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Life in the Southern Ocean
Factfile |
 Life in the Southern Ocean |
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The Basis of the Antarctic Marine Ecosystem
Nowhere on Earth is as cold and windy as Antarctica so the region has no native land animals. Antarctica is such a hostile environment that even a polar bear would have trouble surviving a winter there and no birds or mammals can cope with the terrible cold in the winter at the centre of the Antarctic continent.
But Antarctica supports a unique wildlife that is perfectly adapted to this harsh environment and this is made possible because of a variety of reasons:
- The ocean temperature itself, although cold, is warmer than the land. The water is extremely cold and surface temperatures range from –1.8ºC near the continent to +3.5ºC at the Polar Front or Antarctic Convergence. At the Polar Front the cold Antarctic surface water sinks below the warmer waters of the surrounding oceans (Atlantic, Indian and Pacific). The warm water brought by these oceans heat the Southern Ocean to an average of 3.5ºC at this point. The cold water south of the Polar Front contains rich supplies of oxygen and nutrients.
Humpback whale breaching
- Seawater contains oxygen vital for wildlife survival. When the sea is cold it contains more oxygen than usual. So, the ocean around Antarctica is very rich in oxygen ideal for the wildlife to thrive. Although the top of the ocean, near the land, can freeze, the salt in the water helps to stop this happening so easily further out to sea. Where the frozen ice covering does exist, the plentiful oxygen supply enables life to live below it.
- Antarctica remains relatively undisturbed. This allows for successful breeding. Particularly amongst penguins and seals, who continue to flourish there.
- Antarctic wildlife has adapted to its surroundings in many ways. One main way is the slowing down of their lifecycles. The cold means that food is limited and consequently the wildlife can produce fewer, but larger eggs than if they lived in warmer climates. They also provide better care for their eggs, ensuring the survival of the next generation.
The start of Antarctica’s ocean food chain is phytoplankton. When the ice begins to melt in spring, there is a sudden blooming of microscopic plants. These small plants drift in the ocean currents and the widespread turbulence of the ocean brings a constant supply of vital minerals to the plankton. Combined with the continuous 24 hour daylight of the summer months, the plankton takes the sunlight and converts it into energy for growth through photosynthesis. The plants that exist in the Southern Ocean are tiny and microscopic, often less than 1mm in size. However, the problem for plant life is their dependency on sunlight. Things are fine in the spring and summer months when sunlight is plentiful and they can flourish, but this is not always the case. The presence of ice on the surface of the water all but prevents the sunlight from getting through and plants cannot grow. For this reason, the plant food supply is seasonal. This has had a knock-on effect all the way through the food chain as the other animals have had to adapt to this availability.
Zooplankton under a microscope
The phytoplankton is food for many small marine animals that make up the zooplankton. These are small animals that are unable to swim effectively and so drift freely in the water. The primary example of this in the Southern Ocean is krill. Just 5 cm long, krill can occur in swarms so dense and so large that they turn the surface of the water red/pink for kilometres. There are literally billions and billions of krill in the Southern Ocean at any one time. One estimate is between 300,000,000,000 and 1,000,000,000,000,000 individual krill.
Whilst krill primarily feed on phytoplankton, they can also eat other things including other crustaceans and indeed other krill! This helps them to survive all year round. In the summer they can be found closer to land, where the plant life is plentiful, whereas in winter they are found in deeper parts of the Ocean and continue to eat any plants they can find under the ice.
Antarctic krill
The vast majority of other animals and birds in Antarctica’s ecosystem feed on krill, so their continued existence is vital. Squid, seals, whales, seabirds and penguin depend on krill as their basic food.
At one time, the greatest consumers of krill were the baleen whales. They feed by filtering out plankton from the seawater by means of the horny baleen plates hanging from their upper jaws. Seven of the 11 species of baleen whales come south to feed in the Southern Ocean for the summer and all search for zooplankton and, in particular krill.
Once baleen whales have found a source of krill, their feeding never seems to stop. Each day in summer they catch 3-4 percent of their own body weight in krill. For a 150-tonne blue whale, that means up to 6 tonnes of krill a day.
Sadly, there are now many fewer whales, though no species is extinct. Krill are also fished for human consumption. This fishing is carefully monitored so as not to exhaust the supplies and prevent the natural ecosystem from functioning as effectively.
Southern Right whale
Further information on whales and the effects of whaling in the Southern Ocean can be found in the whales and whaling factfile.
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