Oceans and seas.
The Old Man and the Sea (1952)
1 The salt-water kingdom
1. The continent and the content
1.1 The imprint of history
We are living in a period in which the planet Earth shows a remarkable variety of marine environments. If we had to design an experimental planet that represented the greatest variability of different types of oceans, all we would have to do is copy our planet. There are two poles that are always cold: One (the Arctic) is an ocean, while the other is a continent (Antarctica). There are two oceans, the Atlantic and the Pacific, that stretch from one polar circle to the other. The Atlantic extends across the maximum number of meridians. The Pacific likewise not only covers the maximum number of meridians, but is also extremely extensive 9,320 mi (15,000 km) in terms of the parallels of latitude that it occupies (i.e., east to west). Then there is an ocean that in terms of parallels surrounds the whole planet (the Antarctic Ocean) and another stretching from the tropics to the Antarctic circle (the Indian Ocean). The present-day distribution of the oceans includes all latitudes. These main oceans include a group of minor seas with special conditions that make them very interesting and add to the high variability of the planet's coastal and maritime environments. They are found in all latitudes and in all oceans. In these parts of the oceans, proximity to the continent, and the proportion of coast in relation to the area of the sea, and the reduction of water exchange with the rest of the ocean mean that the minor seas enhance certain local characteristics. Those that receive more freshwater from rainfall and rivers than is evaporated off are brackish (the North Sea, the Black Sea), while those where evaporation is greater than the input of fresh water are more salty (the Mediterranean, the Red Sea).
The oceans have not always been distributed as they are now. There have been periods when an ocean wandered across the equatorial regions of the planet. As regards the age of the world's oceans, the North Atlantic started to open up 165 million years ago, while the Mediterranean is a relict sea, a remnant of the former Tethys Sea that is gradually closing up.
The Earth is constructed on two levels, the continental and the oceanic, because of the nature of the tectonic plates and the solid materials of which they are made. The quantity of water is such that it covers the basins surrounded by the oceanic plates and has led to the ocean bottoms being covered with superimposed ancient marine sediments. These sediments have been--and are--relatively more important on the edges of shallow seas. Now, to a large extent, the sediments also form part of the continents. The plates that make up the continental crust are lighter, and have been more altered by the action of both the atmosphere and of living organisms. Their surface is continuously being washed and eroded, and the intense chemical activity of living beings, as well as that of air and water, has helped to mold them.
The lithosphere and the hydrosphere
The plates have moved--and are still moving--continuously. At their edges, accretion of materials, subductions, and other modifications take place. The rigid nature of the plates has favored the accumulation of stresses that have given rise to later, more sudden, movements in different periods of the planet's history. On this level we can recognize something that is typical of the changes and evolution of the biosphere: the alternation between self-organized changes that are relatively slow, and more sudden events that, from a human point of view, seem to be harder for us to anticipate.
Plate dynamics ensure these changes will never cease. Materials continue to flow out in the Mid-Atlantic Ridge--a structure that surfaces or outcrops above sea level in Iceland--and in the other ocean ridges where oceanic crust is being formed. This is where the continuous supply of materials from the depths--metals and gases (CO2)--to the sea and to the continents is most obvious. The current distribution of land and sea is a time-slice of a continuous process that occurs at about 10-12 times the rate of change in the circulation patterns of the masses of oceanic water.
The oceans of the past
Magnetic alignments, climatic indicators in fossil remains--including indicators of anoxic seafloors--and stratigraphic interpretations make it possible to reconstruct the former position of the fragments of the continents, and at the same time allow us to deduce that the arrangement of the oceans used to be very different from what it is today. In the very distant past the continental blocks were closer together than they are now and as a result there was an ocean, known as Panthalassa, that was much larger and widespread than now. Because it was not subdivided into basins, its circulation could include very large circuits, certainly larger than today's. This might have implied the maintenance of greater potential energy in the circulation of the sea, and also tidal regimes that were very different from those of today.
As an example, we can mention the attempt to reconstruct how the continental masses might have been that existed during the Silurian period (Wenlockian, 430 million years ago), then united in a southern continent (Gondwana) with some segments extending to the Equator, or even somewhat further north (including part of what is now Siberia). The northern edge--in the southern hemisphere--of this large continent was in the westerly wind belt, and this favored the creation of a vast and certainly fertile region of upwelling covering 10,811 sq mi (28,000 sq km) (with upward movements of 8 in [20 cm] or more a day, 0.39 in [1 cm] per hour, which are remarkably high in comparison with current values). Evidence for this is found in the kerosene-rich material, the origin of petroleum, that formed exactly in the positions one would predict.
Later, between 80 and 200 million years ago, the sea was still relatively warm and rich in corals. It was close to the equator and extended over many latitudes. This was called the Tethys Sea, often mentioned when explaining biogeographical distributions both in contemporary seas and on their coastlines (for example, animals living in coastal karstic formations, from Cuba to the Mediterranean islands and those of southeast Asia).
The beginning of the widening of the Atlantic took place in what was a sort of cul-de-sac between nearby continental masses, where conditions were stagnant and highly anoxic. There was a considerable local accumulation of CO2 in the water at the depths, to which gas emitted from the Mid-Atlantic Ridge in the process of formation may have contributed. This build-up of CO2 may have been the cause of the apparent release of calcium carbonate during the Cretaceous period in shallow sea areas where many corals and mollusks lived, all in a symbiotic relationship with photosynthetic protoctists. These corals and mollusks ended up forming enormous fossil masses. These and other facts suggest historical hydrographic regimes that were very different from those of the present day. The marine biosphere has thus changed greatly over time; but we can recognize in it some persistent mechanisms and agents, similar to those acting nowadays, that caused changes in the past.
There is little reliable evidence about the quantitative and qualitative changes that occurred in the past in the composition of the seawater. Nowadays, the total volume of the water in the seas has been estimated to be about 1,369 million km3. Dividing this figure by the surface area of the oceans, estimated to be about 361 million km3, gives a figure of 12,441 ft (3,792 m) as the average depth of the oceans. This volume varies, and probably has varied, over time; it has done so recently as a result of the quantity of water held in solid form, mainly above sea level, especially in Antarctica, Greenland, and other nordic areas. If all this solid water were to melt, it would raise present sea levels by 253 ft (77 m): this is one of the threats of the greenhouse effect, in the worst-case scenario. There has been speculation about variations in the total quantity of water over the course of geological time, but there is no reliable evidence. It is simplest to consider that this volume has remained constant, at least over a long enough period to cover the events dealt with in this book.
The sea, a product of the land
Sea water is currently in equilibrium with the atmosphere. In the past, it must have been in a similar equilibrium with the contemporary atmosphere, which had a different composition (more CO2 and less O2). This means that the solubility of some materials (such as iron, calcium, and magnesium) would have been different, and thus the conditions of life were clearly different from those prevailing now.
Seawater is a very complex chemical solution, containing almost all the elements of the periodic table. It is thought that the water of the oceans is not derived from the condensation of a primitive gaseous atmosphere (which, if it ever existed, was lost many millions of years ago), but that it was an extrusion from the solid part of the earth. The sea can thus be considered an extract of the soluble or dissolved part of the planet's solid crust. Recycling by rain and the ceaseless washing of the continental surface may have led to changes. It should not be forgotten that these modifications were assisted by the existence of life on the continents, which has helped to alter or to accelerate the chemical changes of some of the materials forming the Earth's crust, mainly by initiating and maintaining a relatively strong oxidation-reduction gradient. The most soluble of the resulting materials have, as would be expected, ended up in the sea.
On the other hand, the isolation of basins where evaporation was intense (such as those known from different levels of the Permian and Triassic periods, or more recently, the nearly total drying out of the Mediter-ranean Basin during the Messinian period, a mere five million years ago) removed large quantities of soluble salts from the seas. These salts ended up in the sediments and were later covered with other materials. In this way, for example, the reduction of global oceanic salinity due to this separation of material in the Mediterranean (the halites at the bottom of this sea that are currently covered and effectively sealed by a relatively thin layer of recent sediments) has been estimated at about 2%. The soluble or flexible minerals from saline deposits may have changed location and many can now be used commercially.
Elements whose relative proportions remain constant
The composition of seawater is relatively uniform with respect to those major elements whose relative concentrations remain constant--those that are less influenced by activities of living organisms or by possible changes in the atmosphere. This was revealed by the analyses carried out in the first half of the 19th century, and confirmed by William Dittmar's meticulous analyses of water samples obtained from all over the world by the British ship HMS Challenger during its famous expedition (1872-1876). This constancy is only approximate, but is sufficient for rapid evaluation of the density of seawater. This is done on the basis of the fraction of dissolved materials, which has long been considered to be the same as (or at least related to) what is called salinity (S), measured in parts per thousand by weight, of solid material dissolved in seawater. For a long time this has been calculated by measuring the quantity of chlorides (or rather halogen ions in general), called the chlorinity (Cl) using the formula S = 0.03 + 1.805 Cl. Today, people prefer to calculate chlorinity on the basis of electrical conductivity, which can be measured in situ, by using a cable. In deference to former oceanographers, it must be acknowledged that their chemical analysis of thousands of samples allowed the construction of a picture of the distribution of water masses and of marine circulation that is still valid and accurate today.
In addition to the interest in knowing the exact concentration of salts in seawater, there was also interest in the simultaneous measurement of its temperature in order to deduce its density, which varies according to both its temperature and salinity. Nowadays, measurement of salinity by cable is combined with the simultaneous measurement of temperature and the approximate hydrostatic presure ([sup.a] depth) in the widely-used CTD (conductivity, temperature, depth) apparatus. Density (r) adjusted to the water temperature, often abbreviated and expressed as [s.sub.t] where [s.sub.t] = 1,000 (s - 1), has been of great importance.
1.3 Seawater and life
The study of "water masses," characterized by their specific combination of temperature and salinity, allows a functional breakdown of the oceans. The areas of relative discontinuity, of sections and also of mixing, are the most important for biological production, which is concentrated where there are transitions or relative discontinuities, most of which are of the nature of fronts. This relative discontinuity is maintained by the relative movement and the continuous renovation through friction of the water masses that are in contact. Also common, and possibly of greater biological importance, is the simple rise of a water mass that brings its nutrients into the light.
In each ocean or continuous sea there is a large variety of hydrographic structures, such as currents, large closed eddies, upwellings of deep water, and marine fronts. Many of these typical structures are repeated, in similar and different combinations, in different seas. Very large systems possess great inertia and are highly persistent, and support ecosystems that may remain unchanged over long periods of time, or even indefinitely. Within these systems there is generally included a whole hierarchy of minor structures, either in the form of vortices or eddies that have peeled off from the periphery, like a river forming meanders, or like ball bearings within a bearing case. Near the coasts, the proximity of the seafloor relief channels or deforms these flows. But these structures are never very constant, and year-to-year differences between localities are great, so that it is impossible to identify ecosystems that are as diverse and clearly localized as those seen and described on the continents.
A relatively simple and mappable image, showing the distributions of the major marine descriptors, water masses, and currents, may be useful as a base map of the seafloor or as a model for outlining the distribution of many aspects of the biosphere, but it is as of little value in explaining the dynamics of life as is a static map of terrestrial vegetation. Plankton activity is linked to the occurrence of episodes of change or repeated disturbances: strong gusts of wind, minor vertical mixing processes, changes in the direction of currents and occasional interference between them, etc. This generates a plankton dynamic that, in each case, may leave traces that fit into an overall and seemingly more persistent image, or one that allows the calculation of average values, but which is certainly never static.
When compared with the surface of the continents, the stability of the pelagic environment is very low, given that the organisms of the phytoplankton live from a few hours or a couple of days to a whole week, but rarely longer. During this time, the volume of water they live in may have changed greatly in position and in depth, as well as mixing with neighboring volumes of water, and many external conditions may also have altered. The characteristics of planktonic life have made the segregation of populations difficult, and may have slowed down their evolution. Present day conditions appear to be more important than past history and evolution in conditioning the geographical distribution of the different forms of life and their associations. It is obvious that consideration of the analogies and differences between plankton and forests gives us a deeper and more generally applicable picture of the biosphere (see volume 1, pp. 174-176).
The major oceans and the larger areas of upwelling show considerable year-to-year differences in the location of the areas of greatest production or of comparable hydrographic structures. The very productive areas of the oceans repeatedly exceed (although often irregularly) the highest production values that are found in smaller seas or in marine regions that are more confined by the continents. It also appears, quite understandably, that in the large oceans there is more year-to-year variation in the geographical position of hydrographic structures, such as fronts or eddies, than in smaller seas.
Descending and rising waters
The functional organization of the oceans is such that the water is gradually sinking in over three quarters of the total area. The essential nutrients it contains--mainly phosphorus and nitrogen--fall even faster, especially inside organisms, in their corpses and in their excrement. The areas where water and dissolved materials rise are much smaller and become progressively so the more intense the movement is towards the surface. This leads to a distribution of marine primary productivity that is highly regular in the sense that, if we divide the areas into different groups using arbitrary limits based on increasing biological productivity, we find that the decrease in marine areas included in the successive groups is approximately logarithmic. That is to say, large areas are relatively barren and the higher the values for primary production, the smaller the area in which it occurs. This kind of approximate regularity of distribution is never found on the surface of the continents. The total net primary production of the oceans is estimated at some 35,000 million metric tons of carbon per year. It is clear that the hydrographic structures of great inertia, which give rise to regions of intense upwelling, can only be located in large areas of ocean. The Gulf Stream and the Peruvian upwelling form part of systems that quite simply could not fit into the Mediterranean. Considerations of this type are clearly not applicable to land surfaces, since there are islands with terrestrial vegetation that show extremely high productivity.
The world average for primary production by marine plankton is estimated to be between 80 and 100 g of carbon per square meter per year. Bearing in mind how the measurements are carried out, this overall average value is about halfway between net production and gross production. The total, or gross, production may be 10-20% higher than the estimated net value. Areas of upwelling to the east of the large oceans or in the regions of the currents on the eastern edges show values of more than 500 g carbon per square meter per year. Productive systems with currents on the west of oceanic basins (including monsoon upwelling systems) and those on the Atlantic coast of the Americas, Japan, and Somalia, etc., show figures of at least 150 g carbon per square meter per year. The northwest Mediterranean may approach this figure, and even higher values are found in some parts of the Baltic, the Caribbean and other secondary seas. In shallow seas, such as the North Sea, Georges Bank, to the northeast of the United States, and the banks off Newfoundland, productivity exceeds 200 g per square meter per year. High productivity prevails, possibly to a greater extent, in the region between the southern convergences, for which it is still difficult to give a good estimate of annual production. In other seas at relatively high latitudes, but with short periods of adequate light and without any specific system of intense fertilization, values are between 60 and 80 g carbon per square meter per year.
Most of the water in the oceans is in total darkness, because sunlight penetrates, with sufficient intensity, at most 328 ft (100 m), even in very clear water. This thin sunlit surface layer is essential for life in the sea, not only because it is where plants can photosynthesize, but also because the exchanges with the atmosphere that govern marine circulation always occur at the surface.
Going from the coast towards the center of the ocean, a series of regions with different functional characteristics can be distinguished. This water mass is the pelagic system or division and is called the neritic zone in the region close to the coast, and the oceanic zone when further from the coast and over greater depths of water. The ecological system directly linked to the ocean deeps is called the benthic system or division, with characteristics that depend to a large extent on its depth, as this is related to the main factors that determine the way it functions. The most important of these factors are the supply of external energy (light, turbulence) and the supply of materials (through the sedimentation of organisms that live and die higher up in the water column, or produced on land and transported to the sea). Shallower systems closer to the coast are more dynamic, owing to the greater supplies of materials and energy they receive.
Along the coast, very important environmental changes take place within very small spaces. Light, nutrients, and the movement of the water or its renewal may vary from one extreme to another over a few meters. The ebb and flow of the tide marks clear limits in the distribution of the communities of organisms.
Although almost all primary production of the sea occurs close to the surface--in the presence of light--there are also primary producers at a depth of thousands of meters near the hydrothermal vents found where oceanic crust is forming, and living in cold, dark water at a pressure of several hundred atmospheres. In the absence of light, their energy source is geochemical, the sulfides that emerge in the water, heated by the magma pockets found in the areas of geological activity on submarine ridges. In these areas, a type of oasis of life develops that does not depend on sunlight. The first ecological systems of this type described received names as suggestive as "The Garden of Eden," because of their wealth of beautiful animal forms, also of considerable size (see the section "Hydrothermal oases," p. 206).
2. Marine dynamics
2.1 The displacement of water masses
In spite of its lack of precision, the term and distinction water mass is still widely used, especially when one body of water is very different from another, or when they have different origins. It is also used in a secondary sense, if the two water masses are found along active surfaces as a consequence of various movements that may include ruptures and sliding along fronts, as well as mixing processes.
The study of the geostrophic relation between marine currents and the distribution of water density has attracted, and still does, great interest. Wind is the most important driving force acting on the surface of the water, although the observed movements of the water do not simply respond to the circulation of the atmosphere in the way one might expect. In oceanography an intermediate conceptual level is used, based on an essentially correct physical formula, which refers to water masses, defined by density and located within the gravitational field of the rotating Earth.
The Coriolis force (see volume 1, pp. 153-154) is the local component introduced by the rotation of the Earth, which defines the (always transitory) equilib-rium between the distribution of water masses and the action of the wind. At its most simple it can be stated as: dV/dt = (2W sinF - dD/dx) - R + E where F is the latitude and dV/dt represents the change in velocity (acceleration or deceleration) of the current, which tends towards zero. If it is zero, there is compensation between the Coriolis force, 2W sinF, and the inclination of the isobaric surfaces (those acted upon by the same pressure) transversely to the current, an angle expressed as dD/dx. This description corresponds to what is called geostrophic, or inertial, circulation or flow--the circulation that would be in equilibrium with the rotation of the Earth. To indicate that this does not occur without cost, we add two further terms that compensate for each other: the first, R, designates the energy transmitted from the wind to the water, and the second, E, the dissipation of energy through turbulence that occurs when the water moves. The terms R and E thus remind us that we are dealing with a mechanism that transforms the energy of the wind by modifying advection or transport, and dissipates some of this energy in a series of small eddies that decrease to viscosity, and which play an important role in marine life.
The isobaric surfaces show a (geopotential) topography or relief that does not coincide with that of the Earth. The surface of the sea is considered to be an isobaric surface. Its geopotential topography is expressed as the difference (in meters) in height between two defined isobaric surfaces, as for example the surface of the sea and the surface at 1,000 decibars (a depth of about 3,280 ft, or 1,000 m). This difference will clearly be greater where the density of the water is lower. In accordance with these considerations and the corresponding technique, which need not be given in detail here, it is possible to proceed to a progressive breakdown that sheds a lot of light on the currents that can be expected at different levels and the forces which cause them.
At mid-latitudes, a slope on the surface of the sea of one centimeter per kilometer corresponds to a current of 39 in (100 cm) per second, approximately two knots. The current follows a gradient perpendicular to the line of the greatest slope; looking from the highest point of the surface, the current moves to the right in the Northern Hemisphere, and to the left in the Southern Hemisphere. The process of transmitting the movement from top to bottom, from the wind to the sea, then to the surface currents, and then from these currents to deeper ones, is always influenced by the rotation of the Earth. The result is that the surface current deviates by 45[degrees] to the right of the wind in the Northern Hemisphere and to its left in the Southern Hemisphere. On the equator itself, the dominant winds are easterly and there is some uncertainty: to the north, the current deviates to the right, northwards, while to the south it deviates to the left, southwards. In the center water has to rise from the depths to replace the water removed by surface divergence. An important consequence for the biosphere is that as the relatively nutrient-rich deep water approaches the surface it fertilises the equatorial zone. The relative fertility of its axis contrasts with the general relative biological poverty of the surface waters of warm seas.
Changes in direction in oscillating currents, within the water, are related to the sine of the latitude and thus also correspond with the local rotation of the plane of oscillation of Foucault's pendulum. The movements of buoys, such as those recorded by the large bouee laboratoire (laboratory buoy) anchored for some years off the coast of Provence (France), or the surface currents recorded by, for example, the oil-rigs on the coast of southern Catalonia, clearly show oscillations that cause the direction to change totally in a day and a half, as corresponds to a latitude of about 42[degrees]N. This means that they pass through the same plane of oscillation in half this time (approximately every 3/4 of a day). Within the water, as the surface movement is transmitted downwards, it deviates more and more as it becomes weaker, changing direction in a clockwise manner in the Northern Hemisphere and counter-clockwise in the Southern Hemisphere. It forms what is called the Ekman spiral until it reaches a depth that constitutes the lower limit of the Ekman depth, the level at which the water would move in a direction that is exactly the opposite of that of the wind observed on the surface, and its velocity has, theoretically, been reduced to 1/23 of that at the surface. Net transport is at 90o to the right of the wind direction and is known as the Ekman transport. The theory refers to situations of equilibrium and to water that is not stratified by density, but it can be used as a base for speculations that help to explain many aspects of small-scale marine dynamics (eddies, for example). Nowadays, a well-developed theoretical system has been established based on the formulation of the main relationships established by oceanographers, principally Scandinavian, such as Vilhelm Bjerknes (1862-1951), Sven Ekman (1876-1964), Bjorn Helland-Hansen (1877-1957), Harald Sverdrup (1888-1957) and Carl-Gustav Rossby (1898-1957), but also by scientists of other nationalities, such as the Austrian Albert Defant (1884-1974), the German Georg Wust (1890-1977), the American Henry Stommel (1920-1993), and others who applied Newton's and Laplace's principles to marine dynamics. A good measure of their success is that the more detailed and meticulous information obtained recently has generally confirmed the broad outlines of their predictions. There are, of course, discrepancies, some of which are of the same nature as those that make accurate weather prediction impossible over long periods of time. As water is denser than air and can only be compressed to a very small degree, some of the analogous difficulties found in the study of the atmosphere may be of little importance in oceanography. This discipline, however, has its specific difficulties, such as those due to the irregularities of the bottoms of the marine basins.
Density and vertical circulation
The oceans are like a large engine that transforms various kinds of exosomatic energy (external to organisms) in a variety of ways so that one part of it encourages the continuity and evolution of life and the maintenance of organic synthesis. The surface of the sea receives energy from the sun, via the atmosphere in the form of the force exerted by the wind, as well as directly from the solar radiation that heats and evaporates the water, generating what is called thermohaline circulation. This circulation is a mechanical consequence of local differences in temperature and salinity, and thus density. These movements are combined with the segregation and transport of materials, both greatly conditioned by the mechanical consequences of the rotation of the Earth. The essential conditions for biological production are light and dissolved materials. The function that has the most positive effect on life is that which redistributes the scarcer materials, bringing the nutrient elements (including phosporus, nitrogen, and carbon) into the light and thereby activating one stage of the biological cycle. In order to understand the circulation of seawater it is essential to know the distribution of its density. The density of the water, represented as indicated above by [s.sub.t] increases with depth. The stability of the water, E, is defined by the expression E = (1/s) (ds/dz). The more stable the initial stratification, the greater the work required for vertical mixing of overlying layers. There are often changes from those that occur in differences of level at the surface, when the wind "piles up" the water, to an internalization of forces, in such a way that internal surfaces at equal (isobaric) pressure are more or less inclined with respect to those of equal gravity. This situation may persist due to the forces of inertia associated with the rotation of the Earth and with the general organization of transport. The accelerations acting on volumes of water vary continuously as a result of wind drag, local changes in barometric pressure, the input and output of heat, the intensity of local vertical mixing, and the magnitude of the differences between precipitation and evaporation. Everything is unstable: minor hydrographic structures, whose study is of such importance in understanding planktonic successions, are submerged and are rapidly projected into and deformed in a larger setting within a hierarchy of different-sized circulation structures, with smaller ones fitting into larger ones. It is impossible to "freeze" part of the ocean in time so as to carry out an immediate and thorough study. Research is a path towards understanding this ever-changing system--mobilis in mobile, the motto of Jules Verne's fictional character, Captain Nemo. Many authors have referred to the large range of scales of space and time necessary to deal with such a wide range of phenomena. Differences in scale have meant that the differences in some of these phenomena were exaggerated, ranging from the factors conditioning the mobility of organisms, to the importance of the movement or vertical mixing of the water. Differences in the expression of equivalent phenomena may also depend on the relative size and the mutual fitting together of land and sea, which has changed considerably, as pointed out before, over the course of geological time. This has been the cause of the large variations in marine biological production over the course of geological history, affecting geographical distribution and the level of preservation of part of the materials produced.
Winds and ocean currents
There are now two oceans that stretch almost from pole to pole along a meridian; the Atlantic 33 million sq mi (82 million sq km) and the Pacific 64 million sq mi (165 million sq km); and a third, the Indian Ocean 28 million sq mi (73 million sq km), that is mainly in the Southern Hemisphere. These figures do not include the lesser adjacent seas or secondary basins, that occupy about another 15 million sq mi (40 million sq km). In volume, the Pacific Ocean (723.7 millions km3) accounts for about half the total volume of water in the oceans. The only continuous portion in the direction of the parallels is in the Southern Hemisphere where, where there is a strong general circulation between 50 and 60[degrees]S from west to east with currents of up to 6 in (15 cm) per second (0.3 knots). Two convergences are found there--the subtropical and the antarctic--and between them there is a zone that is thought to be of high productivity. A convergence is a line along which two water masses meet, with the denser water from one side sinking beneath the lighter water of the other side. As they usually have different characteristics, the convergence may be clearly defined, forming what is called a front. A divergence is the opposite movement: a horizontal flow of water in different directions away from a common center or line when it reaches the surface. Divergences are often sites of high biological productivity. Circulation systems reflect the general distribution of winds. Oceans transport heat from the equator to the poles, so that there is a degree of surface transport in this direction. The water that sinks at high latitudes contributes to the oxygenation of the deeper layers. The trade winds generate a circulation towards the west, causing equatorial upwelling. The main oceanic mass is divided into five "semi-oceans," more or less trapezoidal in shape (see volume 1, p. 154). Each of these units forms a large gyre, or circular motion, that rotates clockwise in the Northern Hemisphere and counter-clockwise in the Southern, and is thus anticyclonic in both hemispheres. The centers of these large gyres contain nutrient-depleted water that tends to sink. The main gyres are asymmetrical, when considered in the framework of the parallels running from east to west, with intense southern currents towards the west that transport more water in the Northern Hemisphere than in the Southern, and upwellings in the east that are stronger in the Southern Hemisphere (Peru, Benguela) than in the Northern Hemisphere (California-Oregon, Sahara-Morocco-Western Iberian). Thus strong currents run along the western shores of the oceans, and these currents are faster and more clearly defined--and have been known since antiquity--in the Northern Hemisphere (the Gulf Stream and the Kuro-Shio current with top speeds of up to 2.3 mph). Between these currents and the respective coasts to their west there are very fertile systems whose activity is increased by the inflow of fresh water, which generates a general coastal circulation to the right when looking from land to sea. They have traditionally supported very productive fisheries. The Indian Ocean is a "half" ocean with special characteristics and it is greatly influenced by the monsoon regime caused by the proximity of the raised highlands of central Asia. The winter monsoon, from the northwest, cools the water and increases the fertility of the Gulf of Oman and the Bay of Bengal; the summer monsoon, from the southwest, generates intense upwelling along the coast of Somalia. The southern part of the Indian Ocean contains a large anticyclonic gyre similar to those found in corresponding areas of other oceans.
Circulation in secondary basins
The current arrangement of the continents means that there are marine regions that are not sufficiently continuous or large enough to contain a circulation in total equilibrium with the rotation of the Earth and with large upwellings, nor even a "normal" tidal regime. Their circulation is partially or completely thermohaline, as it partly depends on differences in temperature and salinity, which in turn are often highly dependent on the passing of the seasons. Fertility depends on the size of the fertilizing system. In fact, in small seas this fertility is never very high, but it is often quite good (up to 200 g of carbon per square meter per year). This factor combined with the frequently complex coastline, makes possible considerable ecological differentiation and high biotic diversity. In general, there are always areas where the water is more or less continuously stratified. There are other areas where, for local reasons and especially where favorable climatological situations are of frequent occurrence (for example, cyclogenesis), active marine fronts and even upwelling areas of potentially fertile deep waters repeatedly appear. In the warmer secondary basins (such as the Caribbean and Gulf of Mexico or the Mediterranean) a range of relatively small, but very persistent hydrographic structures maintain essentially stable relationships in space and time. Furthermore, as production is lower than in the favored areas of the larger oceans, it varies less from year to year. The reduced scale may also explain the persistence of some vertical distributions, such as the nitrite-enriched layer just below the nutricline and the chlorophyll maximum. Similar structures on a smaller scale are also found in some relatively calm regions of the main ocean basins. The most notable common characteristic of the secondary basins is above a certain level they are connected with a main ocean, from which they differ in the quantity of water evaporated and rainfall received, and to an even greater extent, in the relative difference between these two quantities. This means there are two possible types of thermohaline circulation with the main ocean. If evaporation in the secondary basin exceeds rainfall, as occurs in the Mediterranean, the surface circulation is from the main ocean to the secondary basin, while at the same level deep water leaves (which is precisely what happens in the Mediterranean). The difference between the incoming and outgoing water is related to the different rates of evaporation, and there is always an apparent excess of water exchanged. This is an example of the general tendency towards systems with high inertia, in which the available energy ends up by circulating a lot of material at a low velocity.
The results are different from those we might expect. In the Mediterranean, the deep saline water escapes through the Strait of Gibraltar carrying a relative excess of nutrients. This is why the Mediterranean is effectively oxygenated right down to the seafloor and does not become eutrophic. On the other hand, the Baltic is an example of a quite different regime. It receives a large quantity of freshwater, and so the living organisms that flourish in the less saline surface water, especially in the large internal gulfs, are derived from (and in part very similar to) those of neighboring freshwater systems. High stratification and high biological activity explain why large areas of the seafloor are anoxic. We may debate the relative care with which the inhabitants of the two basins treat their seas, but in terms of ease of conservation, fortune favors those living around the Mediterranean. Circulation in the Baltic is comparable to that found in a normal or "positive" estuary: Surface water leaves and, to a greater or lesser extent, water enters at the bottom and may form a type of saline "wedge." In the Mediterranean the situation is just the reverse and is often compared to a hypothetical "negative" estuary. The intensity of the tides (which are weak in secondary basins), together with the depth of the sill, largely determine the movement of water in the straits joining a secondary basin to a main ocean.
Traces of former oceans are found in some secondary seas, such as the Mediterranean, or the Caribbean and Gulf of Mexico, which include fragments of the ancient Tethys Sea. This warm, essentially equatorial sea (between 30[degrees]N and 30[degrees]S) encircled the entire Earth during the Mesozoic period, and formed on its shores many of today's hydrocarbon and, in some cases, evaporite deposits.
The Mediterranean has a chronic excess of salinity--its watershed has always been relatively arid--but in some times in the past it has reached very high levels, such as five million years ago, in the Messinian period, when thick strata of evaporites were deposited, which are now covered by new sediments. This segregation of salts contributed, as mentioned above, to reducing the salinity of the world's oceans by about 2%.
The Red Sea is very different. It is the site of recent crustal separation, with the ascending injection of very dense materials, and shows a positive gravity anomaly. The deep brine deposits on the bottom and the excess of metals they contain indicate its deep origin. Its regime has now been affected by the opening (reopening?) of the Suez Canal which, after the gradual elimination of the region's ancient evaporite deposits, has made possible many biological exchanges in both directions.
2.2 The ebb and flow of the tides
All regularly occurring natural phenomena are considered rather mysterious or "magic." The most common ones and the ones that most affect us are the movement of the Sun and Moon across the sky. The Sun, with its unchanging circular appearance, marks a daily rhythm that dominates everything. The Moon, although it performs a similar daily movement, seems to behave in a peculiar way as it is out of phase with the Sun, and thus changes in appearance every day. It repeats a cycle from full moon to full moon (or new moon to new moon), which is of great value for measuring the passing of time--the monthly period. The elevation of the Sun over the horizon varies with the season, and the time of sunrise and sunset changes from day to day, providing a measure for larger periods of time. We can measure years by counting the numbers of springs or winters. It is worth asking ourselves what role these natural clocks have played in the development of human intelligence, but they have undeniably allowed people to organize themselves and to anticipate certain events with great reliability.
This ability to anticipate is, however, not limited to human beings. The entire biosphere dances to the rhythm of the heavenly spheres: to the strains of night and day, the moon, the seasons of the year, and on a larger scale, to the slow beat of the small regular changes in the angle of inclination of the Earth's axis of rotation which would appear to be responsible for the expansion and contraction of the polar icecaps over periods of thousands of years. The lifespan of organisms limits their potential ability to recognize and use the pulsations of the heavenly bodies as a reference clock. In general, small organisms with short lifespans only reflect daily rhythms, while larger organisms with longer lifespans can respond to the seasons of the year. There are no clear periods of a few years, and so there is no natural clock that is useful for measuring periods of between one and a hundred years.
The Sun and the Moon are also the main agents responsible for the tides, another regular rhythm that can be used by organisms as a signal to set their own internal rhythms (see 1.2.1). Tide is the term used for the rise and fall in the level of the sea, with a period of 12.5 hours (or 25 hours), observable on the shore of any of the main oceans. The period and signal intensity (the rise in sea level), however, are not the same everywhere. Along the coast, where tidal effects are most intense, many organisms synchronize their biological rhythms to local tides, and by doing so, they link indirectly, through the sea, to the Moon and the Sun.
Variation in sea level is the most evident effect, but it is not always the most important from a physical point of view, or from that of an organism (including humans, recent users of the sea). The rise and fall of the sea level is also associated with horizontal movements, the tidal currents. In effect, water rises near the coast where currents converge towards the land, and falls where they diverge, that is, water is transported away from the coast. These currents may be very strong, although over a complete cycle the net transport of water (or of floating objects) may be negligible. The intensity of the current means effort and friction against the container (e.g., the bottom of the sea or the edges of a bay), or within the water itself, and this causes (often intense) mixing, and thus promotes the diffusion of substances, whether dissolved or as particles. This mixing and the associated diffusion have an important effect on the functioning of ecosystems subjected to tidal rhythms (by increasing potential biological production or by structuring coastal waters through the formation of active fronts, for example).
The surface separating two fluids of different densities may easily develop undulating movements in response to temporary or persistent deformations. Tides are a good example of this. If the solid part of the Earth were more plastic (i.e., more easily deformed), it would be continuously deformed as a result of the combined gravitational attraction of the heavenly bodies--or at least the closest ones, the sun and the moon--an effect that can be calculated using Newtonian mechanics. The relative rigidity of the solid parts of the Earth means that their response is minimal, although it can be detected. However, the oceans and the atmosphere, being located over a globe that deforms only slightly, undergo periodic and major redistributions of their own masses. This unequal response by the different parts of the Earth has a braking effect, which slows down the Earth's rotation. The length of the year has not changed, but the length of the day has changed, and the number of days in the year increases by one over about 10 million years. This change has been recorded in structures with daily, tidal, and annual periodicities, as seen in the hard parts of past organisms (mollusks, corals). They are comparable to the better-known records of rhythms, such as growth rings in fish otoliths and tree trunks.
The Moon and the tides
The tides are a consequence of the simultaneous action of the gravitational force of the Moon, the Sun, and the joint rotation of the Earth-Moon and Earth-Sun pairs. They should be considered as pairs because the joint center of mass of the Earth and the Moon, around which both rotate, is located within the Earth at approximately a quarter of the way along Earth's radius, on the imaginary axis joining the centers of the two bodies. To begin, it is necessary to understand the composition of the gravitational and centripetal forces of the Earth-Moon pair. They are independent of the Earth's own rotation (centered on the axis running from pole to pole), which has a modulating but important effect on the tides. The Earth and Moon maintain their distance from each other because their mutual attraction, in accordance with Newton's law of gravitation, is compensated for by the centripetal force associated with the rotation of the two objects around their common center of mass, as explained above. However, as the distance from the Moon and the common center of rotation is different for every point on the Earth's surface, the sum of gravitational and centripetal forces is different at every point. In fact, the resulting distribution of forces is radially symmetrical around an axis running from the center of the Earth to the center of the moon. It is worth pointing out that these forces causing the tides are only about 10-7 the strength of the Earth's gravitational field. If such weak forces are important it is because they affect the entire water mass and because they contain horizontal components--ones that are at right angles to the direction of gravity. These horizontal forces can only be opposed by other horizontal forces, and in the sea these are mainly the result of differences in the level of the sea's surface and the associated gradient forces. It is important to note that the horizontal components of these forces are stronger at points on the Earth that are at a certain distance from the line joining the centers of the two bodies, and it is precisely at these points that the water will tend to move horizontally due to the action of tidal forces. So far we have not considered the fact that the Earth is rotating on its own axis. This is why every point on the planet is regularly affected (daily) by tidal forces directed outwards (opposing the Earth's gravity) and inwards (in the same direction as the Earth's gravity), and by forces with larger horizontal components between these two extreme situations.
The influence of the Sun
The Earth-Sun pair produces a series of forces similar to those by the Earth-Moon pair, although their effect is only half that produced by the Moon since, despite the fact that the Sun's mass is vastly greater than that of the Moon, so is the distance that separates it from the Earth. On the other hand, because the Sun and Earth do not move synchronically, the relative positions of the three bodies are constantly changing, and the combined effect of all these forces is very complicated. All these temporal variations (and others not discussed here) are highly periodic, so much so that the final result can be broken down and expressed as the sum of sinusoidal functions, each with its own well-defined periodicity.
Some of the bodies' relative positions are especially important. The sum of forces is greatest when the Earth, the Moon, and the Sun are roughly in a straight line; sometimes the Earth is between the Sun and the Moon (full moon), and sometimes the Moon is between the Sun and the Earth (new moon). The resulting forces are weakest when the Sun is illuminating the Moon on one side or the other (waxing or waning, as seen from the Earth). The variation of these forces with time involves more components than those mentioned so far, since the relative orbits of the Sun and the Moon with respect to the Earth are not perfect circles but ellipses, and their planes of rotation vary in relative inclination. All this means that the heights of the tides are very variable.
The greatest amplitudes corresponding to each sinusoidal component vary according to the place in question, and so there is a very wide range of tidal types at different points on the planet's surface. In any event, bearing in mind that the periods that most affect the tides are diurnal and semidiurnal, we can distinguish between tides dominated by one or other of these two types. Thus, in the range from one type to the other, we will find tides that are basically semidiurnal, mixed (with clearly-defined semi-diurnal and diurnal components), and tides that are basically diurnal.
The prediction of tides
Tidal movements are, above all, the expression of the interaction between the apparent deformation of the sea by tidal forces and the specific shape and size of the coast. One might imagine that these forces deform the planet's fluid covering as if it responded instantaneously to the vertical components of tidal forces (rising in level when these forces act upwards, and becoming depressed when they are acting in the same direction as gravity). However, common sense and experience tell us that the sea has great inertia and that horizontal components have a greater opportunity to move the water than vertical ones. Thus, the most important forces are tidal currents, which in terms of the entire ocean produce forces and generate forced waves, that follow the period of the active forces and not the period that would correspond to the form and size of the oceans, the density of the water, and its depth. The long waves produced by the tidal forces tend to displace themselves through the seas following the forces in question (they are forced waves). At the Equator, our planet's surface is moving at 1,476 ft (450 m) per second with respect to the Moon, while at a latitude of 45[degrees] it is about 984 ft (300 m) per second, continuing to decrease towards the poles. It is necessary to point out, as a point of reference, that in a situation of equilibrium, in an ocean 3.4 mi (5.5 km) deep, the crest of a long wave, such as those produced by tidal forces, would travel at about 755 ft (230 m) per second. As tidal forces are displaced over the planet at the speed of its surface rotation (as explained above), the most common situation is that the wave produced cannot follow tidal forces and is delayed with respect to the movement of the Moon overhead. At low latitudes around the equator, the phase lag is about 6 hours. To be precise, high tide occurs twice, at 6 hours 12 minutes and 18 hours 36 minutes after the Moon has passed overhead. As latitude increases, this difference in phase decreases and, although it varies from place to place, it is constant in each one. However, at a latitude (for example, that of the Antarctic Ocean), where the velocity of the surface of the Earth with respect to the Moon is about the same as the velocity of the tidal waves (not forced ones), these waves can follow the passage of the Moon perfectly. In such cases, high tide occurs when the Moon is overhead and then 12 hours 25 minutes afterwards (when the Moon is precisely at the other side of the planet).
2.3 Other more or less regular movements
Apart from the tides, the sea shows other kinds of movements. The two most typical cases of these displacements are the upwellings of deep water and the to-ing and fro-ing of water caused by the wind.
Waves are produced by the wind, and are propagated for much longer and further away from the place where the direct mechanical action takes place. They are the result of the interaction between two fluids that are separated by their differences in density. The interactions originate and develop from the basis of the irregularities that form at the surface of contact between the two fluids, which are moving at different speeds. Based on significant values for this difference in speed, waves are formed that are, in fact, never perfectly uniform. Moreover, the water is dragged by the wind, the basis of marine currents. Surface friction generates waves, which are a result of the propagation of kinetic momentum.
Waves are characterized by their velocity, C, measured at the crest or highest point (33-49 ft [10-15 m] per second, on the high seas), their length, L (variable, with maximum values in hundreds of meters), their height H, (up to 49 ft [15 m]), and their periodicity, T, (generally between five and nine seconds). These values are related to each other by means of simple equations: L = gT2/2p, C =L/T, T= 2pC/g, where g is the local value for the acceleration due to gravity. This is a first approximation, given that both fluids show turbulent disturbances that complicate their interactions and lead to a wide range of local variations. It has been said that describing the surface of the sea is a task for a poet rather than a mathematician. Obviously, there are other approximations. Thus, for example, the "angle" of reflection of sunlight from the surface of the sea depends on the degree and distribution of the individual reflecting slopes or surfaces. Consequently, information obtained from aerial photographs of the surface of the sea from a known angle allows statistical studies to be made of the distribution of the slope faces at different angles, with interesting consequences.
As we shall see, waves cause eddies of the water below the surface, which continue upwards until they fade out and contribute to the mixing of the water. If the waves approach the coast, they "break." In other words, these lower movements interact with the bottom, the wave becomes shorter and taller, and its top then "falls." All this can be seen clearly from any shore. From the point of view of biology the most important aspect is the effect of the lower part of the waves on the sediment and on the organisms that live in or on it. We cannot ignore this and other effects of the waves on organisms. These forces, added to that of the tides, may have negative consequences, but may also have very positive ones, as far as the exchange of water is concerned, and more precisely with organisms that use and profit from the additional exosomatic energy provided by the most wave-beaten environments. The effects of shearing and pressure (2-3 kg per cm2) are furher, but less positive, manifestations of the same energy.
Internal waves and thermoclines
The movement of the waves is propagated in eddies downwards and mixes a layer of surface water of greater or lesser thickness, making some of its characteristics, such as its temperature, more uniform. In this case, it penetrates downwards and accentuates the thermal gradient below. This thermal gradient, or rapid change in temperature, becomes more abrupt the deeper it is in the water, and decreases the mechanical energy available at that depth. The result is a typical shearing effect leading to the stabilization of the thermal discontinuity or thermocline, often between 98-330 ft (30-100 m) in depth. The thermocline is also a stable pycnocline, a zone where water density increases with depth in response to changes in temperature and that, for precisely this reason, can be crossed by internal waves. As these internal waves flow between two fluids whose difference in density is not as large as that between water and air, they are slower (normally between 1.0 and 2.0 ft [0.3 and 0.6 m] per second), have a longer duration (from five minutes to a few hours) and are correspondingly taller (16-98 ft [5-30 m], for example) than superficial waves. They break like superficial waves and create eddies of great importance to pelagic life. The thermocline eventually stabilizes at a level that depends on the normal local wind intensity. In reality there may be two thermoclines, one that forms every year when the surface water warms up, and a deeper, more persistent one that is the lower limit of the surface layer where vertical mixing often takes place, or is possible. They are always of great importance in dividing the pelagic environment; they are often at a level where superimposed layers may slip over each other, and they sometimes roughly coincide with the limits of the photic layer. Although the thermocline may take months to form from the surface downwards, it can disappear very rapidly: When the surface water cools, it becomes denser, and convection causes it to fall. This is another example of the kinds of asymmetry so common in natural changes.
Fluctuations in upwellings
Everywhere upwellings are, and always have been, fluctuating. Generally, there is a positive correlation between average production and its variance. The less productive marine regions show less variability. The more productive systems are very variable, and the most productive one, off the western coast of South America, is the most variable. There are signs it has changed over the centuries. The northern limit of the upwelling, off the coasts of Ecuador where the climate is now relatively dry as a consequence of the relatively cold water of the upwelling, a former more humid climate may well have left traces on the landscape. There is evidence that the Peruvian guano deposits--so heavily exploited over two hundred years until the use of artificial fertilizers became general in agriculture--are relatively recent, and a few thousand years ago a less dry environment than that of today might have been the result of a slightly less active upwelling system.
It was in this area of coast that L.E. Dinklage, captain of a Prussian ship, made the first accurate description of an upwelling. The Prussian captain wrote in 1874: "I believe that the trade winds are the most important motive forces... they are constantly drawing the water away from the coast... and propelling it in a westerly direction. To compensate this displacement... operating on the surface, water has to rise from below the surface towards the land, rising directly at the coast and reach the surface."
This is not surprising. The normally cold water of the Humboldt Current creates the largest (about 154,440 sq mi [400,000 sq km]) and most productive marine upwelling in the world (between 500 and 1,000 g of carbon fixed per square meter per year). Humboldt himself stated, perhaps with excessive modesty, that all he had done to deserve having the current named after him was to have measured its temperature, as for centuries every child in every fishing family on the Peruvian coast had known how cold it was. Humans, like birds before them, have exploited this remarkably productive area for a very long time. Although it can cause fogs (garua), the cold current generates a descending atmospheric circulation that reduces rainfall, and this has meant that farmers on the Peruvian coast have always had to practice very careful water management.
This descending atmospheric circulation is unfavorable for gliding birds, and this is why these coasts (highly exploited by seabirds, like all coasts with upwellings) are dominated by species that dive from the air or from the water's surface to catch the fish they eat. The most important birds on the Peruvian coast as accumulators of guano are, or more accurately were, the guanay cormorant (Phalacrocorax bougainvillii), the Peruvian booby (Sula variegata), the brown pelican (Pelecanus occidentalis), and the blue-footed booby (Sula nebouxii). There are many birds of other species, including many that can easily fly from one place to another if necessary, making them much more mobile than the local marine fauna. This is in accordance with the unreliability of the local marine production, which cannot support many large and relatively immobile predatory fishes: Their role is taken by birds that can easily change residence. The situation in regions with fluctuating upwellings could be compared to the way migrant birds take advantage of the Nordic spring.
Off the coast of California, where there is a twin upwelling to the Peruvian one, fluctuations (although highly irregular) have been recognized within past fluctuations, by means of biochemical and biological evidence in, for example, the scales of fish preserved in the sediments deposited under the upwelling region.
In many other places there are similar pluri-annual variations. The intensity of the upwelling on the coasts of Galicia also oscillates, and the limits of the more active, colder region of the northwest of the Iberian Peninsula are also reflected in the distribution of coastal algae and mollusks.
At the western edge of the Bay of Biscay, the westerly waters are colder and, if the upwelling is consistently intense, its limits move eastward. Many species living on this coastline also live around the opposite geographical limit on the western coasts of France (the Bay of Biscay is warmer than its two horns). In the 1950s, E. Fischer-Piette started the study of this type of fluctuation and how it is reflected in the organisms found on the coastal benthos, a study that has unfortunately not been continued.
We must remember that the French oceanographer Edouard Le Danois, from 1925 onwards, had recognized movements in the expanse of the warmer centers of the oceans at mid latitudes. He called them "transgressions" because they approached or superimposed themselves on the more coastal bodies of water, as has been reported in the South Pacific, where the phenomenon may perhaps be on a greater scale. It is also less concealed there owing to the different characteristics of the atmospheric dynamics of the two hemispheres.
Comparable fluctuations with a periodicity of six or seven years are seen in other marine phenomena, as happens, for example, in tuna fish populations. This is not surprising because large carnivorous fish travel widely and depend on local prey populations that are in turn related to local upwelling systems. These may show some degree of synchronism in their fluctuations, as they are dependent on a single common atmosphere/hydrosphere system. The fact that the culminating points of all this sort of oscillations are several years apart leads to the consideration of mechanisms not clearly linked to annual cycles, and at a higher level than annual cycles, and based, for example, on the large calorific capacity of the oceans, more than 300 hundred times greater than that of the atmosphere. One might consider that there is an oscillator based on the interaction between the planet's two fluid covers. Now that there are more extensive networks of weather stations and better facilities for observation from space of, for example, the characteristics of the surface water of the Pacific Ocean, we may now expect that, at least, El Nino will cease to be the totally unpredictable mystery it currently is.
1 Mother Earth? Mother Sea? The sea is a lonely place, but full of organisms that stretch our imagination. In our imagination it is also full of sailors (and landlubbers) who between them have seen it as a treasure trove, a den of monsters, a beaten path to adventure, a battlefield, a lifeline, a burial ground, a sewer, a purifying bath, a sports arena, a workplace. Some have loved it, while others have hated it; for some it has been an adventure, for others it has been a nightmare. The gentle curve of the horizon is both a reminder of the limits to our perception and the physical limits of the planet we live on.
[Photo: Jaume Altadill]
2 Plankton blooms, daily vertical migrations, upwel-lings and species extinction due to glaciation are phenomena that seem to lack any clear connection. Stom-mel's diagram, expressing time and space on a logarithmic scale, serves to show semi-quantitatively the variability of the biomass of the zooplankton, represented on the vertical axis. This conceptual model allows us to relate phenomena that occur on very different spatial and temporal scales, but which are not really that different; they range from small patches of high zooplankton density that form and disintegrate on a local scale, through vertical migrations with a daily periodicity (found in both oceans and smaller structures) and oceanic upwellings (occurring on scales ranging from tens to thousands of kilometers, with a roughly annual periodicity), to the transformations associated with a glaciation, affecting the entire planet on a time-scale ranging from tens of thousands to hundreds of thousands of years.
[Drawing: Jordi Corbera]
3 In the tectonically active areas of the Earth materials derived from the mantle emerge and are incorporated into the lithosphere and oceans. This is what is happening to this volcanic lavaflow next to the sea on the southeast coast of the large island of Hawaii, within the Volcano National Park.
[Photo: Philip Rosenberg / Auscape International]
4 The widening of the Atlantic ocean began in the Cretaceous period and had major evolutionary consequences, as it caused a dramatic change in environmental conditions on a planetary scale by altering the relative positions of the continents and the directions and importance of the marine currents. The figure shows the main features of the world's marine circulation during the Creta-ceous period (deduced from the configuration of the continents and from the influence of Coriolis force). Note the size and shape of the Tethys Sea (in violet) which, before the opening of the Atlantic, girdled the Earth at tropical and subtropical latitudes, to the south of Eurasia and North America.
[Drawing: Jordi Corbera, bas-ed on data from Frakes, 1979]
5 The technologies at the service of oceanography have undergone great qualitative improvements since the 1960s. The Johnson Sea-Link II, for example, is a submersible vehicle brought into operation in the 1970s by the Harbor Branch Oceano-graphic Institute, Fort Pierce (Florida, USA). It can reach depths of more than 2,297 ft (700 m), and the crew (properly equipped) can enter and leave at depths of up to 595 ft (180 m). The photo shows it on board the oceanographic vessel Seward Johnson, off the island of Cabrera, one of the Balearic Islands, during a study of the ecological physiology of the alga Laminaria rodriguezii, the Mediterranean's only endemic species of kelp.
[Photo: Enric Ballesteros]
6 Large marine currents such as the Gulf Stream can form meanders, just like rivers. After a time they separate from the main current and turn into an eddy, as shown in the drawings at the top. The differences between two types of water--water from the Gulf Stream is on the outside, while on the inside there is water from the continental slope of the eastern coast of the southeastern United States, or Sargasso Sea--are highlighted by the different distribution of phytoplankton species. The sections reveal how Distephanus (=Dictyocha) speculum is found preferentially in the warmer waters derived from the Gulf Stream, on the eddy's outer circle, while Distephanus pulcra is concentrated in the colder central waters.
[Drawing: Jordi Corbera, from various sources elaborated by the author]
7 Main divisions of the marine environment, in terms of horizontal and vertical zoning.
[Drawing: Jordi Corbera, from several sources]
8 Transitory eddies often form on the edges of sea currents, like these that correspond to the Saltstraumen channel on the north-western coast of Norway, near the Lofoten islands.
[Photo: Richard Matthews / Planet Earth Pictures]
9 Although we usually measure height on land against sea-level, the sea is not in fact strictly flat, but has its own topography that is generally measured in terms of the difference in height and the isobaric surface at a specific reference pressure. This is why this topography is called geopotential, as it expresses the work necessary to move a unit mass between two isobaric surfaces one meter apart in a mass of water at rest. Maps of equipotential topography of the sea surface resemble the isobaric maps used in weather forecasting, and should be read in basically the same way: They show pressure differences and the directions of the movements of the masses of water, and may vary with the reference surface used. The drawing shows the geopotential topography between the Canary Islands and the Cape Verde Islands, the African coast and the meridian 25[degrees] west, measured in dynamic meters with reference to the isobaric surface at 100 decibars (approximately 328 ft [100 m] depth) on the left, and on the right, at 1,000 decibars (approximately 3,281 ft [1000 m] depth); the dots show the observation stations. The curves and the direction of the arrows (showing the movement of the currents in equilibrium with the distributions of mass) do not coincide in the two maps. This is because these currents should be understood as an average of all the displacements of water in the thickness of water considered (328 ft [100 m] in the first case, 3,281 ft [1,000 m] in the second).
[Drawing: Jordi Corbera]
10 Tritium (3H) is a radioactive isotope of hydrogen that has a very short half-life (12.4 years), and is extremely rare in nature. In the atmosphere the natural proportion is about one tritium atom for every 101 hydrogen atoms. The recent presence of tritium in the oceans, as a consequence of atomic explosions since 1945, is an excellent tracer of the recent movements of the planet's fluid coverings, and also shows the latitudes where humanity's destructive capacity is concentrated. This meridian section of the Atlantic Ocean (prepared in 1972 by G. Ostlund of the University of Miami as part of the GEOSECS program) showing tritium produced by atomic explosions clearly reveals the latitudes where nuclear explosions in the atmosphere had taken place up to that time, and also shows the pattern of diffusion of ocean water responsible for its current distribution, penetrating to deeper waters at the latitudes where wind action is strongest.
[Drawing: Editronica, based on data from G. Ostlund]
11 A "bloom," or proliferation of phytoplankton, in the Sea of Cortes, off the coast of Mexico. Contrasts between masses of water of different appearance, mainly due to phytoplankton organisms coloring them green or red, makes it obvious that the waters of the ocean are a source of life, not merely a fluid in motion. Wherever there is upwelling (or nutrient enrichment due to pollution) that allows high production, there is always a set of organisms ready to multiply and proliferate while they can.
[Photo: Norbert Wu / Still Pictures]
12 Engraving by Alphonse de Neuville (1836-1885) in the first edition (1869) of 20,000 Leagues Under the Sea by Jules Verne (1828-1905). Oceanographic re-search could well adopt the motto that Verne's fascinating hero Captain Nemo adopted for his submarine, the Nautilus: mobilis in mobile (moving within movement). Captain Nemo was an archetypal romantic hero fighting for freedom, but also embodied 19th century ideas of science as progress. The sea truly is a dynamic, changing reality, whose regularities can only be recognized by detailed work with a very large number of observations and measurements and the development of ever more complex conceptual methods and techniques.
13 The balance of incoming and outgoing water in two closed seas. Differences between rainfall and the water evaporated, found between secondary marine basins and the adjacent oceans, always lead to exchanges. The Mediterranean and the Baltic Sea are examples of two contrasting forms of this type of circulation. The Baltic receives much more rain and river water than it loses by evaporation, and functions as a large estuary flowing into the Atlantic, through the Denmark Strait and the Oresund Straits. The Mediterranean loses much more water by evaporation than it receives as rain and river water, and functions as a "negative estuary," receiving more water from the Atlantic through the Straits of Gibraltar at the surface than it returns at depth. The Black Sea, like the Baltic, receives more water from rainfall and rivers than it loses by evaporation, and transfers about twice as much water to the Mediterranean through the Bosphorus, as it receives.
[Source: data prepared by the author]
14 The volumes of water exchanged between the Mediterranean and the Atlantic through the Strait of Gibraltar are very large in both directions, much greater than the volumes necessary to compensate the difference between evaporation and input from rivers. This false-color image of the western tip of the Mediterranean uses colors to represent increasing concentrations of photosynthetic pigments: Colder colors (blues and greens) represent low concentrations and warmer colors (yellows, oranges and reds) represent higher ones, indirectly reflecting the biological productivity of the water. The colder, nutrient-rich waters of the Atlantic form a characteristic anticyclonic gyre in the Sea of Alboran, clearly visible in the photograph due to the higher pigment concentration in the waters from the Atlantic. The hydrodynamic mechanisms that cause this gyre are not completely understood, nor is it known why to the east of the sill of the island of Alboran and Cape Tres Forcas (Ras Uarc) the waters that split off from this gyre form a second anticyclonic gyre, also clearly visible as a result of the higher concentration of pigments off the coast of Andalusia. The northwestern Algerian coast deflects a part of this gyre towards the north-east, towards the Balearic Islands, where the waters spread throughout the Mediterranean. The sill between the sea and the ocean is to the west of the narrowest strait between the two coasts, as shown in the bathymetric map and the cross-section of the seafloor. On the surface of the strait, water flows from the Atlantic to the Mediterranean and, at depth, from the Mediterranean to the Atlantic. From March to October the inflow predominates, while from November to February the outflow does, with an annual balance that shows net entrance of Atlantic waters.
[Photo: courtesy of the Global Change Data Center, NASA Goddard Space Flight Center; drawing: Jordi Corbera, based on data provided by the author]
15 Comparative data for some of the most important secondary basins. It shows average and maximum depths (in meters) of the basins and the sills that separate them from the seas or oceans they connect with.
[Source: data provided by the author]
16 Photomicrograph of an otolith of a dolphin fish Coryphaena hippurus showing the daily growth increments (arrows). Rhythmic groupings of growth increments can be seen (triangles), that are probably related to environmental oscillations caused by tidal cycles.
[Photo: B. Morales Nin / CSIC--Institut d'Estudis Avancats de les Illes Balears]
17 Spring tides and neap tides are the result of different combinations of the action of the Sun and the Moon, depending on the phase of the Moon. At full moon or at new moon, when the Sun and the Moon are in the same plane as the Earth, the action of the two bodies is additive, and spring tides occur. When the Moon is waxing or waning and is at right angles to the Earth and the Sun, neap tides occur. In any event, the most intense forces are due to the action of the Moon (arrows in the drawing), that converge on the center of the rotating pair Earth-Moon (or its antipodal point).
[Drawing: Editronica, based on several sources]
18 The theory of tides allows the calculation in general terms, or on a large-scale, of how tides would be in the oceans, as a result of their size and shape. The waves in fact follow a more complicated distribution, but the repeated basic pattern is easy to understand: The waves rotate around centers of zero height, and the further their radial distance from this central point (called the nodal or amphidromic point), the taller they are. If we start at a given moment (0), the crests will have moved to the position indicated by 1, and so on until the cycle is completed. The tidal range is zero around the amphidromic points and increases towards the tips of the crests.
[Drawing: Editronica, from several sources]
19 Can an artist understand waves better than a scientist? Not always, but the great Japanese master Katshushika Hokusai (1760-1849) did. In this colored engraving from his series Thirty-six views of Mount Fuji (1823-1830), as in his other landscapes, the artist manages to express the movement and blind force of the waves, as well as the effort and skill of the frightened fishermen confronting it as they sail to the port of Kanagawa.
[Photo: Archiv fur Kunst und Geschichte, Berlin]
20 Waves do not always pound coasts and stir up sediments everywhere. In the shelter of a bay, or near the coast when a gentle offshore wind is blowing, the sea may be as calm and flat as in this picture of fishermen casting their nets in the Mediterranean.
[Photo: Juan Carlos Calvin]
21 Variation in the oceanographic regime is often the natural cause of rises and falls in fish stocks. This, rather than fishing pressure, was the reason for the decline in the Cape hake or stockfish (Merluccius capensis and M. paradoxus) and the ocellated sardine (Sardinops ocellatus), and the increase in the Cape horse-mackerel (Trachurus capensis) in the upwellings of the Benguela current in the 1960s and 1970s, although the situation reversed in the 1980s. It was also the cause for the oscillations in catches of the Peruvian anchovy (Engraulis ringens) and the South-American sardine (Sar-dinops sagax) in Chilean and Peruvian waters due to the El Nino phenomenon and its effects on the Humboldt Current.
[Drawing: Editronica, from various sources]
22 The relationship bet-ween upwelling and the distribution of fish density, mainly sardines, on the coasts of Senegal, Gambia, Cape Verde. All upwellings vary, and the one on the western coast of Africa, shown in this satellite false color image, is no exception. As the upwelling brings colder, nutrient-rich water to the surface, surface temperatures reveal the surface structure of the upwelling.
[Courtesy of the Global Change Data Center, NASA Goddard Space Flight Center, Greenbelt]