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Organisms that live under sea ice around the coast of Antarctica must be able to survive long periods without light due to the presence of sea ice.
Antarctica is a land of ice. The seas around the coast of the continent are cold, as low as -1.9 degrees C, and often covered with a thick layer of ice, yet this icy marine ecosystem supports a wide range of living organisms as the cold waters are rich in nutrients.
Ice affects life on the Antarctic sea floor in two ways:
The presence of ice on the surface of the sea drastically reduces the amount of light that can pass through to the water below.
Under permanent ice shelves where the ice is several hundred metres thick there is no light at all and so very few marine organisms live there.
Below seasonal sea ice, which is much thinner (usually just a few metres), some light can pass through and so large marine communities can live on the sea floor. The area and thickness of sea ice depends on the season and location. In some locations sea ice forms and melts every year. This ice is usually around 2-3m thick. In other areas the sea ice may stay for several years at 6-7m thick.
Most of the light from the Sun that hits a layer of sea ice is reflected. Layers of snow may also build up on the sea ice further reducing the amount of light that can pass through. Less than 1% of the light above the ice makes it through and an even smaller percentage will reach the sea floor 20m below.
Organisms that live on the sea floor are known as benthic organisms. Those that live in areas that get covered by sea ice need to be able to survive for long periods of time without light. Plants such as the algae that live on the sea floor will only receive enough light to carry out photosynthesis for just a few months per year. For the rest of the time they must live on stored energy reserves. This means that they grow very slowly.
If multi-year sea ice is present then the times when some algae can photosynthesise may be only for short periods every few years.
Organisms that feed on the algae will need to be adapted to a limited food source or switch to feed on other sources.
A simplified view of life under the sea ice in the Ross Sea during spring. Image: Vonda Cummings (NIWA).
Many invertebrates survive under sea ice in low light conditions. This photo was taken in McMurdo Sound. Image: Steve Clabuesch.
A large variety of invertebrates can be found on the sea floor around the coast of Antarctica. Organisms that live on the sea floor are called benthic organisms. Image: Rod Budd (NIWA).
Less than 1% of the light that falls on sea ice will pass through to the water below. Image: LEARNZ.