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Adaption to the Cold
The incredibly harsh environment of the Antarctic continent precludes most life forms. Conditions include air temperatures that average well below freezing all year round, strong winds that increase the effects of the cold, light which varies from months of total darkness to total sunlight, little free water and all but two per cent of the continent covered with ice. Given these conditions, it is surprising anything lives on the continent. Only tiny and primitive plants and animals can cope with these extremes.
The surrounding ocean though, is teeming with life. Conditions for living things are far better than on land - the water, although very cold, remains a fairly constant temperature, there is no wind and plenty of nutrients. However, special adaptations are required to cope with the very cold water temperatures and seasonal variation in light.
Ocean life forms are varied. They range from simple single-celled plants through to mammals. Several groups (seals and birds) take advantage of both the ocean and land.
Plant and animal adaptations to the cold can involve several strategies. Warm-blooded animals have to maintain their core temperature by insulation, changes to their body functions or by behavioural adaptations. Plants and cold-blooded animals are adapted to prevent or reduce the effects of freezing, by either physiological or behavioural adaptations.
The Southern Ocean Top
One of the reasons the seas around Antarctica support so many living things, despite the extreme cold, is the abundance of nutrients in the water.
There is constant movement of water around Antarctica under the influence of the prevailing winds. The Antarctic waters also tend to drift north and on combining with the warmer waters of the Atlantic, Pacific and Indian Oceans, the cold Antarctic water sinks, causing an up welling of nutrients and minerals from the bottom.
The ocean ecosystems are relatively simple and contain few species yet vast numbers of individuals are supported. The nutrients in the cold water are directly used by the phytoplankton. These single-celled floating plants use the sun's energy, water and the carbon dioxide from the water through photosynthesis to form glucose and oxygen. This energy is then able support the next member of the food chain.
Although phytoplankton is microscopic, concentrations of them are visible on the water surface. In summer during the twenty-four hour sunlight and reduced sea ice cover, there is greater light penetration and photosynthesis can be maximised. This is the main source of primary production in Antarctica.
Krill are key animals in the Antarctic marine ecosystem. They are crustaceans like shrimp, living off phytoplankton.
To survive the winter when low light conditions limit photosynthesis and the production of phytoplankton, it is thought that krill either become cannibalistic or shrink and use up their own body's reserves.
Krill is a major item in the diet of crabeater and leopard
seals, Adelie penguins, some fish species, squid and baleen
It is the most abundant animal in the world and plays a major role in the marine ecosystem between the primary production of phytoplankton in top layers of the sea and the vertebrate predators (fish, birds and mammals). Krill is easy prey, as it is quite large (adults are about 7 cm) and is found in swarms kilometres long.
One of the major food sources of the baleen whales, krill increased markedly when whale numbers were reduced during the fifty years of greatest exploitation. Other krill-eating species (seals and penguins) quickly exploited the abundance and numbers have increased rapidly. It may therefore be difficult for the whales to re-establish themselves to pre-whaling levels if other species have increased their populations in response to the greater availability of krill.
The temperature of the Southern Ocean has a narrow range; from about -2°C to 0°C. Sea ice freezes at about -1.8°C which is about 1° lower than the freezing point of many species of fish. Of the 20 000 species of fish world-wide, there are only about 200 species in Antarctica.
Fish of the Antarctic waters show the most remarkable adaptations to these cold temperatures. Many species accumulate sodium, potassium, chloride ions or urea which lower the freezing point of their bodies. Others have glycoproteins which, like anti-freeze, inhibit the growth of ice crystals.
Another fascinating adaptation is the lack of haemoglobin in the Antarctic ice fish. Haemoglobin is the red coloured pigment in the blood which carries the oxygen around the bodies of every other vertebrate species. This is a useful adaptation as oxygen is highly soluble in the cold sea water. This also makes their blood thinner, allowing their metabolism to be slower, which conserves energy.
Ice fish also have extremely efficient enzyme systems which allow them to remain active at low temperatures - their activity at 0° C is similar to that of a temperate water fish at 20°C.
Antarctic fish have been a focus of New Zealand research for many years. The Nototheniidae has speciated widely in the Antarctic region - nearly 75 per cent of the benthic (bottom dwelling) fish and 60 per cent of the coastal species belong to this sub-order of bony fish. They range from small herring-like species to the huge Mawson's cod.
Mawson's cod have a lightweight cartilaginous skeleton, no swim bladder and fatty deposits which allow them to live in middle level waters. They also have retinas that are well adapted to the low light levels. Snow or ice on the surface of the sea, even in summer, reduces light levels to that of the great depths in the open oceans. Other species have large eyes and most species have a well developed sensory system on the surface of the head to help them locate food to compensate for the poor clarity under the water.
The mobility of birds allows them to exploit the benefits of both a marine and land existence. Some escape the rigours of winter by migrating north and most of the birds seen in the Antarctic breed on the warmer sub-Antarctic islands.
The other major group of birds are the penguins, who feed at
sea but breed on the mainland.
Three of the four penguin species breeding in Antarctica avoid the worst of the Antarctic winter by remaining at sea. The emperor penguins however begin their breeding cycle in autumn. The males spend the entire winter huddled in groups while incubating the egg which is kept balanced on top of their feet. This social adaptation reduces the effect of the chilling wind and reduces surface area exposed to the cold.
Seals and Whales Top
These mammals have many of the same adaptations as penguins which reduce the effect of the cold. They spend the winter in the sea, only coming up for air. Their large round bodies have a small surface area to volume ratio, minimising heat loss. Seals have a thick layer of blubber for insulation and small extremities. In summer they come onto shore and ice floes to bask in the sun and to prey on penguins.
All the whales found in Antarctic waters migrate north to escape the harsh winter when krill is unavailable.
Land Plants and Animals Top
As only two percent of Antarctica is ice-free, the niches for plants and animals on the continent is very limited. The extreme cold and wind reduces this further. Two higher plants, a grass (Antarctic grass - Deschampsia antarctica) and a pearlwort, are found in the Antarctic Peninsula area where conditions are more favourable than elsewhere on the continent. They tolerate extreme cold and dry conditions, and can still photosynthesize at freezing point.
The other land plants and animals are all primitive and tiny. Algae are simple plants and are the main type of plant on land in Antarctica. Their tiny cellular structure is able to withstand extended periods of freezing and in summer may be thawed and frozen several times a day.
Algae and simple micro-organisms (cyanobacteria, fungi and yeast) can be found in well protected places (soil, under rocks, in birds' nests and colonies and among mats of moss or algae at the bottom of lakes).
Some algae are found as coloured patches on areas of permanent snow. Others tolerate high levels of nutrients and are found near penguin colonies. Some live inside rock crevices in the Dry Valley region. Others are found under stones, where the microclimate is more favourable than in the surroundings.
Lichens have been found only
400 km from the South Pole. They recover very slowly from freezing
after winter with photosynthesis and respiration levels not
reaching high levels until late spring. Lichens are able to
function with less light and water than other plants and have
a high concentration of pigments and acids.
Last year a New Zealand research team measured a lichen photosynthesizing at -20°C, the lowest ever recorded.
Only a few tiny invertebrate animals have been able to exploit terrestrial Antarctic conditions. Most of these are moss and soil dwelling arthropods - including mites, springtails, midges, metazoa (such as rotifers) tardigrades (minute water dwelling invertebrates), nematodes (worms) and protozoa. Like the lichens, they have compounds like anti-freeze in their cells. Some animals survive through a period of winter dormancy. They are able to cope because the humidity of the soil is maintained by a layer of permafrost.
Most animal species are found on plants on the Antarctic Peninsula and the nearby islands where the warming influence of the sea allows extensive moss and lichen beds and the two species of flowering plants to grow. The largest Antarctic land animal to survive year round is a midge, which grows to a maximum of 12 mm. Lack of free water imposes the greatest restraint on the occurrence and activity of animal life.
Some animals have short life cycles, taking advantage of the
short summer to multiply rapidly. Others spend the winter dormant
in various stages of development, hatching and growing in the
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