5 The Antarctic dominion.
1.1 The Antarctic continent
The Antarctic continent and the waters surrounding it have long been one of the world's most remote, inaccessible, and unknown areas. The Antarctic region plays a vital role in the functioning of the planet as a whole because of the interactions that take place there between the ocean, the atmosphere, the ice, and living organisms. This has been recognized by international programs such as the International Geosphere Biosphere Programme (IGBP), which considers Antarctica as a critical area for the study of global change.
An ancient rocky, icy fragment of Gondwana
Antarctica consists of a continental landmass and several islands, with an area of 13,661,000 [km.sup.2], 97.6% of it covered in ice. The Antarctic continent is a fragment of the great continent, Gondwana, that existed in the southern hemisphere between 550 and 500 million years ago in the Cambrian period. In the Permian (about 275 million years ago), what would later be Antarctica already occupied a position at the pole, a position it has maintained since then without any major changes. Although it is at a very high latitude, Antarctica has not always been covered in ice. During the Permian thick ice caps formed, which melted and did not appear again until 26 million years ago. The center of the Antarctic continent is more or less at the South Pole. The Antarctic continent is divided into three parts on the basis of the surface topography of the inlandsis, that is to say, the large accumulation of ice that covers 99.6% of its surface, and also of its thickness and the configuration and relief of the underlying rocky substrate: Greater (or East) Antarctica, Lesser (or West) Antarctica, and the Antarctic Peninsula.
Greater Antarctica is the largest part, and can be thought of as a large dome of ice with an elliptical base and about 2,796 mi (4,500 km) long. The dome reaches a maximum altitude of about 13,123 ft (4,000 m), and the thickness of the ice exceeds 14,764 ft (4,500 m) in some places. Most of the substrate on which the ice rests, the land surface of the continental landmass itself, is above sea level, but there are some areas below sea level, up to -3,281 ft (-1,000 m) or more. West Antarctica is separated from Lesser Antarctica by the Transantarctic Mountains (whose highest peak, Mount Kirkpatrick, reaches 14,856 ft [4,528 m]), and is a fifth of the size of Greater Antarctica and more irregular in form and relief. The ice is as thick as that in Greater Antarctica, but the substrate is mostly below sea level. The Vinson Massif in the Ellsworth Mountains of the Antarctic Peninsula reaches 16,066 ft (4,897 m) and is the highest part of West Antarctica and the whole continent. The Antarctic Peninsula is a much smaller, narrow, elongated S-shaped peninsula jutting out from Lesser Antarctica.
The continental ice
Antarctica is the site of the world's largest ice-cap. The only other comparable ice mass, the Greenland icecap, covers an area of 663,000 [mi.sup.2] (1,7 million [km.sup.2]). All the world's other glaciers are far smaller, even those in the islands around Antarctica, which in general are domes of ice a few tens, or in some cases hundreds, of kilometers long. Some of these islands are joined to the mainland by ice banks.
The volume of ice in Antarctica is very large. Calculations by different authors give differing results, as the underlying relief supporting the weight of ice is not precisely known. Yet it has been estimated that the volume must be around 25,000,000 [km.sup.3]. If this volume of ice melted, it is estimated that sea level would rise throughout the world by about 217 ft (66 m). Much of Antarctica's rocky substrate is below sea level, but if the ice melted and removed this weight from the continent, it would gradually rise and this would leave East Antarctica, as well as much of West Antarctica, above sea level, except perhaps for some depressions.
There are three reasons for the presence of this huge ice mass. First, the earth's temperature range permits the presence of water in its three phases: solid, liquid, and gas. The three phases of water are found on no other planet in the solar system, and this is due to the earth's distance from the sun. Second, the position of the earth's axis of rotation in relation to the sun means the polar areas receive little incoming solar energy, thus allowing the formation of ice sheets (Antarctica and Greenland). Third, land is present at the pole (Antarctica) or near the pole (Greenland) on which the ice caps can form.
Yet there have not always been polar ice-caps (inlandsis in the southern hemisphere, or a frozen sea in the north) over the course of the earth's history. There is proof that they existed during parts of the pre-Cambrian, the Ordovician, and the Permo-Carboniferous. The climate started to cool at the end of the Cretaceous about 65 million years ago, and the first permanent ice formed in Antarctica about 26 million years ago. There have been, however, long periods of the earth's history when there have been no ice-caps. Their presence or absence seems to be conditioned by the geographical distribution of the continents, a variable that, as shown by the theory of plate tectonics, influences the circulation of the seas (see volume 1, pages 57-73). This circulation is a very effective at transporting and distributing heat, but the way in which the continents are arranged relative to each other may prevent it from reaching all the earth's surface. In any case, it is known that in the last 26 million years the Antarctic ice cap's size has fluctuated several times, most recently in the Quaternary glaciations. These fluctuations seem to be related to astronomical factors like variations in the ellipticity of the earth's orbit and slight changes in the inclination of the earth's axis of rotation.
The dynamics of the inlandsis
The inlands is is supplied with water in the form of the snow that accumulates on its surface. But snowfall is scarce and highly variable in Antarctica, ranging from 24 in (600 mm) on the coastline to a mere 1 in (30 mm) per year in the center of the continent. This very low precipitation is the result of the presence over Antarctica of a permanent high-pressure center, caused by the global atmospheric circulation. The amount of precipitation in the Antarctica Peninsula is greater.
The snow is gradually compacted under its own weight and is transformed into small grains until it eventually becomes ice. Two of the deepest soundings in the Antarctica have taken samples of ice from depths of more than 6,562 ft (2,000 m). One of them, the Vostok, produced continuous samples of ice thought to cover the last 150,000 years. Analysis of air bubbles trapped between the ice crystals in the samples has provided direct information on the composition of the atmosphere at the moment the air bubble was trapped in the ice, and the variations in its composition over the intervening period. Analysis of the ice's 18 [degrees] to 16 [degrees] oxygen isotope ratio and its deuterium to hydrogen ratio allows the environmental temperature when the snow in the sample was deposited to be deduced. Chemical analysis of the sample provides information about the composition of the atmospheric aerosols. To sum up, study of the ice allows us to learn more about the atmosphere and its changes over the last few thousand years, as well as valuable data on the way in which the climate has evolved.
The ice mass' own weight subjects it to internal deformation (basal sliding) that causes it to flow slowly downhill. This flow, as a result of deformations, is often accompanied by slippage of the ice over the rocky substrate. The typical speed of the inlandsis varies from 3.3 ft/year (1 m/year) in the central part of the inlandsis to 656 ft/year (200 m/year) near the coastline. Speeds of up to 4,593 ft/year (1,400 m/year) have been locally recorded. This movement of the ice gives rise to the formation of coastal ice shelves where the spreading ice floats over the sea. The low temperatures mean little of the surface ice melts. The ice may, however, melt at the base where it is in contact with the rocky substrate, but little is known about this and it is very difficult to evaluate quantitatively. Basal melting also occurs in the coastal ice shelves owing to contact with the seawater. In some points, however, just the opposite has been found to occur, that is, ice of marine origin grows under the iceberg. The largest losses of ice occur when icebergs break off (the phenomenon known as calving), but they are also very difficult to evaluate.
Naturally, this scarcity of data makes it very difficult to give a precise balance of the inputs to and outputs from the Antarctic inlandsis. Little is known about the inputs and less about the outputs. Even so, most authors tend to consider the result must be in balance or slightly positive. Nobody, however, doubts the importance of this balance in the regulation of the global climate, as the inlandsis, the atmosphere and the ocean are interdependent and form a dynamic whole. The climate is one of the clearest manifestations of this relationship.
1.2 The Antarctic waters and the sea ice
For oceanographers, Antarctica includes the marine area south of the Antarctic convergence, or Antarctic polar front, between 48[degrees]S and 60[degrees]S. The Antarctic convergence is where the cold dense surface waters of Antarctica meet the warm waters of the Atlantic, Pacific and Indian Oceans and sink beneath them. The center of the polar front roughly corresponds to the 37[degrees]F (3[degrees]C) isotherm. The area of the pack ice, which varies greatly from one sector of Antarctica to another and over the course of the year, determines the latitude at which the front occurs.
The water masses
The typical surface salinity of the Antarctica Ocean (between 33.8% and 34.5%) is lower than that of the surrounding oceans, but high enough for water to reach temperatures of 1[degrees]C below zero without actually freezing. The vertical temperature and salinity profiles of the Antarctic Ocean show few abrupt changes. In general, as far down as 656 ft (200 m) the water column show a minimum temperature associated with the level at which the vertical change in salinity is greatest (the halocline). Below this minimum temperature, the lower limit of what is known as the Antarctic Surface Water, the temperature increases to 32[degrees] F (0[degrees]C) and salinity to 34.5%, characteristic of what is known as the Upper Deep Circumpolar Water, at a depth of between 984 and 1,640 ft (300 and 500 m). Below a depth of 3,280 ft (1,000 m) is the Lower Deep Circumpolar Water, which has a higher salinity (34.7%) and a temperature above 32[degrees]F (0[degrees]C).
The slight differences between the hydrographic characteristics of the two water masses, found all around Antarctica, is due to the fact that they do not come from a common source. The source of the upper deep circumpolar water is the deep water of the North Atlantic, while the source of lower deep circumpolar water is the subtropical waters of the Atlantic, Pacific, and Indian Oceans. In the polar front zone, the layer between the surface Antarctic water and the upper deep polar water is called the Antarctic Intermediate Water and is the result of the subduction of cold surface waters from southern latitudes under the relatively warm surface masses of water of these three oceans.
The abyssal depths of the Antarctic Ocean are occupied by the Antarctic Bottom Water, the coldest and densest body of water in the planet's oceans. Its temperature may be below 29[degrees]F (-1.8[degrees]C) and its salinity above 34.6%. These waters are derived from restricted areas relatively close to the continent, which are called the Antarctic seas (mainly the Weddell Sea and the Ross Sea), and spread through all the ocean basins of the southern hemisphere and part of the North Atlantic. The formation and transport of the bottom Antarctic water is of great climatic importance: as it sinks and flows towards other basins, it reduces the average temperature of more than half the world's oceans by about 36[degrees]F (2[degrees]C). Furthermore, as the Antarctica water is rich in oxygen due to its not very deep origin (the cooling of the surface water of the Antarctic seas takes place at the beginning of the southern winter), this process helps to oxygenate the ocean depths.
The main dynamic feature of the Antarctic Ocean is the Antarctic Circumpolar Current, the only ocean current that truly flows round the entire planet. It flows in an eastward direction on its route round the world, covering 14,913 mi (24,000 km) in a band between 124 and 621 mi (200 and 1,000 km) wide, located between 40[degrees]S and 60[degrees]S. The current's intensity is not uniform within the band; on the contrary, it is possible to distinguish different nuclei within the strip that are separated by areas where the flow is very low, or even in the opposite direction.
The origin of the Antarctic Circumpolar Current is the wind from the west, and in the English language it is often called West-Wind Drift. More precisely, the interaction of this wind and the earth's rotation causes the surface water to move north, and this creates a north-south pressure gradient, the immediate cause of the current. This is why the Antarctic Circumpolar Current affects a considerable depth, unlike currents directly induced by the wind that are rarely detectable more than 492-656 ft (150-200 m) below the sea surface.
There are no very accurate data on the amount of water transported by the Antarctic circumpolar current because of the variability of flow over time at different scales and the lack of in situ measurements, meaning the only calculations that can be made are simply estimates. The most reliable published estimates suggest that the average flow of the Antarctic circumpolar current at Drake Passage (between the southern tip of South America and the northern tip of the Antarctic Peninsula) is between 95 and 158 Sverdrup (1 Sverdrup=106 m3/s), although more recent data from the oceanographic campaigns within the World Ocean Circulation Experiment suggest the true value may be higher.
To the south of the Antarctic Circumpolar Current is the Antarctic divergence, an area where there is upwelling of deep relatively warm, saline, nutrient-rich water that displaces the relatively cold, low-salinity surface water. The Antarctic divergence is delimited to the north by the reach of the current from the east, the East-Wind Drift, which is a discontinuous current, weaker than the Antarctic Circumpolar Current, that flows west over the Antarctic continental shelf. This flow is derived mainly from the winds from the east that form over the continent.
During the southern winter, almost 50% of the Antarctic Ocean is covered by a layer of floating pack ice, the Antarctic ice-field, but this is reduced to about 10% at the height of summer. This seasonal variation has major consequences for the functioning of the Antarctic ecosystem. Sea ice forms when surface water freezes at temperatures descending below 29[degrees]F (-1.8[degrees]C) (the temperature at which water with a salinity of around 34% freezes). First, small hexagonal crystals form that later develop, if the water is not disturbed by the winds or the waves, into needles or plaques of ice that tend to agglomerate by forming an oily-looking film known as "grease ice." As it gets thicker, the layer freezes, and the wind and waves then break it into discs that collide, forming what is known as "pancake ice."
When the ice starts to form, it consists of almost salt-free grains between which there are small channels of unfrozen brine, expelling salt into the neighboring water and increasing its density. In summer, however, when the ice melts, it forms a layer of relatively low salinity and density that creates "lenses" that do not mix with the underlying layers. Major phytoplankton blooms occur in these stable and well-lit lenses. If it is not covered by snow, pack ice is transparent enough for light to penetrate, meaning that the channels make excellent habitats for autotrophic microorganisms. Furthermore, the ice offers shelter to many types of consumers, such as krill, which also find an excellent source of food there.
Sea ice as a whole is known as pack ice. Most pack ice is less than one year old, and is normally between 2 and 5 ft (0.5 and 1.5 m). In coastal areas, however, there may be areas of pack ice formed more than one year ago that do not melt in the southern summer. This ice may be much thicker than the first year pack ice and takes longer to melt. The area of the Antarctic pack ice varies from season to season over the course of the year. The area of pack ice is greatest in September, about 5,791,509 [mi.sup.2] (15 million [km.sup.2]), and least, covering about 1,158,302 [mi.sup.2] (3 million [km.sup.2]), at the end of the southern summer (the end of February). In fact, the area within the confines of the pack ice may be greater, as much as 6,949,810 [mi.sup.2] (18 million [km.sup.2]). This difference is due to the areas that remain free of ice during the winter, called "polynyas," that vary in size and location.
The seasonal fluctuation in the area of pack ice is about the same as the total surface area of the Antarctic continent. This huge fluctuation greatly affects the world's climate and weather. The freezing of the surface of the sea changes its albedo and moderates the exchange of heat and moisture between the cold polar atmosphere and the relatively warm ocean. The formation of sea ice increases the salinity of the surface layers of water, causing vertical mixing processes. The melting of the pack ice during the summer increases the stability of the water column, which favors primary production.
Not all the ice that floats in Antarctica's waters is strictly speaking pack ice. Near the coast there are often large banks of ice, "ice shelves" that can be seen as the transition between the frozen Antarctic ice cap and the ocean environment surrounding Antarctica. An ice shelf is an area of ice from the continental landmass, of variable thickness, that floats on the sea and moves with the tide. Ice shelves are thus attached to the coastal side by an embayment where the ice starts to float, and on the seaward side by a front where smaller pieces, icebergs, break off.
The ice shelves are the result of the balance between inputs of ice from the continental landmass from precipitation as snow and from basal freezing, and the losses due to basal melting and the calving off of icebergs. The Ross Ice Shelf covers an area of 536,000 [km.sup.2] and has a volume of 23,000 [km.sup.3] (30% of all the ice in the Antarctic ice shelves), and is the world's largest ice shelf. It drains a basin of 2.86 million [km.sup.2], including the area of the bank itself, that is to say 21% of the total area of Antarctica. On the front, the shelf is between 328 and 656 ft (100 and 200 m) thick, with an average thickness of around 1,312 ft (400 m). Other important banks include the Ronne Ice Shelf and the Filchner Ice Shelf in the Weddell Sea, and the Amery Ice Shelf at the mouth of the Lambert Glacier in Greater Antarctica.
2. Life on Antarctica and in the surrounding sea
2.1 The Antarctic seas: not so well supplied as formerly thought
Research into the microorganisms of the Antarctic plankton can be said to have started in the mid-19th century with the observations by Christian Gottfried Ehrenberg (1795-1876) and Joseph Dalton Hooker (1817-1911) of the diatoms collected on the voyage of the Erebus and the Terror (1839-1843) commanded by James C. Ross (1800-1862) with Hooker as surgeon-botanist. Several expeditions between then and the early 20th century sampled the phytoplankton, but generally used nets that retained only the larger microorganisms, and so their work was mainly centered on diatoms.
Things took off with the expeditions of the Discovery (1901-1903), under Robert F. Scott (1868-1912). With data from these campaigns, between 1934 and 1942 J.S. Hart published very useful studies on the spatial and seasonal distribution of the Antarctic phytoplankton. There were other important studies in the 1930s, including those by Ernst Hentschel, and more recently, those by G.R. Hasle (1969). Since the 1950s, however, marine research in general has changed greatly owing to the development of new techniques (such as the quantitative determination of chlorophyll levels, the use of 14C to calculate primary production, and the use of remote sensors) and because of efforts to carry out interdisciplinary campaigns to study the ecosystem as a whole. In the last few years, there has been intense international collaboration in the Antarctic Ocean in programs such as Biological Investigations of Marine Antarctic Systems and Stocks (BIOMASS) or Southern Ocean Global Flux Studies (SOGFS).
The trophic webs
More recent studies have shown that the classic scheme of a more or less linear trophic chain with phytoplankton, zooplankton, herbivores (krill and other groups), and secondary consumers needs to be replaced, in both the Antarctic Ocean and in other seas, by the image of a trophic web in which the relationships between the components are not so simple. In many areas where krill is not abundant, organisms such as copepods or salps develop more complex trophic webs with the other components of the community. Although in some areas the copepods are able to consume only 1 or 2% of total daily primary production, in other areas the copepods or even the salps may exert a pressure of 50%-60%, a level of consumption comparable with that of krill. In particular, the role of the microheterotrophs, protoctist microorganisms that feed on microalgae and bacteria, may be of great importance in the food webs of the Antarctic Ocean. The substances they excrete (like those of the zooplankton) return to the primary producers some of the nutrient salts they have used. The phytoplankton also excretes dissolved organic matter that may stimulate bacterial production. This forms what is known as the microbial food web, from which little organic matter is exported to higher trophic levels.
The circumstances leading to the dominance of the classic model of trophic web structure (phytoplankton-zooplankton-higher levels) or of a microbial trophic web is now a major research issue and is far from being resolved. It can be affirmed, however, that the classic chain tends to be more important in conditions of high primary production, while the activity of the microbial trophic web is more stable, even in conditions of low production.
Sampling limitations in the first work on the Antarctic phytoplankton gave excessive importance to the microplankton, that is to say, organisms greater than 0.008 in (200 [micro]m), especially the diatoms, and tended to ignore other groups. Yet more recent research has shown the important role of the smallest components of the plankton, the nanoplankton and the picoplankton, in Antarctica's marine ecosystems.
The smallest components of the plankton
In fact, the development of techniques such as electron and fluorescence microscopy have shown that virus, bacteria, and other autotrophic or heterotrophic microorganisms of the size that is now collectively known as picoplankton (< 0.00008 in [< 2 [micro]m]) and nanoplankton (between 0.00008 and 0.0008 in [2 and 20 [micro]m]), are well represented in Antarctic waters, where their role may be as important as it is in temperate regions. Recently introduced methods, such as measuring the uptake of tritium-labeled thymidine, have made it possible to calculate the rate of bacterial production in very different environments. Even so, currently available techniques for the study of microbial processes are far from perfect, and many gaps remain in our knowledge of this subject.
It is not possible to talk of a typical Antarctic ecosystem, although there are some features they all have in common, such as the low temperature and the marked seasonal differences. In addition to the more or less concentric areas of open sea between the Polar Front and the Antarctic divergence, there is a variety of smaller- scale environments, ranging from the hypersaline interstices of the sea ice to the hydrothermal ecosystems in regions of underwater volcanic activity. In terms of variables like the number of cells, productivity, and the specific rates of growth of the bacteria, the measurements taken in the waters of Antarctica cover the entire range of variability recorded from temperate and tropical ecosystems. Some authors thus consider that Antarctic bacterial communities cannot be said to be intrinsically different.
One of the most interesting problems raised by the physiology of Antarctic bacteria (and polar bacteria in general) is their response to cold. Some authors have pointed out the presence of both bacteria adapted for peak growth at low temperatures and others that tolerate them but which grow better at temperatures between 59- 68[degrees]F (15[degrees]C and 20[degrees]C). The factors that determine the abundance of one or other are not yet known. Another issue is the relative importance of the effects of the low temperatures on autotrophic processes (less strongly affected) and heterotrophic processes and the repercussions they might have on the flow of dissolved organic matter in the marine ecosystem.
Diatoms, flagellates, and other protoctists
The most abundant genera of diatoms in the Antarctic phytoplankton include Chaetoceros, Corethron, Rhizosolenia, and Thalassiosira. Several species of Fragilariopsis are also usually found in high numbers near pack ice. The dinoflagellates, with autotrophic and heterotrophic forms, have been studied less, and the works published deal mainly with the thecate forms. The nonthecate dinoflagellates, which are sometimes very abundant, are very difficult to classify.
Among the most frequent organisms belonging to other groups are the chrysophyte Distephanus [= Dictyocha] speculum and haptophytes of the genus Phaeocystis. It has recently been shown that archaeomonads are present in the Antarctic waters, forms that have been interpreted as the cysts of as yet unidentified chrysophytes. These cysts, which until recently were only known as fossils, have been found in marine ice, although it appears that their formation takes place in the water column. Another group of chrysophytes with cells similar to these cysts are the Parmales; yet in this case they would appear to be vegetative forms. One of the best studied groups of heterotrophic flagellates is the choanoflagellates. Although many groups are little known, it seems clear that the Antarctica ocean is one of the marine areas with the highest levels of endemism. One especially interesting biogeographical problem is that of bipolar distribution (in the Arctic and in Antarctica, but not in the lower latitude areas between) of some organisms. According to some recent studies of coccolithophorids, which have some genera in both polar areas but are of different species, there might have been relatively recent exchanges (possibly during the Pleistocene glaciations) by means of a deep-water route.
Biomass and production
There are still many gaps in knowledge of the geographical and seasonal variations of the Antarctic phytoplankton, essentially due to the logistic problems of working in southern waters. One of the classic studies is by Hart, who observed that areas close to the Polar Front typically show two peaks of phytoplankton abundance, one in spring and one in autumn. At higher latitudes, the spring peak tends to occur later. This is generally valid, but it must be borne in mind that the seasonal and the interannual distribution of the Antarctic phytoplankton is highly variable.
The open waters of the Antarctic Oceans are poor in phytoplankton, despite the abundant mineral salts. The greatest concentrations of biomass are in the areas of continental platform, especially close to some islands and the coastline of the Antarctic Peninsula. Dense phytoplankton communities have also been found in the area of some fronts, such as the confluence of the Weddell Sea and the Scotia Sea and that associated with the continental shelf in the Weddell Sea. One particularly interesting case is the phytoplankton blooms at the edge of the ice. When the ice melts in the spring, this leads to the formation of a highly stable superficial layer of desalinated water that is a highly favorable habitat for phytoplankton growth. The bloom associated with the edge of the ice is transitory, but it occurs over a very large area (about 10[degrees] of latitude) as it moves towards the south, following the melting ice front.
The factors limiting primary production
The fact that many of the first studies of primary production of the Antarctic plankton were performed in neritic zones, near the coast, and the image of fertility suggested by the large populations of animals of the higher trophic levels (such as krill, seals, and marine birds) in some sites, led to the belief that the Antarctic oceans, as a whole showed high primary production. As has been pointed out, this idea (which was criticized by Hart half a century ago) is now untenable. According to the most recent calculations, the Antarctic oceans, which occupies 20% of the world's seas, produces only 100-150 ton/moles of carbon per year, less than 5% of the total production of the world's seas. In many Antarctic open sea areas, phytoplankton growth does not reduce the concentrations of the main nutrients, phosphorus, nitrogen, and silicon, to levels that are limiting. This fact, known as the Antarctic paradox, has still not been explained. The proposed mechanisms include the lack of light due to great vertical mixing, the intense consumer pressure exerted by the herbivorous zooplankton and microheterotrophs and the lack of another micronutrient, iron.
The hypothesis that primary production is limited by the shortage of iron is based on the low atmospheric supply of this element in the Antarctic high seas. It has even suggested that 300,000 t of iron be added to the Antarctic Ocean to stimulate primary production and the retention of organic carbon in the sediment. A reduced version of this large-scale experiment was performed, not in Antarctica but in a localized area of tropical upwelling where it is thought that iron might also be a limiting factor. Although the iron stimulated the growth of the phytoplankton, it rapidly disappeared from the photic zone and did not cause a great increase in biomass. The role of iron in Antarctic waters has yet to be thoroughly explained; as some studies consider that it is a limiting factor, but others consider that it is just one more control factor.
In general, the seasonal blooms of phytoplankton are explained by the relationship between the phytoplankton's rate of growth and the intensity of the mixing of the water column. If the mixed layer exceeds a certain depth (the critical depth), the amount of light received by the phytoplankton does not allow a growth rate high enough to compensate for losses (including sedimentation and consumption by predators). When spring arrives, the increasing stability of the water column and the increasing light mean the mixing layer is above the critical depth and the biomass of the phytoplankton increases. These concepts have been applied successfully in temperate regions but did not provide useful results in Antarctica until new optical and physiological data made it possible to calculate more accurately the critical depths and showed that they were nearer the surface than was generally thought. Obviously, the low water temperature must be borne in mind, as this limits the rate of cell division to below 0.5 divisions per day.
There are few reliable data available on the consumption of phytoplankton by herbivores. It has been shown that in a given locality, the krill may spectacularly reduce the phytoplankton biomass, but the distribution of the swarms of krill is very heterogeneous and it is difficult to make global calculations. The impact of other groups of predators must also be taken into account.
Given the complexity of the food webs and the heterogeneity of the Antarctic marine systems, there must be interactions between the different mechanisms regulating primary production, and in different situations they may be of greater or lesser relative importance. In this context, recent studies of the pigments of the phytoplankton in the Antarctic ocean, based on data from the CZCS satellite, suggested that the high chlorophyll concentrations observed downstream of the currents, near certain areas of the platform, were due to the input of iron from the land. Chlorophyll concentrations were low in the areas of high sea subject to strong winds, with deep mixing layers.
The surface distribution of the water and the ice masses means the Antarctic zooplankton can be divided into three large latitudinal zones. The most oceanic area, away from the influence of the winds from the west, shows low zooplankton biomass and biodiversity. At the southern edge of the Antarctic convergence there is a great increase in zooplankton density and biomass but these both decrease towards the south.
Distribution and composition
The communities of the intermediate area below the Antarctic convergence are dominated by copepods (more than 60%), although there are also abundant salps, members of the krill family, chaetognaths and amphipods. This is the most productive area of sea around Antarctica, and krill (Euphausia superba) is usually very abundant here in summer. The largest swarms of krill are found where the convergence of two masses of water gives rise to mixing. This occurs in the area of the continental shelf in Bransfield Strait and the confluence, mentioned above, of currents from the Weddell Sea and the Scotia Sea. The area further to the south is covered by ice almost all year round, and both zooplankton biomass and density are low; the species dominating the community are neritic, such as some members of the krill family and fish larvae.
The zooplankton in the area between the Scotia Sea and the Bransfield Strait is dominated by copepods and euphuasiaceans, although in some areas there are large populations of salps that are absent in the areas where waters arrive from the Weddell Sea. There is a mixture of Antarctic and sub-Antarctic species in this area. The Weddell-Scotia confluence in fact marks the southern limit of the distribution of most sub-Antarctic species. The abundant later developmental stages of copepods and euphausiaceans in this area will later move to the Weddell Sea. Where there are no dense swarms of krill, the zooplankton community is dominated by copepods (up to 95%), and among the copepods, by cyclopoids (between 40 and 80%).
Krill mainly develops where there is mixing of waters, especially near the South Georgia Islands. The reproductive adults prefer the edge of the continental shelf, whereas the juvenile forms swarm at the edge of the continental platform of the Antarctic Peninsula. The continental platforms of the South Shetland Islands have always been considered one of the largest krill spawning areas, and the larvae are then passively borne from there to the Scotia Sea. The distribution and density of the krill masses in this area show great variability in space and time. In 1981, for example, dense populations, especially of larvae, were found at the Weddell-Scotia confluence; in other years, however, hardly any krill was found in the area but was present close to Elephant Island or in the eastern sector of the Antarctic Peninsula. The greatest concentrations of krill have been found in the meanders of the gyres caused by the easterly circumpolar winds acting on the Antarctic sea currents, as occurs in the Bellingshausen Sea (to the east of the Antarctic Peninsula) and in the north of the Ross Sea.
Other species of the krill family may also become as abundant, or more abundant, as krill. There are dense swarms of species such as Euphausia crystallorophias, a neritic species, concentrations of which have been found in some bays on King George and Deception Islands. The area around the Palmer Archipelago is rich in zooplankton but not in krill, while the area's least abundant krill communities are found in the Bransfield Strait.
In the Weddell Sea, the clockwise circulation caused by the winds from the east conditions the spatial distribution of the zooplankton. Their abundance decreases from the northeast tip to the southwest, although the developmental stages of the copepods and krill are more abundant in the southern area during the summer owing to the late development of the southernmost zooplankton communities. Recent studies have shown three distinct epiplanktonic communities. The first occurs on the southern platform and the second over the northeastern platform, while the third is the oceanic community in the center of the Weddell Sea.
The coastal convergence that separates the cold waters of the platform from the deep and less cold waters marks the limit of the distribution of the oceanic species, mainly copepods with circumpolar distribution and with a remarkable abundance of gelatinous species of zooplankton, such as medusas, siphonophores, salps, and sea butterflies (pteropod mollusks). On the eastern coast, a divergence occurring at the latitude of Halley Bay separates the communities of the north and south of the platform, which are both dominated by the larval stages of more coastal species of copepods and euphausiaceans. The macrozooplankton includes dense swarms of chaetognaths and of Euphausia crystallorophias over the platform and of E. superba at the border of the platform and the slope. The current that flows over the platform transports many krill larvae and may be the cause of major fluctuations over short periods of time (two or three weeks) in the density of larvae in some areas, such as the northeastern tip of the Weddell Sea. One of the general features of the distribution of zooplankton swarms in the area is that it is highly variable, meaning that dense populations of krill may be caught at a given moment and spot, but after a few days there is not a single krill to be found nearby.
The Ross Sea remains frozen almost all year round, except in summer when an anticyclonic gyre affects the surface current. The waters nearest the coast are relatively poor in zooplankton, but the zooplankton biomass clearly increases below a depth of 656 ft (200 m). This increase is because tidal currents bear coastal water enriched in nutrients and zooplankton towards the bottom and towards the open seas. The zooplankton of the Ross Sea is dominated by copepods, although there are also many pteropods, euphausiaceans, ostracods, polychaetes, and radiolarians. The peak abundance of copepods is related to a platform-slope front. The areas of the Ross Sea not influenced by the Antarctic circumpolar current, such as the southwest tip, have a high proportion of coastal species and the larval forms of benthic species.
Seasonal variation and trophic chains
The abundance of the Antarctic zooplankton as a whole does not show major seasonal variations, except in some particular areas, such as the waters surrounding the South Georgia Islands, where in winter the zooplankton biomass is only 65% of that found in summer. Most of the biomass is below a depth of 656 ft (200 m) in summer, but is at a depth of 1,640 or 3,281 ft (500 or 1,000 m) during the winter. The months of October and December show the greatest biomass of zooplankton, and this decreases in January and then increases again in February and March.
One of the largest known copepods, Rhincalanus gigas, may have two reproductive periods a year, one in summer and the other in autumn, and remains almost inactive over the rest of the year. One of the most spectacularly abundant zooplankton organisms is Salpa thompsoni. Its rapid growth of stolons in response to the abundance of phytoplankton means that this species can develop enormous aggregates of more than 1 individual/m3. Other important species in the Antarctic zooplankton, such as the hyperiid amphipod crustacean Themisto gaudichaudii, grow much more in the summer and produce a generation at the same time every year. Most Antarctic organisms take advantage of the short periods when phytoplankton is available to feed and to accumulate fats that will be their reserve for the rest of the year.
The most distinctive feature of the Antarctic zooplankton is the high biomass that may be reached by a single species, the krill Euphausia superba. It is so abundant that its biomass exceeds that of all the fisheries in the world's seas, although its abundance shows large seasonal variations, with very good years and very bad years for fishermen. This species is vital to Antarctic food chains, because it is the largest component of the diet of whales, penguins, seals, and other marine mammals and seabirds.
Swarms of krill are highly mobile. The adult animals can move at speeds of 12 in/sec (30 cm/sec), swim against the current and move great distances. The swarms may also be very large (more than 62.1 mi [100 km long]) and very dense. One of the largest swarms known is near Elephant Island, with about 2 million t of krill distributed in an area of about 174 [mi.sup.2] (450 [km.sup.2]). These swarms are formed by individuals of different sizes and ages, in which dominance by males or females varies with the geographical location. The swarms in the eastern sector, for example, are dominated by large males (more than 2 in [50 mm]). Krill populations are usually found between a depth of 66 and 164 ft (20 m and 50 m), although they may descend to a depth of 200 m. The different swarms perform vertical migrations at night and may feed near the surface. It seems that the largest individuals reach the top first and perform the largest displacements, up to 656 ft (200 m) in a few hours. The larval and juvenile forms are concentrated in the middle part of the water column and take longer to reach the surface to feed.
Krill mainly eat phytoplankton, especially that with large cells, but it may have a very varied diet (including copepods and flagellates) when phytoplankton becomes scarce. Grazing by krill may drastically reduce the phytoplankton biomass in Antarctic waters. Their trophic pressure is particularly high in some areas at certain times, to the extent that they may consume 50-80% of total daily primary production. In coastal areas krill may even consume the detritus deposited on the sea bottom, which is very negative for the benthic fauna (see volume 10, "Whale food").
The diet of most of the common Antarctic birds, fish, and mammals consists mainly of krill. The main consumers are the animals resident in Antarctic waters all year round, such as several seals, the Weddell seal (Leptonychotes weddelli) and the Ross seal (Ommatophoca rossi); some penguins, such as the Emperor penguin (Aptenodytes forsteri) and the Adelie penguin (Pygoscelis adeliae); and other birds, such as the Antarctic petrel (Thalassoica antarctica) and the great skua (Stercorarius skua). Hundreds of kilometers from the edge of the ice, during the winter there are also other major predators, such as beaked whales (Mesoplodon), Antarctic fur seal (Arctocephalus gazella), southern elephant seal (Mirounga leonina), or several petrels and fulmars (Macronectes giganteus, Fulmarus glacialoides, Daption capense, Pagodroma nivea).
In these oceanic waters, predators capture not only krill but also eat squid, small shrimps, and lantern fish (myctophs). Lantern fish seem to play an important role as intermediate steps in the Antarctic trophic chain, as they are among the most important consumers of krill in oceanic waters and are then eaten by whales, sea lions, and seals when they rise near to the surface.
During the summer, the Antarctic waters are invaded by many predators, such as the rorquals (Balaenoptera), sperm whales (Physeter macrocephalus), killer whales (Orcinus), and large numbers of birds and seals. The rorquals and sperm whales hunt in the waters at intermediate depths, whereas all the others hunt in the surface waters, moving as far south as they can reach, as far as the continental landmass itself in the case of seals and birds. The lantern fish and squid consume a lot of krill and at the same time are consumed by a wide range of the predators that also eat krill. The highest level of the trophic chain is occupied by the killer whale and the leopard seal (Hydrurga leptonyx), which mainly eat penguins, other seals, and sea lions and fish.
Almost all krill-eating whales carry out annual migrations to Antarctica's waters in the summer; they spend the rest of the year in warmer waters, where they calve. In fact, it appears that whales almost eat only during the Antarctic summer, and during the rest of the year there is almost no food in the stomachs of caught specimens. Not all cetaceans mainly eat krill; for example, sperm whales (Physeter macrocephalus), have specialised in catching cephalopods, which they locate using their sophisticated radar. A sperm whale can dive for a long time to a depth of more than 3,280 ft (1,000 m). Unlike other whales, which have baleen to filter the zooplankton, sperm whales have jaws with strong teeth that they use to catch large squid, and their stomach contents include many species of benthic fish and crustaceans as well as sediment, showing that they feed by trawling the sea bed.
The pack ice communities
Polar pack ice, with its variety of structural type and physical and chemical conditions, makes a major contribution to the ecological diversification of the environments available to the microorganisms. The fact that the microorganisms of very different groups interact and form associated trophic webs within the ice has led authors to refer to sea-ice microbial communities (SIMCO).
The organisms forming these ice microbial communities include bacteria, choanoflagellate protozoa and other heterotrophic flagellates, ciliates, and microalgae of most of the groups represented within the plankton. Diatoms are often very abundant and are one of the best studied groups of microalgae. The typical forms from this environment are pennate (bilaterally symmetrical) forms of the genera Nitzschia, Amphiprora, and Navicula, but there are also the centric (radially symmetrical) species. The most common autotrophic nanoflagellates include the haptophyte Phaeocystis pouchettii (and probably some other species of the same genus), which shows a colonial phase with immobile cells and another phase with solitary flagellated cells. Many of the ice microalgae can also be found in large numbers in the water column. One as yet unsolved problem is how much the release of the microalgae when the ice melts contributes to phytoplankton blooms within the water column. In terms of the relative importance of primary production and heterotroph activity in sea-ice communities, published research indicates a wide variety of situations, possibly due to changes related to seasonal succession.
Little is yet known about the communities of organisms that dwell in the Antarctic benthos, and as in all other seas, these communities vary greatly depending on local conditions. They consist mainly of species with ecological characteristics that can be considered to adopt a "K" strategy (see volume 10, page 79): low fertility, slow embryonic development, larvae lacking a pelagic phase, and so on.
This dominance by "K" species is not surprising: benthic organisms in the Antarctic sea bed live in very constant conditions of temperature and salinity. Except for very precise areas, the temperature at the sea bottom varies by at most about 36[degrees]F (2[degrees]C), and salinity varies only between 34.6% and 34.9%. The input of continental sediment is low, and only occasionally do blocks or stones arrive, transported from the continent when an iceberg breaks off. This environmental stability is even greater in the deeper areas, which also show a high level of insularity with respect to the surrounding oceans, the result of the action of the circumpolar currents.
Other environmental factors do, however, vary sharply with the seasons and have repercussions on the structure and dynamics of the benthic communities. For example, the light regime restricts primary production in the water column to certain periods of the year, and these in turn determine the chance of food reaching the bottom of the sea. The periods of darkness or twilight are also influenced by the formation of surface ice, as this restricts the incident light reaching the benthic flora.
In coastal areas, the ice blocks that break away from the sediment may cause serious damage to the benthic communities as they scrape off the organisms and smooth the substrate. On the other hand, the changes in the regime of the local currents condition the distribution of organisms on the sea bottom. The intensity and direction of the currents determine the areas of greater or lesser deposition of different types of sediments and especially regulate one of the most important hydrodynamic processes affecting the organisms of the Antarctic benthos--the resuspension of sediments.
There has been much speculation about the apparent adaptation of these benthic organisms to low temperatures, but this appears to be little more than a cliche and has recently been questioned by several authors. Life in a cold environment is not unusual for marine organisms, especially those living at great depth. In any case, the species of the Antarctic benthos are clearly able to live in extremely cold temperatures, although the price they must pay is that they are highly stenothermic, that is to say, they are very sensitive to small changes in temperature.
Most materials that fall from the surface appears to be immediately consumed by the dense populations of suspensivorous organisms. Despite everything, in the periods of abundant input of materials, much of this settles on the bottom and then becomes a food source for the suspensivorous organisms, thanks to the resuspension that occurs as a result of local currents. The irregularity of the input of suspended materials in the water mass or from the surface means that the most important food source for the benthos is the organic matter deposited (often in large quantities) on the surface of the sediments.
In some benthic communities there is an apparent discrepancy between the seasonal limitation of food and the abundance of organisms, now resolved by the description of a large variety of trophic strategies that make use of the corpses or organic matter that settle on the bottom. Thus, when there are macroalgae, excavating organisms, surface grazers, sedimentivores, carnivores, omnivores, and even herbivores are not uncommon. Some isopods, for example, have been shown to be independent of the fluctuations in primary production, unlike other organisms, such as some amphipods. There is a general tendency to develop trophic strategies to make the benthos organisms independent of the fluctuations in the external food supply. As suspensivores may act as limiting factors for other trophic strategies, many organisms have become adapted to live on and colonize the colonies and body of suspensivorous organisms, and this has been called "living on the second sea bottom." Many of these colonizing organisms are mobile organisms, such as amphipods or foraminifers.
Biomass and biodiversity
In the areas of the continental platform covered by ice, the macrobenthic communities, such as those in the Weddell Sea, are poor. The mobile fauna is several hundred kilometers from the edge of the ice. Even so, in areas near the edge of the continental platform where the slope of the platform is steep, and under the surface ice layer, for example, in the Ross Sea, the communities that form have high biomass. In the coastal areas where ice blocks are continually breaking off, there are almost no benthic communities. Generally, the biomass of the benthic communities increases with depth, leading to zonation of different communities depending on the type of substrate. This zonation is not observed clearly on soft seafloors. In sublittoral and bathyal seafloors, the communities are dominated by suspensivorous organisms distributed over a broad bathymetric range; there are abundant sponges, cnidarians, molluscs, amphipods, and echinoderms. The community of suspensivores are very abundant on the eastern coast of the Weddell Sea, whereas on the opposite coast the dominant communities have lower biomass but greater trophic diversity.
The classic view of the communities of the Antarctic benthos is that they show high biomass and diversity, especially in comparison with the Arctic Ocean. But although densities of more than 120 individuals/m2 have been found at the American McMurdo Base, on the western coast of the Weddell Sea, the density of organisms in other sites is lower by one or two orders of magnitude. The biomass varies from less than 1 g/m2 to more than 1,500 g/m2 in the Weddell Sea, or, for example, between 9 g/m2 and 55 g/m2 around the Antarctic Peninsula. There are very few abundant species, and most species are present at very low densities. Locally, high densities of amphipods, Pegellidae, and bivalves have been recorded. Other groups, such as sponges, sea pens (pennutalaceans), sea cucumbers (holothuroids) and small shrimps have been found in some sites in dense groups. Production by benthic organisms in Antarctica is very low compared with that of other oceans. In the species studied, the ratio of annual production/biomass (P/B) is normally less than 0.5.
Several authors have recently questioned the common idea that the Antarctic's benthic communities show high diversity. It is generally considered that the apparent species richness of the water of the Antarctic can be shown only in some groups, such as amphipods or polychaetes, while in other groups, such as sponges, bryozoans, bivalves, gastropods, and isopods, diversity is lower, and some groups are almost absent, such as decapod and cirripede crustaceans.
From the trophic point of view, most benthic communities are dependent on the downwards flow of the unconsumed fraction of production in the surface layers. Antarctica's waters show periods of short but intense surface production, and this greatly influences benthic production. It has recently been suggested that the organisms living in the ice are a source of energy for the benthos when they fall to the bottom after the ice melts. For example, a large density of benthic diatoms derived from the ice mass is a major food resource for many benthic filterfeeders.
Growth and reproduction
As a result of the conditions of their habitat, most organisms of the Antarctic benthos show low metabolic activity and low rates of growth and reproduction. But no increases have been found in the respiration of this fauna at low temperatures. This has led different authors to propose that their low activity is not the result of metabolic adaptation but that it is mainly caused by the scarcity of food. There is very little information at the molecular level about the degree of metabolic adaptation in the polar benthic organisms. In shrimps, however, it has been shown that the energy needed to activate proteolytic enzymes is much less in Arctic species than in those from lower latitudes. In any case, the low activity of many Antarctic animals appears to be a response to limitations of food supply rather than as a response to the cold. The short periods when there is a supply of food condition the rates of growth and reproduction, and many of these organisms also become fertile at later ages.
Although the annual average growth of the different species is low, this does not mean growth is slow when enough food is available. This observation has led several authors to suggest that many species are biochemically adapted to grow faster when enough food is available. It has even been shown that when they receive some type of trophic stimulus, their rate of activity changes, alternating apparent lethargy with extremely rapid movements. In any case, the low rates of population growth observed in Antarctica's benthic communities suggest that a long time must pass for structural changes to be observed. This means they are highly persistent but that they are also very fragile in the face of environmental disturbances. Studies of succession carried out in McMurdo Sound showed that submerged plates remained almost clean five years after their installation. Colonization rates were slightly faster in some cases, but it is generally accepted that years must pass before significant changes can be observed in the configuration of benthic communities.
Benthic organisms show the same patterns in their reproduction as in their growth. Most show great seasonality and link their periods of gamete production to those of food inputs. Their low metabolic rates may mean that they may not need to store lipids to be able to grow or reproduce at times when there is no nutrient input. Embryonic development also appears to be slowed down. For example, it has been observed that isopods and shrimp larvae show very long developmental periods and take four to six years to become sexually mature adults. In other groups, such as gastropods and echinoderms, the minimum time taken to attain sexual maturity is from three to seven times greater than that needed by other species of the same group living in temperate waters. Slow development is often found in species that produce large eggs and have a low reproductive fertility.
Mainly owing to the uncertainty of completing their larval development, most benthic animals have a strategy of reducing the pelagic phase. The tendency followed by many species is to incubate the larvae and then launch them into the environment as individuals in a metamorphic stage when they can rapidly colonize a nearby environment. For example, more than 90% of the Antarctic species of groups with rapidly developing pelagic larvae, such as bivalve mollusks, do not have plankton-eating larvae. Low annual growth, growth to a large size, low mortality, and long lifespan are thus typical features of the organisms of the Antarctic benthos, and shown among the sponges, bivalve mollusks, shrimps mysidaceans, and echinoderms, which all grow almost exclusively in the summer months. Even so, the above tendency is not universal, of course, as some species are capable, for example, of growing faster than other species in the same community because they have more direct access to the food in suspension, such as some sea squirts and sponges.
The predators of the benthos
The benthic fauna has been found in the stomachs of many fish and warm-blooded animals. The diet of whales and some penguins includes shrimp, pegellids, mysidaceans, and cephalopods. In fact, benthic cephalopods are the only food for whales in deep waters. Other predators, such as the Emperor penguin (Aptenodytes forsteri), do not feed on benthic organisms. However, fishes do eat benthic crustaceans, selecting them from the infauna, which is less accessible and has a more heterogeneous distribution. In Antarctica, it appears that the most relevant factor in explaining variability at the level of the community is predation rather than the other factors controlling the abundance and distribution of benthic organisms at other latitudes, such as competition for substrate.
The data available on the flow of biogenic particles in Antarctic waters, obtained by sediment traps, are limited but suggest major differences from other maritime areas. The flow of particles is highly seasonal, as in the Arctic and temperate areas, but the flow is much smaller than that recorded in Arctic environments. This might be related to the relatively low fertility of the Antarctic ocean, but very few sites have been studied and they can hardly be considered to be representative of large areas.
Discussion of the ocean's role in controlling the excess of CO2 emitted into the atmosphere since the early industrial revolution has recently led to an enormous increase in scientific interest in the Antarctic ocean's role in the global carbon cycle. There are still, however, many unanswered questions. For example, it is still not precisely known whether the net overall balance of CO2 between the Antarctic ocean and the atmosphere is positive or negative. Intense international research activity into the flow of carbon in Antarctic ecosystems will soon contribute to a rapid improvement in our current knowledge.
2.2 The Antarctic continent; a frozen desert
Antarctica is a large continent covering 15,274,517 [mi.sup.2] (13,661,000 [km.sup.2]), representing about one tenth of all the world's dry land. Yet almost the entire surface is occupied by large glaciers, and only the nunataks (rocky outcrops rising above the ice), some coastal areas (concentrated especially in the Antarctic Peninsula and the adjacent islands), and some extremely dry inland areas are free of ice and snow during the short Antarctic summer. In fact, adding together all the areas potentially able to be colonized by plants would give a total area of about 115,831 [mi.sup.2] (300,000 [km.sup.2]), little more than the state of Michigan but sparsely scattered within an area larger than Europe. This means the plant communities are highly isolated and very fragile. In reality, the Antarctic's terrestrial and marine ecosystems, subject to very harsh climatic conditions, show little diversity of organisms and very low production. In the southern hemisphere, the latitudes where there is tundra in the northern hemisphere are mostly occupied by ocean. So it is not possible to talk of a landscape resembling that of the Arctic tundra, except in a very few sites in the Antarctic Peninsula and in some of the adjacent islands, and in some sub-Antarctic islands, such as the South Sandwich Islands; the rest of Antarctic mainland not permanently occupied by ice is more similar to the Arctic deserts. Animal life is only present on the seashore.
The almost total absence of vascular plants
Antarctica is a botanical paradox. It is very poor in vascular plants but very rich in mosses and lichens, which despite their small size may form in some places communities with a high biomass and biodiversity. There are only two species of flowering plant to the south of 60[degrees]S, the endemic grass Deschampsia antarctica and Colobanthus quitensis (Caryophyllaceae), which also grows in the Andes. Both species occur only in small sunny refugia in the western part of the Antarctic Peninsula and onsome nearby islands. This is why this small region of Antarctica, which is also very rich in mosses and lichens, is considered to be biogeographically distinct, and it is known as maritime Antarctica, to distinguish it from the rest of the continent, which has an even colder climate and is totally lacking in flowering plants, and is known in biogeography as mainland Antarctica. In maritime Antarctica at least, the extremely low number of species of flowering plants cannot be explained by isolation from other continental land masses; within moss cushions on King George Island, near the Antarctic Peninsula, there are large quantities of pollen of several species and fern spores from Tierra del Fuego. The cause for this would appear to be some sort of climatic barrier, and this hypothesis is confirmed by the consistent failure of attempts to acclimatize species from Tierra del Fuego or Patagonia in Antarctica. It is worth pointing out that major forests and crops thrive at these latitudes in the northern hemisphere, and there are large cities, such as Stockholm or Moscow, whose average summer temperatures may easily exceed 59[degrees]F (15[degrees]C). But in the warmer areas of maritime Antarctica the average temperature of the two middle months of the southern summer (January and February) is between 32[degrees]F and 37[degrees]F (0[degrees]C and 3[degrees]C), and during the rest of the year it is below freezing point.
Domination by lichens
This climatic frontier that vascular plants cannot cross has little effect on lichens and mosses. A relatively large number of moss species, nearly a hundred, live in Antarctica. Surprisingly, lichen diversity does not decrease with respect to relatively close areas such as Tierra del Fuego, but actually increases, reaching the almost incredible total of almost 400 species described by 1994. Nowhere else in the world is there such a disproportion between the vascular plants, on the one hand, and the bryophytes and fungi on the other. Furthermore, although the immense majority of bryophytes are species common to the cold areas of the planet that have probably reached Antarctica as postglacial immigrants, almost a quarter of the lichen flora consists of species endemic to Antarctica. This suggests a process of independent speciation, probably in the remote past, related to a progressive adaptation to the Antarctic environment.
The adaptations of lichens to the Antarctic environment
As is well known, the lichen symbiosis consists essentially of a fungus, the mycobiont, and an alga, the photosymbiont. In 80% of lichen species the photosymbiont is a member of one of several genera of chlorophytes, while in the remaining 20% the photosymbiont is a cyanobacterium belonging to one of several genera. In maritime Antarctica lichens with cyanobacteria are very common (for example, Leptogium puberulum and Placopsis contortuplicata), especially in areas supplied by meltwater runoff or with small streams. Their role in the Antarctic terrestrial ecosystem may be relevant, given the capacity of cyanobacteria to fix molecular nitrogen and the almost total lack of any other organisms able to fix nitrogen.
It is well known that the lichens are the most resistant of all living organisms to extreme conditions. It has been shown that there are species from cold areas that can be exposed to temperatures close to absolute zero (-459[degrees]F [-273[degrees]C]) for short periods without suffering any apparent metabolic damage. Dehydrated and at low temperatures, lichens can survive several years of dormancy, but need only a few hours in favorable conditions to resume activity. Despite everything, the great resistance of the lichen symbiosis alone cannot explain the success of lichens in colonizing the Antarctic areas not covered in ice. Antarctic lichens are also adapted to live and thrive under the harsh climatic conditions of the southern continent. Some species show net photosynthesis at temperatures as low as -0.4[degrees]F (-18[degrees]C), while peak assimilation occurs at temperatures of 32-41[degrees]F (0[degrees]C to 5[degrees]C), the average temperature range of the summer in maritime Antarctica. These lichen may obtain water from snow, rain, mist, or even the water vapor of the atmosphere. They are able to make use of very low light conditions, such as the light that reaches them through a 8 in (20 cm) layer of snow in many sites of Antarctica in the winter months. Lichens also show a noteworthy ability to adapt to microclimates that allow them to obtain the greatest advantage from the changes, often drastic, that rocky biotopes often introduce into the general climatic factors.
The Antarctic environments colonized by lichens
Lichens, as a result of the physiological adaptations, prosper not only in maritime Antarctica, where they are often the dominant feature of the landscape, but also in the drier continental landscapes called 'dry valleys' near the Ross Sea, one of the world's most inhospitable regions, where the air temperature in the summer months never exceeds 18[degrees]F (-8[degrees]C) and the relative humidity is normally between 15 and 47%. No rain is thought to have fallen in this region for thousands of years, and the winds are so strong and the environment so dry that when snow occasionally falls, most of it sublimates before it reaches the soil. Even so, lichens have managed to take the only chance available in this desolate region. The rocks that are exposed to the sun for the longest time (north-facing surfaces) may reach temperatures of up to 45[degrees]F (7[degrees]C) on clear windless days. If the rock is sandstone, moisture from the snow enters through its porous texture and is present for a while in the surface layers. In a layer located between 0.4 and 0.8 in (1 and 2 cm) from the surface of the rock, there is a curious endolithic symbiosis of unicellular algae, ascomycete fungi, and bacteria. The light that enters this biotic zone is very weak, but enough for these tiny oases to show measurable photosynthesis.
In the South Shetland Islands in the maritime Antarctic, climatic conditions are much more favorable, and even optimal, for the lichen symbiosis. Here, lichens do not need to hide within solid rock, although endolithic forms are present here too, as both the rocky outcrops and the earth are often covered by a thick layer of lichens in which the most varied growth forms live together. The first and most conspicuous type are the small clumps that, like tiny bonsai, form "lawns" of several colors dominated by the yellowish-green thalluses of beard lichens Usnea and the blackish thalluses of Himantormia lugubris; this growth form is known as fruticose. The umbilicate growth form is also common throughout the Antarctic, and includes all the species of Umbilicaria. Foliose types are also common, such as Parmelia, Physcia, Cetraria, and Xanthoria. In areas where there is least growth of larger lichens, the crustose growth form can prosper. Crustose lichens have a thallus that is firmly attached to the substrate, is often brightly colored, and includes most of the species described so far; the genera Buellia, Lecidea, Rinodina, and Caloplaca show the greatest species diversity.
The lichens of the South Shetland Islands have been studied by numerous specialists. The main monographs published on Antarctic lichens also deal with them and pay them special attention. Yet the distribution and taxonomic data available on many species is far from complete. One hundred fifty species have already been described from Livingston Island, one of the islands of the South Shetland Archipelago. The best represented genera are Buellia, Caloplaca, Lecidea, Cladonia, Umbilicaria, and Usnea. Apart from confirming the area's enormous lichenic diversity, the results showed the presence of species not previously recorded in this area of Antarctica, such as Fuscidea asbolodes, Buellia latemarginata, and Carbonea assentiens, and even more interestingly that of other species never previously recorded in Antarctica (Sporastia testudinea, Umbilicaria africana, U. krascheninnikovii, U. nylanderiana).
The distribution of species such as Umbilicaria africana and Coelocaulon epiphorellum suggests that they must have belonged to the mycota of the ancient continent of Gondwana, making them living fossils with an age of about 70 million years; Antarctica formed the core of this continent, and its floristic exploration should provide further important new biogeographical data. Antarctica's lichen and moss diversity is greatest in coastal areas, where the cover may exceed 100%, with lichens and mosses struggling for all the available space. Towards the interior there is a decrease in species diversity and a reduction of cover; isolated lichen thalluses grow on the naked rock, and there are almost no mosses.
Succession, growth, and regression
The early stages of ecological succession are common in ice-free areas of Antarctica. The harsh climate of some areas, as well as the instability of the substrate and the frequent advances and retreats of the glaciers and snow cover, often explains the limits of succession. Yet, it has recently been shown that there have been more permanent retreats by the ice cover in the whole of maritime Antarctica, especially in the South Shetland Islands. This led the lichens and mosses to colonize new substrates in a highly selective process, to which only some species have shown themselves to be well adapted (the pioneer species).
In recent glacial moraines (about than 40 years old) formed by relatively large blocks of rock, the pioneer community that forms consists mainly of lichens, mostly crustose forms (Caloplaca sublobulata, Acarospora macrocyclos, Buellia latermarginata, Aspicilia glaciaris, and Rhizocarpon geographicum), together with some small thalluses of the fruticose biotope of Usnea antarctica. If the moraine or the area around the glacier contains soil, mosses are more frequent, especially Drepanocladus exanulatus, and among the lichens the abundant thalluses of Placopsis contortuplicata contribute effectively to immobilizing small stones. In some areas of moraine where it has been established exactly when the glaciers retreated, it has been possible to estimate the rate of growth of the lichen thalluses, and major differences have been found between different species. Caloplaca sublobulata is the fastest-growing species with an annual growth rate of 0.04 in (1 mm) in diameter, whereas Rhizocarpon geographicum is one of the slowest-growing, growing a mere 0.013 in (0.34 mm) per year.
These slow rates of growth mean that many of the lichen thalluses forming this Antarctic tundra are hundreds of years old. They appear to be ancient lilliputian forests that have grown in the harsh Antarctica environment for centuries, and they are extremely fragile to damage by the boots of visitors, which on this scale can be compared with magical "seven-league boots." Perhaps this is why the populations of lichens and mosses in the areas surrounding many of the human research facilities in the Antarctic are in such bad condition. It is essential, especially for the scientists working in the area, to try and to reduce this impact to the minimum. Despite Antarctica's great size, the same few ice-free spaces must be shared by humans, lichens, penguins, and other plant and animal species.
Amphibious birds and mammals
On the coast, the main animal life is the large populations of seals and penguins. They spend much time on dry land, but they exploit the marine environment and are considered as marine predators. Although there are about 50 species of sphenisciforms (penguins), procellariiforms (albatrosses, petrels, etc.) and charadriiforms (gulls, terns, skuas, etc.) the seven species of penguin account for 90% of the bird biomass of Antarctica. In fact, strictly speaking, only the emperor penguin (Aptenodytes forsteri), the Adelie penguin (Pygoscelis adeliae), and the chinstrap penguin (P. antarctica) truly nest on Antarctica. The Ellsworth penguin (P. papua ellsworthii) and the macaroni penguin (Eudyptes chrysolophus) have more sub-Antarctic distributions and nest in some points of the Antarctic Peninsula. The king penguin (Aptenodytes patagonicus) and the rock-hopper (Eudyptes chrysocome) are exclusively sub-Antarctic. The most numerous penguins (and also the most conspicuous because of their orange crests) are the macaroni penguin (Eudyptes chrysolophus), with a total population of more than 20 million, a quarter of them living on the Antarctic Peninsula and the adjacent islands, and the Adelie penguin, with a population of 4 or 5 million, most of them on the Antarctic mainland and adjacent islands. The emperor penguin is the largest of all the penguins (43-47 in [110-120 cm tall]) but is much rarer, with a total population of 275,000 to 300,000.
Most penguins eat basically krill. Only the emperor penguin and the king penguin mainly eat fish and cephalopods. They all live in large colonies. The reproductive adults form stable pairs in which the male and the female generally alternate the long incubation of the (one or, at most two) eggs, which may last from one to four months, depending on the species. The young are protected and fed for a period of 3 to 4 weeks in the case of the Adelie penguins. The emperor penguin eggs are exclusively incubated by the males, which stay on land without feeding for two months of the harsh winter while incubating the single egg laid by the female.
There are four strictly Antarctic seals, although some individuals may stray as far as the southern coasts of South America, Africa, and Australia, such as the crabeater seal (Lobodon carcinophagus), the Weddell seal (Leptonychotes weddelli), the Ross seal (Ommatophoca rossi), and the leopard seal (Hydrurga leptonyx). The southern elephant seal (Mirounga leonina) and the Antarctic fur seal (Arctocephalus gazella) are sub-Antarctic in distribution. The four truly Antarctic species rarely go on dry land and usually live in the water and on the banks of Antarctic pack ice. They feed and mate in the water, calve on the ice, and at least in the case of the males of the Weddell seal, they mark and defend their territory under the pack ice.
The Antarctic crabeater seal (Lobodon carcinophagus) is the most abundant species of pinniped on the planet and probably the most numerous of the large mammals. (Low estimates suggest 15 millions, while others suggest more than 40 million.) It lives on the outermost pack ice and in the nearby waters. It is a little more than 7 ft (2 m) long and moves with great agility underwater and on ice and snow. It has a distinctive specialized dentition that is adapted to its diet, 90% of which is krill.
The Weddell seal (Leptonychotes weddelli) is even bigger, 8-10 ft (2.5-3 m) long, slightly more in the females. It is probably the southernmost of all mammals. It occupies the ice close to the coast, where it makes holes that it keeps open so it can reach the water. It mainly feeds on fish, preferring Dissostichus mawsoni, a species of Antarctic cod icefish (family Nototheniidae) that may weigh as much as 66.1 lb (30 kg).
The Ross seal (Ommatophoca rossi), however, is more or less the same size as the common seal (although the females are slightly bigger) and usually lives on the pack ice far from dry land and the open sea. The Ross seal was thus the last species of seal to be discovered (1840) and is still little known. It is known that its diet is mainly cephalopods (64%), though it also eats fish, krill, and other marine invertebrates.
The leopard seal (Hydrurga leptonyx) is the only seal that attacks warm-blooded animals. It is as big as the Weddell seal or bigger, and its area of distribution is much larger than those of other Antarctic seals, reaching the sub- Antarctic islands. It catches many seals, especially the common seal, and penguins, generally inexperienced young individuals, as adult seals and penguins are usually very agile swimmers and escape. It also eats krill (in fact, almost as much by weight as warm-blooded vertebrates) and small quantities of fish and cephalopods.
The vertebrate faunas include some species of birds and mammals that live mainly on the resources of the coastal waters and do not go far from the coastline, but most of the much more numerous invertebrates prefer the habitats provided by the nunataks of Antarctica's interior. One, the acarid mite Nanorchestes antarcticus, has been found in a nunatak at a latitude of more than 85[degrees]S, making it the animal known to live closest to the pole.
Most of Antarctica's terrestrial invertebrates resist the very low temperatures owing to the presence of glycerine in their bodily fluids, which prevents the formation of ice crystals and freezing even at temperatures of -40 or - 58[degrees]F (- 40[degrees]C or -50[degrees]C), although their high basal metabolism helps them to deal with the intense cold of the environments where they live. These are generally in the interstitial spaces of the soil, in cushions of moss and of fruticose lichens. There are also some that live in freshwater, and there are of course also several parasites of vertebrates that live in a much more comfortable environment. One unusual adaptation shown by many Antarctic invertebrates is their ability to survive long periods without oxygen, a common situation in the interstitial spaces where they live, because when the water freezes they are trapped in a very small space. Many organisms with rapid metabolisms may quickly use up all the oxygen of a space like this, but studies performed on some species, such as the acarid mite Alaskozetes antarcticus, have shown that 80% of the population can survive period of 28 days at a temperature of 32[degrees]F (0[degrees]C) in the absence of oxygen.
Like the vertebrates, which may form very large populations on the shores of the Antarctic ocean, the few existing species of Antarctic invertebrates occur in large populations. For example, there are only about 70 species of nematodes (apart from parasites), but all of them show very high population numbers, in the order of millions of individuals/m2, very similar to the densities they reach in temperate regions in similar habitats. Tardigrades, rotifers, acarid mites, and insects (mainly springtails and flies) complete the range of groups found, together with some heterotrophic protoctists traditionally considered as protozoans.
3. The permanent human presence
3.1 The precarious nature of the Antarctic installations
The wonderful Terra Australis inhabited by fantastic creatures was a fiasco for our arrogant species. After imagining for centuries that there was a large southern continent, the desolate landscapes and harsh conditions forced a change in established ideas. There is no stable human population in Antarctica, and there seems never to have been one. The absence of dry land ought to have been enough to rule out the idea of the Terra Australis, but for a long time this imaginary geography prevented people from accepting the observations that were made, with all their implications. Science has revealed the true riches of the frozen continent, precisely because Antarctic exploration has been closely linked to scientific and commercial interests.
Human life on Antarctica has always been very difficult. It is a very isolated region, separated from the rest of the world by the stormy Antarctic oceans and subject to a climate that makes most activities impossible. The cold is so intense and blizzards so frequent that it is necessary to shelter in isolated, resistant dwellings, which creates a sensation of confinement and monotony in an environment lacking any privacy. In addition, the long periods of dark and light alter the notion of day and night. These problems have to be dealt with by present-day expeditions, but it should be remembered that the first explorers had poor technical facilities, rudimentary transport, and limited medical knowledge.
There are three types of human settlements in Antarctica: permanent bases, seasonal camps, and shelters. There are now many permanent stations in Antarctica where small, highly specialized and trained detachments carry out a wide range of tasks. Their reduced presence shows the tenacity of the human species and their small role in Antarctica. The bases are constantly threatened by fire or collapse under the weight of accumulated ice. The bases located on ice shelves may end up as part of a drifting iceberg. A less obvious but serious danger is the risk of psychological disorders. Several research programs into the effects of confinement on personality have been carried out in Antarctic bases. One noteworthy conclusion is that the presence of women reduces tension in traditionally all-male groups.
But with all these problems, who wants to live in Antarctica? There are no more whalers in need of a factory to process their catch, nor shipwrecks that isolate people for long periods of time. The reason for the bases lies in the consequences the race to the pole had on international politics. The intense rivalry of different states reflected and reinforced their conflicts of interest, and these interests can only be defended if the state has a colony in Antarctica, however small it is. The Antarctic Treaty acted as a brake on disputes, but it recognizes that there is no solution to the territorial question, which can be interpreted in a variety of ways. This is why the bases proliferate. There are now more than 40 permanent bases, together with another 20 or so that operate only during the summer months.
Settlement for strategic reasons
Scientific activities are the best-known, but they have generally sought to explore the continent's economic potential. Military reasons, especially during the years of the cold war, prevailed over all other considerations. This led to a race between the United States and the Soviet Union for control of the Antarctic. In 1939, U.S. President Franklin Delano Roosevelt clearly stated the need to colonize Antarctica permanently. Immediately after the Second World War, in Operation High Jump, 5,000 men played war games in the polar regions. There are now eight permanent American bases, strategically located throughout Antarctica, including at the South Pole itself. McMurdo Base was established in 1956 and often houses more than 1,000 people, making it the largest human settlement in Antarctica.
The Soviet Union also built a total of 12 bases, seven of them permanent, that surround the entire continent. Poland joined the Soviet undertaking, providing finance and ceding them a base that was already closed. Another Polish base is now in operation, mainly for the assessment of exploitable resources.
German claims to most of Atlantic Antarctica were ignored after the Second World War. There are now four permanent German bases. Japan also had imperialist ambitions in Antarctica but is now the only country that has formally stated that it has no territorial ambitions, in accordance with the 1951 San Francisco Peace Treaty. Japan does, however, have major economic interests in the area, and these were the main aims of the studies carried out in the three bases that Japan maintains in the area claimed by Norway.
Argentina has a policy of colonizing Antarctica, based on ambitious projects with major environmental impacts. To show their determination, the entire military junta of General Lanusse went there in summer 1973 and declared one base a provisional seat of government; there have been births in this base and its population is increasing. Not wishing to lag behind, the military junta led by Augusto Pinochet built landing strips, shops, banks, and hotels and received high-level official visits. In Brazil, things have not been so different: In support of a theory according to which a portion of Antarctica equivalent to the longitude covered by its southern coast corresponds to Brazil, it maintains a small base in South Shetland.
This attitude by the South American countries is not without its reasons and is partly the result of the territorial designs of the European powers. Great Britain presented the first territorial claim in 1908, but owing to an unfortunate calculating error the claim included southern Argentina and Chile. In the 1950s, this tension led to exchanges of fire, but without any serious consequences. Over the last 50 years, there have been a total of 21 British bases, not all of them in operation at the same time. After the Falklands War (1982), the British government's decision to maintain its claims to Antarctica was strengthened with even more ambitious plans. The budget, however, is conceded on condition that emphasis is placed on research into possible economic exploitation of natural resources.
When Australia became independent, it inherited many of the British claims and defends them with an ambitious program clearly intended to maintain its right to sovereignty. New Zealand was in a similar situation and now has a permanent base and two summer bases in the area of the Ross Sea. South Africa has one permanent station and two summer bases in the Norwegian zone, whose sovereignty it tacitly recognizes. It also has two bases in the sub-Antarctic Gough and Marion Islands.
Belgium is an anomalous signatory to the Antarctic Treaty, as it has no base or current program, although several expeditions and a variety of activities were performed under the Belgian flag in the 1950s and 1960s. Despite the many expeditions by Norwegian whalers, and although the first man to reach Antarctica and the first to reach the South Pole were Norwegians, the situation of Norway is also strange, as it has no permanent base in Antarctica, although it has carried out major expeditions with relative frequency. France has shown intermittent interest in Antarctica, but in the 1980s it started a grandiose extension of its single but large base in Adelie Land.
The nonaligned countries, led by India, rejected the exclusive spirit of the Antarctic Treaty for decades. In 1981, India established two bases and a prospection program and received criticism from the countries that had shared its opposition to national claims on Antarctica. Following India's lead, in 1983 China sent an expedition of 591 people to construct a base that caused serious damage to the environment and to several scientific programs that were being carried out. After a preliminary diplomatic offensive, Uruguay entered the Antarctic club in 1985 by establishing its own base.
Italy and Spain have recently arrived in Antarctica, justifying their presence with sporadic scientific journeys. The Italian base in the New Zealand claim has been in operation since 1986, while the Spanish base is located in the now densely occupied South Shetland Archipelago.
Greenpeace is the only nongovernmental organization that has maintained a base in Antarctica. For five years, the World Park Station gave environmentalists a voice and voting rights under the Antarctic Treaty. This base was dismantled rigorously and in an exemplary fashion in 1992, after it had achieved the aim of a moratorium on extractive activities in Antarctica.
3.2 A world natural park?
Antarctica and the surrounding Antarctic Ocean contain very unusual, and fragile, ecosystems. The hostile conditions have not prevented the proliferation of many species of microorganisms, plants, and animals, all with their own adaptations. Life is concentrated on the coastline, precisely in the ice-free areas where human beings have needed to establish their bases. The fact that the polar wildlife is extremely sensitive to human disturbances has led to the idea of declaring Antarctica as a world natural park. It is worth preserving Antarctica intact for its intrinsic scientific and practical value.
The coastline was for a long time the scene of the activities of whalers and seal trappers. The indiscriminate slaughter had a great impact on the species exploited and on the region's trophic webs. When they first arrived for the time, the hunters already had an unenviable reputation. The intensity of exploitation is clearly shown by the fact that Steller's sea cow (Hydrodamalis stelleri [= H. gigas]), an enormous but peaceful inhabitant of the remote Bering Sea, was extinct by 1767, within 30 years of its discovery. The hunters behaved no differently at the other end of the planet. The hunting did not stop before whaling had left several larger species of cetacean, on the brink of extinction. The interests of the states followed those of the hunters.
The International Geophysical Year (or year and a half, to be precise) in 1957-1958 was an important step towards settling these conflicts. A total of 67 states participated and formed the International Scientific Committee for Antarctic Research, a non-governmental organization that advises on activities in Antarctica but has no power to enforce its directives. To avoid conflicts and after long negotiations, the Antarctic Treaty was signed in 1959. This treaty admits that the problem of the overlapping territorial claims is insoluble, thus obliging the signatory states to reach directives by consensus. Before the Antarctic Treaty came into force, some general guidelines were drawn up to protect nature at latitudes above 60[degrees]S. In 1975, a list of especially protected sites was drawn up. But all this added up to little more than mere declarations of good intentions: If there is a conflict between installing a base and protecting the environment, the environment usually loses. One clear example is the destruction of the lakes region of the Fildes Peninsula, where the Soviet Union and Chile installed bases. In 1968 the protected area was reduced to single lake, and the area ceased to be protected in 1975 as a result of the area's degradation. The spread of bases has worried the defence ministries of several countries, but the consequences of all this activity on Antarctica's fragile ecosystems concerned almost nobody. The parties to the Antarctic Treaty signed a document in 1964 to prevent "the uncontrolled destruction [of] or human interference" with native mammals and birds. The construction of bases and landing strips in highly sensitive sites shows how little attention has been paid to this restriction. The landing strip built for the French base required the leveling and joining together of several islands by the use of explosives and heavy machinery in the middle of the penguin colonies. Ironically, the base's original purpose was to study the area's biological riches. Another picturesque and absurd matter is the introduction of pigeons to the Chinese base. In the 1960s the McMurdo Base (U.S.) received a nuclear reactor, but this was a bad investment, as it had eventually to be returned to the United States, together with 11,000 m3 of contaminated soil and rock. In 1975, consensus was reached on a Code of Conduct for wastes, and this was considerably revised in 1989. It obliges the signatories to clean up or eliminate their toxic or radioactive waste, but most bases are in fact surrounded by dangerous materials stored in the open air and large quantities of litter and rubbish. Wastes are frequently incinerated, often without any control; yet incineration in sophisticated furnaces only transfers the problem of local water and soil contamination to a larger area in the form of atmospheric pollution. Sewage is not treated. In 1987, after several recommendations, consensus was reached that an environmental impact study should be carried out before any base is constructed, but in fact the precise site is often chosen only when the expedition to build it has already set foot in Antarctica.
Tourism is an phenomenon that also has a severe impact on Antarctica. This flourishing business has had catastrophic effects at some of the most important tourist sites. The increasingly numerous occasional visitors disturb nesting birds, trample the slow-growing carpet of lichens and mosses that retains the limited soil, remove natural objects as souvenirs, and leave garbage. The staff of the different bases have also caused serious damage through inappropriate recreational activities.
The need to reconsider
It is obvious that state governments as a whole have not adequately maintained Antarctica's integrity. This is why the idea of direct international management has been gaining strength. There was a proposal to place the entire continent under the jurisdiction of the United Nations, but it affected too many interests and was unsuccessful. Since 1982, other countries lacking Antarctic outposts have repeated the proposal, forcing a re-examination of the question.
The initiative to declare Antarctica a world natural park began with the Second World Congress on Natural Parks and Protected Areas in 1972. A request was made to the signatories to the Antarctic Treaty, but after three years only New Zealand, with the support of Chile, was in favor of the idea.
The environmental organization Greenpeace continues actively promoting this initiative. In 1991 there was a meeting in Madrid of the 26 member states of the Antarctic Treaty. After very long sessions it was agreed to prolong the current protective measures for 50 years and to establish a 50-year moratorium on the exploitation of mineral resources. After that, the continent's status will be revised by majority decision. This represented a partial victory for the environmental movement but is a definite step towards the world park.
The proposal for a world park does not foresee the creation of more bureaucratic structures, but rather the remodeling of the existing structures and the rationalization of current policies. Without affecting the delicate question of sovereignty, all harmful activities would be prohibited, including military and nuclear activities as well as the dumping of toxic rubbish and wastes and the destruction of native mammals and birds. Other activities would be allowed, but under strict control: tourism, commercial fishing, the construction of new support buildings, and even the operation of the bases. The Antarctica World Park should have an internationally acceptable legal structure to fulfill its aim of protecting wildlife and ecosystems. Scientific cooperation would be for useful purposes in long-term projects, performed in all the bases by people from all over the world. Antarctica would be an area of peace, where the whole of humanity would collaborate in scientific progress and respect for nature.
83 Antarctica is dominated by ice that has gradually accumulated over thousands of years and there is now about 25 million [km.sup.3], which represents 90% of the ice on earth. The layer of ice is so thick (with an average thickness of 7,874 ft [2,400 m]) that only the highest mountains and ranges rise above it. This huge accumulation of ice provides a good record of past events. When the snowflakes fall they take up small particles of atmospheric dust that are trapped in the ice, together with microscopic bubbles of air. Scientists drill through a piece of ice, counting and measuring the different layers, and then take samples at different depths to analyze their composition, allowing them to deduce the prevailing rainfall regime and temperature when the different layers of ice were deposited. They can also detect the presence of pollutants and other particulates in the air, such as clays, salts, and volcanic dust.
[Photo: D. Parer & E. Parer-Cook / Auscape International]
84 The area of the Antarctic ice shelf in winter and in summer. In very cold winters, the sea freezes over far above the Antarctic Circle, and the ice shelf is largest in the months of August and September. The ice starts melting when the summer arrives, and the area of frozen sea is least in January.
[Drawing: Editronica, from several sources]
85 Isometric projections of the Antarctic continent produced with data from satellites and aerial radar. The image on the left shows Antarctica with its ice cover, while the right-hand image shows the underlying bedrock, which has in fact sunk due to the weight of the ice. The red line A-B is a cross section from the Ronne Entrance to Colvocoresses Bay (the line crosses the Antarctic Mountains but not at the highest point). Surprisingly the ice is thickest at distances of only a few hundred kilometers from the coast. Note the great thickness (about 3,281 ft [1,000 m]) of the permanent sea ice in the Ross Bay.
[Drawing: Editronica, from J. May, 1989]
86 Icebergs that break off turbulent and fractured glaciers are irregular in shape and have alternating layers of ice (light) and rocky sediments (dark) that the glacier has scraped off during its movements. When this iceberg in Adelie Land fell off the continental ice mass, it tilted, and these alternating layers are now vertical. The horizontal line where the ice has been worn away is a former flotation line, indicating that it has recently risen a little in the water, probably because of a sudden breakage. A new flotation line is forming below. As the submerged ice melts, the iceberg gradually rises.
[Photo: Colin Monteath / Auscape International]
87 Convergences and water masses in the Antarctic Ocean. From a biogeographical point of view, it is considered that the northern limit of the Antarctic marine ecosystem is the Antarctic Convergence, or Polar Front, where the cold, dense surface Antarctic waters meet the deep circumpolar waters, which are warmer, and sink beneath them. The region between the Subtropical Convergence and the Antarctic Convergence is generally known as the sub-Antarctic region.
[Drawing: Jordi Corbera, based on Squire, 1987]
88 "Grease ice" and "pancake ice," are the first two stages in the formation of a layer of ice in oceanic waters. When the sea begins to freeze, the crystals rise to the surface forming a very thick, soft, pastelike layer known as grease ice. If the wind and waves do not disturb it, this mass will freeze into a solid layer a few centimeters thick. But when the sea moves a little the ice rapidly fragments into many smaller plates (lower photo), forming what is known as pancake ice. If the air is cold enough, the plates will join together (upper photo) and the sea will completely freeze over.
[Photos: Rich Kirchner / NHPA and Benoit Tollu / Jacana]
89 Ice shelves enclose more than 90% of the Antarctic coast and are formed by the accumulation of ice and snow over the millennia. The Ross Ice Shelf (upper photo) is a continuation of the continental inlandsis of the same name; it is almost as big as France and it is very thick (between 656 and 820 ft [200 m and 250 m]). In summer, the icebergs that break off the ice shelves drift away from the coast and gradually melt. These icebergs are usually tabular (lower photo), with a flat surface and straight edges, and in general they are much larger than those in the Arctic. They may rise 65 or 98 ft [20 or 30 m] above the level of water, and the most southerly ones may cover areas of more than 39 [mi.sup.2] [100 [km.sup.2]].
[Photos: Tui De Roy / Auscape International and Amans Medialp / Explorer]
90 Distephanus [=Dicty-ocha] speculum) is a very abundant silicoflagellate in Antarctic waters. Like all silicoflagellates (unicellular algae that are members of Chrysophyta), it has a siliceous internal skeleton, which in this species shows hexagonal symmetry inside which there are several chloroplasts. Sometimes D. speculum appears in large numbers and contributes greatly to the phytoplankton biomass and to primary production in the Antarctic seas.
[Photo: Marta Estrada and Antoni Fauquet (x 1900)]
91 The main primary producers in the Antarctic marine ecosystem are the autotrophic microorganisms of the plankton that live in suspension in the water column. For simplicity and also for historical reasons, they are known by the generic name of phytoplankton (plant plankton), even though the organisms present include some prokaryotes (bacteria and prochlorophytes) now classified as Monera and some eukaryotes ("microalgae") now classified as Protoctista (see also volume 1, page 75 and volume 10, page 56). Diatoms are present in the phytoplankton of the Antarctic waters. Most of them are cosmopolitan species, but some have exclusively Antarctic distributions. Diatoms have an external skeleton of silicon dioxide (silica, SiO2) called a frustule, formed of two pieces (theca, plural thecae) that fit together and have small pores on their surface that allow the exchange of substances between the cytoplasm and the exterior. The thecae may be radially symmetrical (centric diatoms) or bilaterally symmetrical (pennate diatoms). Antarctica's waters contain centric forms, such as Actinocyclus (upper photo) and Coscinodiscus (middle photo), and pennate ones, such as Nitzschia kerguelensis (bottom photo), which is normally found in colonies of individuals joined at the flat face of the frustule, as shown by the three specimens in the photo.
[Photos: Marta Estrada and Antoni Fauquet (x4100 above, x2000 middle, x1980 under)]
92 Krill is a major component of food chains in Antarctic seas. This name is usually applied to the crustacean Euphausia superba (see also volume 10, pages 98-99). Krill feeds on the phytoplankton in the food column or associated with the sea ice and is the basic diet of some whales, seals, and penguins. E. superba has a long lifespan, normally two years, but individuals may easily live for eight years. It grows more rapidly in Antarctic summer than in the winter, and the period of time between molts is 15 days in the summer and up to 50 days in the winter. The rate of growth depends on the availability of food and the temperature. The adult female may produce up to three generations of eggs, which it lays near the surface. The eggs fall to the bottom, where they hatch, and when the larvae develop into the second larval stage they start to rise. During the Antarctic winter, before they are a year old, the larvae reach the phorcilia stage (the third larval stage, immediately preceding the juvenile stage). The phorcilia and juveniles concentrate near the edge of the ice and feed until they turn into juveniles and subadults, respectively, in the spring. In summer, the swarms move away from the edge of the ice, but at the end of the summer, they return to spend the winter.
[Photo: Jean-Paul Ferrero / Auscape International]
93 The amphipod crustacean (Themisto gaudichaudii) is abundant in Antarctic waters. Many different species occur in the Antarctic zooplankton, varying greatly in their size, population density, distribution, and behavior, although crustaceans, including krill, account for most of the biomass. In an ocean where zooplankton is very heterogeneously distributed, patterns of coexistence between species appear that ensure their survival when they all live together. This is the case of T. gaudichaudii, which lives in association with colonies of gelatinous zooplankton, such as salps, which they often also eat.
[Photo: Josep M. Gili]
94 The top of the food chain in the Antarctic seas is occupied by secondary consumers, such as the killer whale (Orcinus orca, lower photo), the largest of all predators of warm-blooded prey. Its favorite prey are marine mammals (cetaceans and pinnipeds), although it also eats fish, cephalopods, and other animals. The photo shows a killer whale near a group of emperor penguins (Aptenodytes forsteri), another of its prey. But not all secondary consumers are mammals; the south polar skua or jaeger (Stercorarius skua maccormicki, upper photo) can also act as one. The two individuals in the photo are eating an emperor penguin chick. The Antarctic skua normally eats krill and sea fish, but also eats the corpses of any seals or whales it finds on the beaches, and it often takes eggs and chicks from penguin colonies.
[Photos: Michael Whitehead / Auscape International and Cristophe Guinet / Bios / Still Pictures]
95 Microbial communities within the sea ice cause the differences in color that it may show. Different types of communities occupy the habitats within the ice (the surface, the middle and the bottom). The surface communities appear at the interface between the ice and snow, and are apparently related to the layer of ice that forms when seawater floods the top of an iceberg. The middle communities, the cause of the colored layers seen in this core sample of ice, are formed by the concentration of plankton organisms when the water freezes or when a bottom community is included within the ice when another frozen layer is added underneath. Bottom communities form in the bottom layer of ice when this receives sufficient light and are the best studied ones. They have been shown to play an important role in sustaining the krill and the other large consumers that eat them.
[Photo: Marta Estrada]
96 The benthic communities of the Antarctic seas vary greatly between the littoral and the sublittoral zones. The much poorer littoral zone contains very few animals that live attached to the bottom, as they would be easily swept away by the ice. Just a few meters lower, the sublittoral zone shows much greater biological diversity, as the ice does not cause so much damage, and there are several groups of organisms that live attached to the substrate. These include many sponges, with more than 300 species recorded in Antarctic waters. The photo shows Rossella racovitzae, a hexactinellid glass sponge with an exclusively circumpolar distribution, which was taken in Marguerite Bay (Bellings-hausen Sea), at a depth of 476 ft (145 m). It has been colonized by several crinoids (Pomachocrinus kerguelensis) and a starfish (asteroid).
[Photo: Julian Gutt and Andreas Starmans / Alfred-Wegener-Institut, Bremerhaven]
97 In trophic terms, most of the animals of the benthos communities in Antarctica are suspensivores: They mainly eat particles of organic matter that settles or becomes re-suspended in their surroundings. Many other organisms take advantage of their presence to settle nearby, and so some of Antarctica's benthic communities are especially rich, in sharp contrast with the low diversity of the terrestrial fauna. The photo features one of these communities, showing an actinarian sea anemone (Utricinopsis), the long stolons of the gorgonacean sea whip Primnoella, some small isopod crustaceans (Antarcticus fucatus), and a pair of echinoderms (a starfish and a sea cucumber).
[Photo: Philip Sayers / Planet Earth Pictures]
98 Crustaceans are very abundant in Antarctica's waters, for example the genera Glyptonotus, Ceratoserolis, Serolis and Antarcturus. This specimen of Ceratoserolis, was caught in the Weddell Sea at a depth of 1,312 ft (400 m).
[Photo: Armin Svoboda / Planet Earth Pictures]
99 The fishes of Antarctica's waters use several physiological tricks to avoid the freezing of their body fluids. Some channichthyid crocodile fish, such as the specimen caught by these Brazilian scientists in Admiralty Bay, on King George Island, have reduced their blood's hemoglobin content and the number of red blood cells, making their blood a pale yellowish color. Other species have specific macromolecules, called glycoproteins, that prevent the nucleation, or formation, of ice crystals, by lowering the freezing point.
[Photo: Colin Monteath / Auscape International]
100 The most remarkable characteristic of the Antarctic mainland is its lack of "land." This is not because there is no land, but because it is almost all covered by ice. On King George Island, when the snow melts, a very uniform vegetation appears that consists almost entirely of lichens and mosses, showing the low diversity of the Antarctic terrestrial habitats. In other sites, however, when the snow melts, it uncovers nothing more than bare rock.
[Photo: Jean-Paul Ferrero / Auscape International]
101 The compact cushions of Colobanthus quitensis, (Caryophyllaceae), a small Antarctic caryoph alternate with mosses and Antarctica's other flowering plant, the endemic grass Deschampsia antarctica. The photo shows the maximum development of flowering plants in the most favorable areas.
[Photo: Leopoldo G. Sancho]
102 Crustose lichens are tightly attached to the surface of the substrate, mainly on coastal cliffs and rocks but also in dry areas in the interior of Antarctica. They do not look very showy, but these organisms allow the existence of a microscopic fauna by providing shelter and food for many small animals.
[Photo: Jean-Paul Ferrero / Auscape International]
103 The vegetation of Antarctica's terrestrial habitats consists basically of lichens, the only organisms that are generally abundant and diverse in Antarctica. It is not uncommon, however, to find moss-covered areas, which are occasionally visited by penguins, such as this bay in Shingle Cove, on the South Orkney Islands.
[Photo: Tui De Roy / Auscape International]
104 Seals are the most characteristic marine mammals of the Antarctic fauna. They are represented by six species: the crabeater seal (Lobodon carcinophagus), the Weddell seal (Leptonychotes weddelli), the Ross seal (Ommatophoca rossi), the leopard seal (Hydrurga leptonyx), the Antarctic fur seal (Arctocephalus gazella), and the southern elephant seal (Mirounga leonina). The most common is the crabeater seal, which can easily be watched in small groups resting on the ice shelves. The seal that reaches furthest south is the Weddell seal, which spends most of the winter under the water; to breathe, it makes a hole in the ice with its teeth, making use of the cracks. The Ross seal is also relatively easy to observe; it emits characteristic sounds to defend its underwater territory and for long-distance communication. The final Antarctic seal is the leopard seal, the only pinniped that hunts warm-blooded prey, although its varied diet also includes krill, fish, and cephalopods; one of its favorite preys is the young of the crabeater seal. The other two species, the Antarctic fur seal and the southern elephant seal, have a sub-Antarctic distribution. The Antarctic fur seal can move with great agility on land, on a smooth surface it can reach a speed of 12 m/h (20 km/h), and can surely go faster when swimming. The southern elephant seal is the largest of all pinnipeds; the adult males have an inflatable proboscis above the mouth that acts as a resonance chamber when they wish to drive away other males. Fights between males often leave conspicuous scarring on the neck.
[Drawing: Anna M. Ferrer, from several sources]
105 Without a doubt, penguins are the most typical Antarctic birds. They soon became popular with the first explorers because they are so unusual, and also because they were a tasty and easily caught fresh food. In Lusitania Bay on Macquarie Island, the king penguin (Aptenodytes patagonicus) forms dense breeding colonies. In November, one egg is laid per pair, and it hatches in mid January. The parents care for the chick, and after five or six weeks, when it is covered by thick brown down (unlike the gray down of the emperor penguin, shown in the photo on the preceding page) and has developed a large reserve of fat that makes it look chubby, the chick enters a sort of nursery regime. In early April, the chicks are as large as their parents and start to change their brown down to the black and white adult plumage.
[Photo: Graham Robertson / Auscape International]
106 A Russian icebreaker boat unloading material on an iceberg for a helicopter to collect and take to a base. There have long been shelters and bases on Antarctica and supplying them has not been easy. Whenever possible, supplying is by sea using icebreakers, but at some times of year in some areas, the frozen sea prevents the passage of even these ships, and then airplanes must be used.
[Photo: Tony Howard / ANT / NHPA]
107 All the bases that have been established in Antarctica have to deal with the same problems related to the low temperatures, the wind, the snow and the isolation, and each base resolves the problems in its own way. The teams and facilities in some bases are better equipped than in others, but the most effective way of surviving in such a hostile environment is for all of them to cooperate.
[Photo: Jonathan Scott / Planet Earth Pictures]
108 The countries that are signatories to the Antarctic Treaty and the International Scientific Commission for Antarctic Research (SCAR), and the year in which they joined. The first bases for the Treaty were elaborated on December 1, 1959, but did not come into effect until June 23, 1961 (the date of the ratification, approval and acceptance of the Treaty). The management committee consists of the 12 original members (OM). The consultative members (CM) are the 12 original nations plus the 14 that attained this status after actively participating in the exploration of Antarctica; the remainder are the members who have adhered to the Treaty (AM). As of 1994, 44 countries had adhered to the Treaty. The associate members (ASC) of the International Scientific Commission for Antarctic Research number 15, and the full members (FM) number 25. This commission was formed on February 3, 1958, and constitutes part of the International Council of Scientific Unions (ICSU). Within SCAR there are full member countries and associate countries that participate in other ways.
[Source: Scott Polar Research Institute]
109 The Antarctic claims of different nations and the sites of the Antarctic bases. In 1995 there were 69 bases belonging to 18 nations. Of these 69 bases, 42 function all year round and 27 are only occupied during the short Antarctic summer.
[Drawing: Editronica, from several sources]
110 Viewing this spectacular crater on the island of South Georgia is one of the attractions for tourist cruises around the southern islands. Tourism began in Antarctica more than 30 years ago, and the use of larger and larger cruise ships and airplanes has allowed unmanageably large groups of tourists to land on the continent, with a resulting impact on flora and fauna.
[Photo: Hans Reinhard / Bruce Coleman Limited]