swamps, and so, in a way, they are subsidized by the production of other systems. Water passively transports sediments, organic and inorganic nutrients, and organisms. The most productive areas are near the mouths of the rivers and near marshes. If river flow is low and water exchange with the ocean is limited, productivity diminishes. The high values are mainly caused by the movement and entry from other ecosystems of water bearing a large quantity of nutrients; the presence of primary producers showing year-round activity, physiological adaptations, such as tolerance of changes in salinity; and behavioral adaptations, such as migrations, which are responsible for high biomass inputs at certain moments of the year, the wide range of habitats, the large number of different life cycles and a highly complex, interconnected food web.
Diversity and ecotones
Estuaries and lagoons have many external and internal frontiers. Their external frontiers are where they border the sea, the freshwater, the neighboring terrestrial systems, and the atmosphere. The internal ones are frontiers between water and sediments, between aerobic and anaerobic conditions, between fresh and salt water, between the water from the marshes and the water of the main watercourse, between shallow and deep waters, and so on. This diversity gives the system complexity and stability. A frontier implies the existence of a gradient, as the changes are rarely drastic. Thermodynamic activity occurs when there is a gradient and therefore many gradients may make the system more productive. In these environments, the system's hydrology is essential for the maintenance of the gradients.
However, it is very difficult to separate different coastal ecosystems from each other. Mangrove swamps are very closely related to estuaries. Mangrove swamps supply nutrients and energy to the estuaries, and much of the estuary's fauna seeks protection and shelter among the mangroves. In turn much of the sediment not deposited in the estuary flows into the sea and then to the beach, where it forms part of the dunes. Low marsh swamps, ranging in width from a few meters to tens of meters, occupy the area between the dunes and the mangrove swamps. They have some species in common and there is a gradual transition between the areas unaffected by salt water to the floodable areas, such as mangrove swamps. Other animals, such as birds and mammals, circulate between these environments in search of food, shelter, and nesting sites.
It is very difficult to separate estuaries from mangrove swamps, since the mangroves often surround the estuaries and form a gradient from the water to the dry land. Even so, the most flooded parts of the estuary have their own dynamic and particular species composition, which changes with the season of the year, and as a result the salinity gradient and the hydrological conditions of the water change. One of the most relevant elements are the fish, reflecting these ecosystems' high complexity and high productivity. There are freshwater species that occasionally enter brackish waters, migratory fish that only cross through them, estuarine fish that complete most of their life cycle in the estuary, marine fish that use the estuary to spawn and enter only occasionally, and marine fish that when adult continually enter the estuary in search of food. Whether herbivores or primary or secondary carnivores, they all form part of the food chain.
227 Intertidal meadow of flowering plants and seaweeds left uncovered by the low tide on the Californian coast of the Pacific Ocean. The photograph shows the strap-like leaves of a marine flowering plant of the genus Phyllospadix, the brown thalluses of the fucaceous seaweed Pelvetia fastigiata, and some specimens of the starfish Patiria miniata. All these intertidal organisms share the ability to withstand the fleeting exposure to air twice a day at low tide.
[Photo: Norbert Wu Photo-graphy]
228 Intertidal benthic community attached to a rock on the European Atlantic coast (Cornwall, United Kingdom) with several typical mollusks, attached crustaceans, calcified algae, mussels (Mytilus edulis), limpets (Patella), acorn barnacles (Chthamalus [=Euraphia] depressus), and articulated coralline algae (Corallina officinalis). Similar communities are common on all temperate rocky coastlines.
[Photo: P. H. Ward / Natural Science Photos]
229 Browsers, detritivores, and macrophagous organisms all live in the intertidal zone, as in this benthic community on the coast of the Pacific Ocean (Point Lobos, California). The two starfish Pisaster giganteus (above) and Patiria miniata (bottom), and the anemone Tealia piscivora stand out against a base of encrusting and articulated corallinaceous algae.
[Photo: Enric Ballesteros]
230 The intertidal detritus eaters include the common and well-known sand flea (Talitrus saltator), normally present among the natural debris on the beach.
[Photo: Ramon Torres]
231 The distribution of the main genera of Laminariales (kelp) in the world's seas. The geographical distribution of the Laminariales is greatly influenced by the temperature of the surface water, which is in turn influenced by the surface ocean circulation. As the western coastlines of the continents are bathed by currents transporting cold water from the polar regions, the Laminariales extend further towards the equator on the western coasts of the continents than on the eastern coasts, where warm currents are transporting equatorial waters to higher latitudes. Species of the genus Laminaria dominate on the Asiatic coasts of the northern Pacific and northern Atlantic (and even the Mediterranean where there is an endemic species, Laminaria rodriguezii). The genus Macrocystis dominates on the Pacific's American shores in both hemispheres and in other areas of the southern hemisphere, where its dominance is shared with the genus Ecklonia. Other genera (Nereocystis, Pelago-phycus, Alaria, Pterygophora) may be present in a secondary role, and may even locally displace species of the genus Macrocystis.
[Drawing: Editronica, from several sources]
232 Diagram of the intertidal zone in situations of different wave exposure near San Diego, California.
[Drawing: J. Corbera, based on data from Stephenson and Stephenson]
233 Grazing periwinkles, eating black encrusting li-chens of the genus Verrucaria. These littorinid snails are a typical feature of the fauna of the intertidal space. The periwinkle (Littorina littorea) is a black Atlantic snail greatly appreciated as seafood by the Spanish (who call it bigaro) and the French (who call it bigorneau).
[Photo: J.L. Mason / Ardea London Ltd.]
234 Tropical reefs close to the water's surface in the intertidal part of Isla Larga, in the Caribbean (Venezuela). These Atlantic coral reefs are always submerged, while in southern seas in the Indo-Pacific they may be exposed for several hours.
[Photo: Adolf de Sostoa & Xavier Ferrer]
235 The intertidal com-munities of the European Atlantic coast are usually dominated by seaweed of the family Fucaceae. This is visible in this photo of part of the coast of Galicia (Spain) at low tide, showing a fringe of Fucus on the rocks and a dense floating covering of the unmistakeable strap-like Himanthalia elongata. Just breaking through the water's surface are the large laminate thalluses of Saccorhyza and Laminaria.
[Photo: Juan Carlos Calvin]
236 The intertidal sea-weeds of the Pacific coasts usually belong to the fucaceae and can reach a large size. This photograph of the rocky shore of the Olympic National Park (state of Washington, US) shows the erect palmate thalluses of Postelsia palmaeformis and the deeply-cut thalluses of Egregia menziesii with their buoyancy bladders.
[Photo: Adolf de Sostoa]
237 The confluence of ocean, marsh, and river environments at the mouth of the Murray River in the Coorong National Park, near Victor Harbour (South Australia), on the Great Australian Bight in the Indian Ocean. Sandy soils, dunes of pure sand, and varying water salinity create very unusual conditions for life in these fluctuating and geologically unstable environments.
[Photo: Ralph & Daphne Keller / NHPA]
238 The start of colonization of coastal sands by seedlings of pioneer species. The larger one is marram grass (Ammophila arenaria), a species that is adapted to growing on sand. Its powerful root system enables it to grow in shifting substrates and is also well adapted to the dry, salty air of coastal sandy ground (see photo 241).
[Photo: Francesc Muntada]
239 Volcanic sand on a beach on El Hierro (Canary Islands). This black sand contrasts with the general idea of white or gold sand in coralligenous, calcareous, or dioritic areas.
[Photo: Joaquim Reberte & Montserrat Guillamon]
240 The combination of the wind and an obstacle is the normal cause for the formation of small, dune-like cumuli by the displaced sand. The photo shows a beach on the Ebro delta, where small bivalve shells form a barrier to the wind action.
[Photo: Teresa Franquesa]
241 The fixation systems of dune-colonizing plants, such as these marram grasses (Ammophila arenaria), usually imply the presence of rhizomes, stems that may be underground runners or just creep below the ground level, and which give rise to roots or aerial shoots. Thus, apparently separate plants are really part of the same individual, connected by underground rhizomes. This gives it stability and makes it a successful colonizer (see fig.
[Photo: David Woodfall / NHPA]
242 Dune system com-pletely fixed by vegetation. It is on the coast of the Bass Strait, on the division between the Indian and Pacific oceans (Philip Island, Australia). After the pioneer fixing plants, such as marram grass, a woody vegetation establishes itself that gradually forms what is called the coastal forest mantle.
[Photo: Oriol Alamany]
243 Vegetation on tropical beaches is lush as is the rest of the vegetation at these latitudes. The photo shows the bay on Qamea Island (Fiji archipelago) in the south Pacific, covered by the leaves and flowers of Ipomoea pes-caprae).
[Photo: Jean-Paul Ferrero / Auscape International]
244 The tropical coastal forest mantle includes two very typical woody plants, cocoloba ("beach grape") and screw pines. The upper photo shows Coccoloba uvifera on the Caribbean island of Mosquitos (Venezuela). The lower photo shows the screw pine Pandanus pedunculatus in the National Park of Bundjalung (Indonesia). The screw pines are monocotyledons and thus show little lignification, but are arborescent, or tree-like in form. They have unmistakeable fruit and a well-developed root system often showing many prop roots.
[Photos: Xavier Ferrer & Adolf de Sostoa and Jean-Paul Ferrero / Auscape International]
245 Spontaneous colonization by coconuts (Cocos nucifera) on a coral, volcanic beach in the Eori Islands (Fiji archipelago) in the south Pacific. The coconuts and other beach plants are clearly different from the vegetation of the rocky volcanic part.
[Photo: Jean-Paul Ferrero / Auscape International]
246 Many marine species reproduce on beaches, such as the horseshoe crabs (Limulus polyphemus) shown in this photo taken in the Moluccas. These remarkable arthropods of the Atlantic coasts of North America and the Indo-Pacific turn up on the beaches to spawn in very large groups. In spite of their name, they are not crabs. They may settle on the hulls of ships and be transported long distances to new areas.
[Photo: Francois Gohier / Ardea London Ltd.]
247 Many costal and marine birds nest on beaches. The photo shows a specimen of the variable oystercatcher (Haematopus unicolor) tending its clutch of two eggs laid in a depression in a sand beach in New Zealand.
[Photo: Darran Leal / Auscape International]
248 Not a sea serpent, but a sea snake. Sea snakes are members of the Hydrophiidae and their adaptations include valvular nostrils (usually on the upper part of the head) they can close when underwater, and a laterally compressed tail that functions as a fin, both visible in this photo. Taken in the eastern Great Barrier Reef, it shows a specimen of Aipysurus laevis out of water at low tide and almost unable to move until the tide comes back in again. Not all snakes that live in water have difficulties moving on dry land, and some leave water to lay their eggs. The Pacific species Hydrus [=Pelamis] platurus can actively swim up to a few kilometers offshore, but other Hydrophiidae (Hydrophis, Laticauda) remain in coastal waters. Some coralline sea snakes, such as those on Gato Island in the Philippines, have attractive skins and are hunted. Many have poison glands and some species have the most lethal venoms of all the snakes, but they are not always dangerous to human beings due to their small size (less than 4.9 ft [1.5 m], except Hydrophis spiralis which can reach 8.2 ft [2.5 m]). All Hydrophiidae are viviparous and eat fish.
[Photo: Valerie Taylor / Ardea London Ltd.]
249 Many mangroves are plant palafittes (stilt houses) with crowns isolated from the salt water by their ingenious root systems. This is clearly shown by these specimens of the Pacific red mangrove (Rhizophora stylosa) on the shore of Queensland, Australia. They show the stilt roots typical of the genus, which maintain them above sea level and give them stability on a shifting substrate. Their lenticel system means they also play a respiratory role by ensuring gas exchange continues when the soil is flooded.
[Photos: Reg Morrison / Auscape International and D. Pareer & E. Parer-Cook / Auscape International]
250 Excretion of common salt (NaCl) on a typical red mangrove (Rhizophora mangle) seedling from a Venezuelan mangrove swamp. Many mangrove species can excrete the excess sodium chloride entering their tissues, giving rise to these salt efflorescences on the plant.
[Photo: Jordi Bartolome]
251 Respiratory roots called pneumatophores stand out from the soil at low tide in this mangrove swamp in Bako National Park on the island of Borneo (Indonesia). These simple pencil-shaped pneumatophores are typical of the black mangrove (Avicennia). Unlike red mangroves, black mangroves do not have stilt roots to provide support and help respiration. Instead, their basically underground root system gives rise to many pneumatophores.
[Photo: R. Seitre / Bios / Still Pictures]
252 Viviparity is common in mangrove species, although it is especially clear in the red mangrove (Rhizophora mangle). The seeds germinate on the mother plant, and the seedlings hang from the peduncle that supported the fruit, as shown in the upper photo. When the dart-shaped seedlings fall, they embed themselves in the soil (lower photo), increasing their chance of rooting firmly. This ensures that development of new individuals is fast and efficient, but greatly limits their potential dispersal.
[Photos: Adolf de Sostoa & Xavier Ferrer]
253 Different mangroves have very different root systems. Mangroves of the Combretaceae and especially the Rhizophoraceae (the typical mangroves) have prominent stilt roots, visible in the upper photo showing the rhizophoraceous Bruguiera in a mangrove swamp in Port Barton, on Palawan Island, Philippines (see fig. 249). Mangroves of the Verbenaceae and Sonneratiaceae have conspicuous pneumatophores (respiratory roots) (see also photo 251), while those of the Caesalpineaceae and Theaceae usually have trunks with buttress roots, like the Pelliciera rhizophorae in the lower picture, a theaceous plant found on the relatively stable coastal soils of the Ensenada de Utria National Park in Colombia.
[Photos: WWF / Ronald Petocz / Still Pictures and Adolf de Sostoa & Xavier Ferrer]
254 An abundant sessile fauna settles on mangrove roots, especially filter-feeding mollusks, such as these characteristic mangrove swamp oysters (Crassostrea), which are collected and consumed as fresh "seafood." Those in the photograph are from a mangrove swamp in Elenkina, in the mouth of the River Gambia (Senegambia).
[Photo: Adolf de Sostoa & Xavier Ferrer]
255 Two birds typical of American mangrove swamps, the egret (Egretta tricolor) and the scarlet ibis (Eudocimus ruber). The egret (on the left) is found on both Atlantic and Pacific tropical American coasts in the northern hemisphere, and also slightly to the south of the equator; it eats mainly fish and small terrestrial vertebrates. The specimen in the photo is from a mangrove swamp in the Bay of Chetumal, in the Yucatan peninsula (between Mexico and Belize) above the Caribbean. The scarlet ibis (on the right) mainly eats crustaceans and mollusks and lives in the Atlantic mangrove swamps of Colombia, Vene-zuela, Guyana, and the Amazon delta, as well as inland salt or freshwater marshes on the equator, and even reaches some points on the coast of Santos (Sao Paulo, Brazil). The photo shows juveniles (adults are scarlet with black beaks) in the mangrove swamps of the Cuare reserve (Venezuela).
[Photos: Tony Rath / Natural Science Photos and WWF / Roger Leguen / Still Pictures]
256 The mammals that live in, or visit, North Ame-rican mangrove swamps include the racoon (Procyon lotor), a forest carnivore and excellent swimmer. It is common from Canada to the Caribbean, including the Antilles. It has the unusual habit of carefully cleaning the fish it catches before eating them. The specimens in the photo are from the coast of Chiapas, Mexico.
[Photo: Adolf de Sostoa & Xavier Ferrer]
257 Photo of nipa palms (Nypa fruticans) and list of the dominant species in the mangrove swamps of the New World (Colombia) and the Old World (Madagascar). Only two species in the list, the fern Acrostichum aureum and the malvaceous shrub Hibiscus tiliaceus (neither of which is a true mangrove) are found in the mangrove swamps of both the Old and New World. In all the other cases there is a clear floristic separation between the two main mangrove areas, which have different species and even families. Nipa palms, however, grow on muddy substrates in small sheltered coves where mangrove swamps do not form, in areas such as the islands in the Sonda strait (the photo shows specimens in an estuary in Borneo), in the Bay of Bengal, and in Australia. Apparently the oscillations of the tides that partially submerge the leaves are key determinants of the germination of the seeds but not the life of the plant. The nipa palms are a strange group with a fossil record stretching back over 100 million years. They are highly valued by local people because the leaves supply thatching and fencing, the fermented sap provides an alcoholic beverage, and the green fruits are edible.
[Photo: P. Burton / Natural Science Photos; table: Edi-tronica from data provided by Prahl, 1990]
258 The rarest and most endangered species of the fauna of the mangrove swamps is probably the proboscis monkey (Nasalis larvatus). This cercopithecoid primate eats mainly leaves and fruits, and is found exclusively in mangrove swamps and forests near marshes on the island of Borneo (Indonesia). The proboscis monkey received its name for its distinctive big nose, visible in the photo, which is especially prominent in adult males. Its intestinal bacterial flora can digest cellulose and neutralizes the toxins in the poisonous plants that form part of its normal diet. It can swim very well, both above and below water, and it is considered to be the best swimmer of all the simians.
[Photo: Jean-Paul Ferrero / Auscape International]
259 Estuary and delta of the Yangtze in the China Sea (31[degrees]N), as seen in a false-color multispectral scanner (MSS) image taken from a Landsat satellite. The city of Shanghai is visible (the dark patch in the center of the lower half) on the banks of the river Huangpu. The plain of the delta and the intensively cultivated islands in the estuary are a consistent bright red. The nutrient- and sediment-laden water is different shades of blue, depending on the specific characteristics and temperature at each spot.
[Photo: AGE Fotostock]
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|Publication:||Encyclopedia of the Biosphere|
|Date:||Oct 1, 2000|
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