The antarctic

Antarctica is a big place. It is the Earth's fifth largest continent, accounting for 10 per cent of its land surface and covering an area of some 14 million square kilometres. It is twice the size of Australia and half as big again as the USA. Surrounding it is the Southern Ocean which covers an area twice that of the Antarctic continent and which isolates Antarctica from other land masses. As might be imagined, while there are some common features to Antarctic environments, there is also a tremendous variety. We can recognise some broad distinctions, however, relevant to life in Antarctica. Antarctic marine environments are less extreme than terrestrial environments, due to

Marine Phytoplankton Antarctica

figure 2.5 Map of the Antarctic region, showing the approximate position of the Antarctic Polar Front (solid line) and of the Subantarctic, Maritime Antarctic and Continental Antarctic regions. The dots indicate the approximate position of the major Subantarctic and Maritime Antarctic islands: (1) S. Georgia; (2 ) S. Orkney; (3) S. Sandwich; (4) Bouvet; (5) Marion; (6) Prince Edward; (7) Crozet; (8) Kerguelen; (9) Heard; (10) Macquarie.

figure 2.5 Map of the Antarctic region, showing the approximate position of the Antarctic Polar Front (solid line) and of the Subantarctic, Maritime Antarctic and Continental Antarctic regions. The dots indicate the approximate position of the major Subantarctic and Maritime Antarctic islands: (1) S. Georgia; (2 ) S. Orkney; (3) S. Sandwich; (4) Bouvet; (5) Marion; (6) Prince Edward; (7) Crozet; (8) Kerguelen; (9) Heard; (10) Macquarie.

the lack of exposure to desiccation and the buffering effects of seawater temperatures. The Antarctic Polar Front (Antarctic Convergence) is recognised as the northern boundary of the Southern Ocean (Figure 2.5). This is where the northerly flowing currents from the continent sink beneath the warmer currents which circulate around the Southern Ocean. This not only represents a change in seawater temperature but also in chemical composition and serves to isolate many Antarctic marine organisms. The marine habitat varies with latitude and depth and in its relation to sea ice.

Terrestrial Antarctic environments are much more extreme and life only occurs in sparse areas where the conditions are favourable for its survival. Three broad ecological zones are recognised in terrestrial Antarctic environments. The subantarctic consists of a scattered ring of small islands which lie within, or close to, the Antarctic Polar Front (Figure 2.5). These experience a wet, cool oceanic climate with temperatures above freezing for at least six months of the year and an annual precipitation of over 900 millimetres. The maritime Antarctic consists of the west coast of the northern part of the Antarctic Peninsula, its nearby islands and some island groups, such as the South Sandwich, South Orkney and South Shetland Islands, which lie in the Scotia Basin. This region has mean daily summer temperatures above 0°C and winter temperatures rarely below —10 to —15 °C. The remaining part of the Antarctic, and its associated islands, is considered to be part of the continental Antarctic region. I will focus on life in this region, since this is where the most extreme environmental conditions are found.

The harshness of the climate of the continental Antarctic increases as you travel towards its centre. At the coast, summer temperatures exceed 0 °C for up to one month in summer, and winter means range from — 5 °C to below —20°C. On the ice plateau, which reaches elevations over 3000 metres, mean monthly temperatures are below —15 °C all year round, falling to below — 30 °C in winter. Life is mainly associated with ice-free areas. These are in short supply with less than 1 per cent of the continent ever free of snow and ice. There are three types of bare ground. Some areas, notably the Dry Valleys of Victoria Land, are permanently free of snow and ice due to local climatic conditions. Other areas, mainly on the coast, have some snow cover in winter which melts during summer to provide liquid water. In places, exposed areas of rock project above the ice sheet (nunataks) and are associated with mountains which are isolated among the snow and ice.

Antarctic terrestrial organisms

When you say 'Antarctic animals', most people think of penguins and seals. These are, however, part of the community of marine organisms.

They breed on the continent, but return to the sea to feed and, in most cases, to escape the rigours of the Antarctic winter. The true animal residents of the continent, which live there all year round, are fully terrestrial and microscopic, with the largest (mites and springtails) being no larger than a pinhead (1-2 millimetres in length). The highest forms of terrestrial animal life are two species of midge, which are found in a few sheltered areas on the west coast of the Antarctic Peninsula and on the South Shetland Islands.

When you visit one of the ice-free areas of the continental Antarctic, the overwhelming impression is one of barren rock, gravel and scree. Yet, look carefully and, in a few favoured sites, there is life even here in one of the harshest environments on Earth. The ground may be tinged green by the growth of algae and black crusts on the soil surface indicate the presence of microorganisms. There may even be more obvious growths of lichens and small cushions of moss. In the maritime Antarctic, where conditions are less harsh, the mosses form extensive mounds and banks and even grass is found growing. The mosses of the continental Antarctic are much more sparse and grow no bigger than the size of a drinks coaster. Living in among the growths of mosses, algae and microbes are microscopic invertebrates such as nematodes, tardigrades and rotifers (Figure 2.6). Microarthropods - mites and springtails - browse on the growth in surprising numbers. These simple communities of organisms rely on local conditions to provide the opportunity for their survival and growth. Even among the largely barren soils of the Dry Valleys, there are microhabitats which provide conditions favourable for life. Although the air temperature may be freezing, a dark rock absorbs enough heat from the sun for any ice beneath it to melt and for liquid water to become available.

The vegetation of the continental Antarctic consists of mosses, lichens and algae, which develop in the moister areas. Lichens are more widespread than mosses. Crustose lichens cover the surface of rocks and pebbles like smears of paint. Foliose (leaflike) and fruticose (shrublike) lichens may have growths which project above the surface of the soil. Mosses and lichens have been recorded as close to the pole as 84 ° S

figure 2.6 Scanning electron micrograph of an Antarctic nematode (Panagrolaimus davidi). The worms are about 1 millimetre long.

and 86° S, respectively. Growths of algae are relatively abundant in coastal areas where there is a seasonal supply of water from melting snow, and in and around shallow melt streams and ponds. In drier areas, vegetation can still occur under small rocks and pebbles which are transparent enough to allow sufficient light for photosynthesis. Stands of vegetation are more extensive in the milder conditions of the maritime Antarctic. In favourable places, mosses form turfs, banks and carpets and there are even two species of flowering plants: the Antarctic Hair Grass (Deschampsia antarctica) and the Antarctic Pearlwort (Colobanthus quietensis).

Microorganisms, such as algae, fungi, yeasts, bacteria and actino-mycetes, occur in association with the plants but also in areas which do not support visible vegetation. The microbial communities are dominated by microscopic algae and cyanobacteria which are the most predominant photosynthetic organisms. These provide food for browsing springtails and mites in areas where there is no obvious vegetation. Microbes also colonise other specialised Antarctic habitats. In very dry areas, microbes can still survive by living within fissures or cracks in rocks (chasmoendolithic) or actually within the rocks, beneath its surface (cryptoendolithic), finding protection against desiccation and other hazards of their environment. These endolithic communities consist of a variety of organisms including algae and fungi (which are often closely associated and may form lichens) and cyanobacteria. These organisms lie dormant in a state of cryptobiosis for most of the time and can only grow when the occasional snow falls and its melt supplies them with sufficient water.

Some microorganisms even colonise habitats in snow and ice. Snow algae, which grow on the surface of snow and ice, are well known from more temperate areas of the world but are rare in the Antarctic, with most reports coming from the maritime Antarctic. Here, they produce patches of red-, green- or yellow-coloured ice and snow due to the presence of millions of algal cells, threads or spores. Wind-blown dust and debris can accumulate in holes on the surface of glaciers. As this material is dark, it absorbs heat from the sun and produces melting. This deepens the hole, allowing it to collect more debris and may expand to form pools several metres across. The hole also collects meltwater running over the surface of the glacier. These holes and pools are colonised by algae and cyanobacteria. Systems of melt ponds and streams also form on the surface of ice shelves and sheets during summer. These are colonised by algae and cyanobacteria and even by microscopic invertebrates such as nematodes and rotifers. Microorganisms have even been isolated from the surface of the central Antarctic ice plateau. These are mainly inactive forms which have been blown there, but it is possible some microbes could have brief periods of activity during the year. Living bacteria have even been isolated at the South Pole.

Summer meltwater from snow and glaciers forms streams on icefree land. In some areas, the streams unite and provide a sufficient flow of water to justify calling it a river. The best known of these is the Onyx River in the Wright Valley of the Dry Valleys, which flows for up to three months a year in summer. The Onyx River is fed by glacial melt-water and flows away from the sea, discharging into Lake Vanda.

Antarctic lakes, and also streams and rivers which receive regular flows, provide perhaps the most favourable habitats for life in the continental Antarctic. Lake Vanda is 75 metres at its deepest point and is permanently covered by a sheet of ice up to 4 metres thick. Despite air temperatures which do not rise above 5 °C, and which remain below freezing for most of the year, the temperature of the lake increases with depth, reaching 25 °C at the bottom. The lake acts like a giant solar heater, absorbing heat from the sun during summer and trapping it beneath its layer of ice during winter. As well as changing in temperature with depth, there are also changes in salinity. The freshwater flowing in from the Onyx River lies on top of the saline water at the bottom of the lake.

Both freshwater and saline lakes and ponds are found in Antarctica. Some saline lakes were formed from seawater trapped as the ocean receded, others from the concentration of dissolved salts by the evaporation of water. The lakes of the continental Antarctic are permanently covered with ice, although the ice may melt around the edge of the lake during summer. Lakes in the maritime Antarctic may be free of ice for several months each year. A variety of organisms inhabit Antarctic lakes both in the water column and at the bottom of the lake. Most vegetation occurs as thick mats of algae and cyanobacteria at the bottom with protozoa and microscopic invertebrates (rotifers, nematodes and tardigrades) living in these mats. Some lakes have aquatic crustaceans, such as fairy shrimps, which can feed in the water column.

Lakes of unfrozen water even occur under the central ice plateau. These were first discovered in the mid 1970s. The largest is Lake Vostok, which is roughly the size of Lake Ontario and is covered by 4 kilometres of ice. How Lake Vostok formed is unknown. It may have been sealed by the formation of the polar ice sheet or it may have formed due to the melting of ice by its movement over the bedrock. Estimates of the age of Lake Vostok vary from hundreds of thousands to millions of years and it may well have been isolated from the atmosphere for all that time. It is perhaps the most extreme, isolated and pristine environment on Earth and may contain unique forms of life.

Studies of ice cores from the ice above Lake Vostok have yielded viable microorganisms that are 200000 years old. Scientists are currently trying to develop drilling techniques which will enable them to collect samples from the lake without contaminating it.

Cold is the most obvious challenge to life in the Antarctic, but, as well as being cold, most of Antarctica is very dry. This may seem surprising given the amount of ice and snow associated with the continent. However, frozen water is not available for organisms to use, unless they can melt it, a feat which few organisms can spare the energy to achieve. It is the availability of liquid water that determines where life can exist. During the brief summer, temperatures can rise sufficiently to melt accumulations of snow and partly melt glaciers and the edges of frozen lakes and ponds. This supplies enough liquid water for life. The supply of water is, however, transient and terrestrial Antarctic organisms have to survive periods of desiccation. Conditions favourable for growth may only exist for a few weeks or even days each year. Desiccation may, in fact, protect against freezing, since in a desiccated organism there is no water to freeze. However, during the summer, liquid water is present which, when it freezes, may result in the freezing of the organism. Freezing, or the threat of freezing, and freeze/thaw cycles are thus major hazards for terrestrial Antarctic organisms. Other threats include wind, unstable substrates and high levels of solar radiation (especially ultraviolet radiation, made worse by the ozone hole over the Antarctic). Resistance adaptation predominates among terrestrial Antarctic organisms; they lie dormant during adverse conditions and become active when conditions are favourable. There are, however, some examples of capacity adaptation. Antarctic lichens, for example, can continue photosynthesis at temperatures as low as — 10°C.

Antarctica has an extremely sparse flora and fauna. Only 74 plants and other macroflora and 186 animal species have been recorded from the maritime Antarctic (by 1993) and much fewer from the continental Antarctic. Geographical isolation is thought to be the major cause of this low biodiversity. Antarctica has had no land bridge with any other continent for more than 25 million years. The nearest land mass, South America, is over 1000 kilometres away. As well as distance from other land masses, the Southern Ocean and the circulation of air around the continent act as barriers to potential immigrants. The only route for immigration (other than largely accidental human introductions) is for seeds and other propagules to be transported by air currents or by birds. Any immigrants that do arrive have to establish themselves in the face of the harsh environmental conditions.

The study of life in Antarctica is important for a number of reasons. The communities of organisms consist of only a few species, making it feasible to attempt an overall understanding of their ecology. The organisms are adapted to survive some of the most extreme environmental stresses on Earth, making them good models to study how life can exist in extreme conditions and perhaps providing us with new techniques for storing and preserving biological materials. Since they are living at the limits of life on Earth, any change in their growth patterns may be a particularly sensitive indicator of global climate change. Polar regions may also warm first and so any effects of climate change could be seen earlier here than in other regions of the Earth. Finally, some Antarctic habitats are among the closest analogues we have on Earth to other bodies in our solar system which may contain life. The Antarctic has proved an important location to develop techniques which could allow us to search for and recognise the presence of life on other planets.

Antarctic marine organisms

The marine environments of the Antarctic are less extreme than their terrestrial counterparts. Marine organisms, by and large, are not exposed to desiccation and the thermal buffering effect of the water restricts variations in temperature. The marine environment is, however, not without its hazards. Organisms have to cope with low temperatures, the presence of ice, low light levels during winter, seasonal food supply and, in some places, high salinity.

The melting point of seawater is — 1.9°C and organisms which live close to sea ice have to function at this temperature. The presence of ice is ubiquitous in many marine Antarctic habitats, both as solid bodies of ice and as crystals floating in the water. The body fluids of marine invertebrates have much the same composition as seawater. They would freeze at the same temperature as seawater and are thus not at risk of freezing, unless the sea froze solid. Most Antarctic fish, however, have body fluids which are more dilute than seawater. This, together with the low temperature and presence of ice crystals, means that they are at constant risk of freezing. How they cope with this hazard will be explored in Chapter 5. Low temperatures slow the rate of biological processes and many marine organisms are slow growing and long lived.

Seasonal variations in both temperature and light intensity are major features, particularly for marine organisms living close to the continent. The temperature affects the extent of the pack ice which in winter covers an area seven times that in summer, forming a belt up to 1900 kilometres wide completely encircling the Antarctic continent. The pack ice not only affects the temperature of the water immediately beneath it but also the penetration of light, particularly if it is covered by snow. In winter, there is permanent darkness for several weeks. Photosynthesising organisms, such as algae, thus have to survive over winter with little or no light. When ice forms, the salt remaining becomes concentrated in unfrozen pockets, exposing any organisms trapped within them to very high salinities.

The seashore of Antarctica is relatively barren since much of it is permanently covered by ice or is kept clear of life by the scouring action of floating ice crashing against it. In some sheltered areas, the shore may be colonised by ephemeral seaweeds and by mobile molluscs which can escape ice damage by migrating. Permanently attached organisms, such as mussels and barnacles, which are common on seashores in other parts of the world, are not found on Antarctic shores. The actions of ice on the land produce large quantities of sediments which are deposited into the ocean. A rich community of organisms develops on these sediments where they form stable areas of the sea-floor (benthic organisms). Conditions for benthic organisms are fairly stable, with low but steady temperatures and constant salinity. These organisms rely ultimately on phytoplankton from the waters above them as a source of food. This input of food is large but short lived, generally limited to when there is open water in summer. In contrast to the land, the ocean floor supports a rich and diverse community, especially in relatively shallow coastal waters.

The open waters of the sea are very productive, supporting a relatively simple food chain. Phytoplankton, mainly diatoms and algae, are fed on primarily by krill - a shrimp-like crustacean. Krill is a key organism in the food chain, being eaten by whales, seals, fish, squid and birds. Trapped within, or in close association with, the sea ice are diatoms, algae, protozoa and bacteria. The irregular surface under the ice can support thick growths of diatoms and other microorganisms. These live at the water/ice interface, become packed into channels within the ice itself or hang beneath it. Sea ice communities are thought to contribute substantially to the productivity of the phyto-plankton by extending the period of the year over which it is active and providing a population which is liberated into the open water when the ice melts. The marginal-ice zone, at the edge of the receding pack ice, is thought to be particularly significant in this respect.

Most marine mammals (seals) and birds (penguins, skuas and other seabirds) that breed on the Antarctic continent do so only in summer and avoid the winter by migrating north. A dramatic exception is the Emperor penguin. This is the largest of the penguins and its large size helps it cope with the cold. Its size, however, poses a problem in rearing its chicks. Penguin chicks cannot go to sea and feed themselves until they are fully grown. The summer is too short for Emperor penguin chicks to complete their development, as the chicks of small penguins do. Emperor penguins have solved this by breeding before the start of the winter. The birds spend the summer feeding at sea, building up large stores of fat. The single egg is laid in May to early June, at the start of the winter. The females return to the sea shortly after laying the egg which is transferred to the male bird to incubate. The male holds the egg on its feet and covers it with a fold of skin and feathers which pro-

figure 2.7 Male Emperor penguins incubating eggs. At over a metre tall, this is the world's largest penguin. Drawing by Jo Ogier.

tects it against the weather. And there the males stand, on the surface of the sea ice, incubating the egg during the depths of the winter and awaiting the return of the females some two months later (Figure 2.7). The chick hatches around the time the females return and are ready to go to sea by January or February when much of the sea ice has melted and food is readily available.

How do the males survive their long winter vigil? They do not feed, since they cannot abandon their egg to reach the distant open sea. They rely on their food reserves built up during the summer. Standing on the sea ice, rather than the land, probably helps since the sea beneath the ice (which is at — 1.9°C) keeps them considerably warmer than they would be on land. The penguins are superbly insulated by their feathers and by the layer of fat beneath their skin. Their large size also helps them to retain heat. The males form creches and huddle tightly together, decreasing their exposed surface by as much as five-sixths and further reducing heat loss. They generate heat internally by utilising their food reserves. Their heat retention mechanisms are so efficient, however, that they only lose 15 per cent of their body weight during the nine weeks of incubation.

Fur, feathers and fat beneath the skin also provide the insulation needed by other penguins, seabirds and seals during their summer breeding season on land and during their activities in the water. Penguins are so efficiently insulated that on a sunny day they are in serious risk of overheating. They deal with this by increasing heat loss from their bodies by fluffing up their feathers and extending their flippers. Most birds do not have feathers on their legs and feet and a bird standing on cold ground or ice is at risk of losing heat through their feet. Indeed, if their feet were as warm as the rest of their body, they would melt the ice beneath them and become trapped as it refroze. The legs and feet are allowed, therefore, to cool to a lower temperature than that of the rest of the body. This is partly achieved by a countercurrent mechanism in which cold blood in the veins returning from the legs picks up heat from the warm arterial blood. The veins and arteries in the legs are close together to give the maximum area for this heat transfer to occur and thus to conserve heat within the body.

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