The marine environment
What is the marine ecosystem like near Antarctica? Why are the surrounding seas so much more productive than the Antarctic terrestrial ecosystems? How have organisms adapted to living in the cold Southern Ocean?
The phytoplankton underpin the food web by fixing the energy from the Sun into chemical energy. Although much of their biomass is consumed by pelagic organisms shown in the diagram, some sinks down to the bottom of the ocean providing sustenance for benthic (bottom dwelling) organisms and the rest simply accumulates on the sea floor. As this happens, the carbon that was fixed through photosynthesis becomes part of the marine sediment. This is an important part of the global carbon cycle, sometimes referred to as the ‘biological pump’ which transfers carbon from the atmosphere into long-term storage in rock. In this way, the Southern Ocean is an important ‘sink’ for atmospheric carbon dioxide.
The main consumers of phytoplankton in the Southern Ocean are zooplankton (consumer micro-organisms such as protozoans) and crustaceans such as copepods and krill.
Antarctic krill is a shrimp-like crustacean, measuring about 6cm long, that dominates the primary consumer level in the Southern Ocean ecosystem. Watch krill swim and feed on the “Krill cam“.
Although numbers of krill fluctuate, there are likely to be over 500 million tonnes of krill in the Southern Ocean. They begin their life feeding on algae beneath sea ice (in ‘krill nurseries’) and during the summer they leave the shelter of the ice and form swarms with millions of individuals (sometimes containing over 10 000 individuals per cubic metre of sea water). Krill plays a central role in the ecosystem as a key source of food for many other animals:
- Fish – E.g. the Antarctic cod.
- Squid – Very abundant in the Southern Ocean. There are many species of squid and they can range in size from 15cm to over 15m in length.
- Whales – Baleen whales such as the Blue, Humpback, Fin, and Minke have evolved mouths with fibrous plates instead of teeth that enable them to filter out huge quantities of krill from the ocean. During the main feeding season, a Blue Whale consumes about 3600kg of krill a day!
- Whereas the Blue Whale is only numbered in the hundreds, the Minke is by far the most abundant of the baleen whales today, with perhaps a million or so in the Southern Ocean.
- Seals – The Crabeater seal in particular relies almost exclusively on krill. The ‘crab’ in its name comes from the word ‘krebs’ in German which is a general term for crustaceans, not because the seal eats crabs! It is the most abundant species of seal in the world, and consumes more krill than all of the baleen whales put together.
- Penguins – There are 17 different species of penguin (all in the Southern Hemisphere) but only five breed along the continent and/or the Peninsula (Emperor, Adélie, Gentoo, Chinstrap, Macaroni). In the Antarctic, penguins (especially the Adélie) feed on krill as well as on fish and squid.
In turn, these krill eating animals are the prey for animals higher up the trophic ladder. For example, among other things, Killer whales hunt seals, squids, and penguins; and seabirds (such as the Albatross) hunt fish and squid.
Student activity 4
Using the text and any of the suggested web links above, create a fact file for any group of Southern Ocean organisms of your choice (e.g. penguins, seals, whales, seabirds). Include the names of the different species, their habitats, their life cycles, their food sources, and any threats that they may be facing.