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Abiotic Factors of the Arctic
The Arctic Ocean presents many challenges to marine life. It is very cold, with water temperatures that hover around freezing. Ice covers much of its surface all year round. There is continuous sunlight during the summer, and continuous darkness during the winter, which limits photosynthesis, and therefore primary production, to only half the year. Even so, life survives and even thrives.
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Ice Algae and Phytoplankton
At the base of the Arctic Ocean food web are algae and phytoplankton, which capture energy from sunlight through photosynthesis. Algae grows on the undersurface of the ice, some of them in long strands, beginning each spring as sunlight returns.
Phytoplankton are one-celled organisms that free-float in the water column; they function much like plants in terrestrial ecosystems, but are evolutionarily unrelated. They include diatoms, dinoflagellates, and protists. Some are species of algae.
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Plankton are any living thing of any size in the water column that swim too weakly to directly control where they go. Zooplankton are planktonic animals, and the most common ones in the Arctic Ocean are small arthropods - copepods, amphipods, and krill. Other types include jellyfish, larval finfish, and the larvae of many bottom-dwelling invertebrates. Zooplankton feed on phytoplankton and smaller zooplankton.
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As the ice algae ages and dies, it falls off the undersurface of the ice and sinks, along with other dead organisms and fecal pellets. This detritus from above provides food for marine animals living on the ocean bottom where sunlight doesn't reach, allowing a complex benthic ("bottom") ecosystem to flourish. Clams, mussels, sea urchins, sea stars, and crabs are some examples of invertebrates found at the bottom.
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There are around 240 species of marine fish in the Arctic Ocean, with salmons, cods, and halibut probably the most well-known. A few live near the surface right under the ice, and are famous for having antifreeze compounds in their blood. The majority live near the ocean bottom. Small fish feed on ice algae and zooplankton. Large fish eat smaller fish, jellyfish, and bottom-dwelling invertebrates.
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Seabirds mostly come to the Arctic Ocean during the spring and summer when there is food, then leave before winter. Some species fly as far south as Antarctica for its summer. 64 species breed in the Arctic, including gulls, terns, jaegers, ducks, loons, cormorants, and alcids such as puffins. (Penguins are Antarctic birds.) Seabirds filter for zooplankton, catch fish and squid in the water column, and dive to the bottom for the invertebrates living there.
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Seals, Walruses, and Whales
Several species of seals, walruses, and whales are found in the Arctic Ocean. From the top of the water column to the bottom: there are harp seals which eat the fish living just under the ice, bowhead whales which filter small zooplankton with their baleen, ringed seals which eat zooplankton and small fish, narwhals, beluga whales, ribbon seals and hooded seals which catch larger fish and squid in the water column, and spotted seals, bearded seals, walruses and gray whales which eat fish near the bottom and bottom-dwelling invertebrates.
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The polar bear spends most of its time on land (or on top of the ice) but is counted among the marine mammals, because it functions as the top predator of the Arctic Ocean food web. It mainly eats seals, but will also eat fish and seabirds when seals are unavailable. Due to global climate change, polar bears might be endangered.
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Indigenous peoples of the Arctic are not marine life, but have traditionally hunted seals and whales for food and shelter, and can also be considered a top predator of the food web. Until modern times when humans outside of the Arctic arrived to also hunt seals and whales, humans did not take more than the ecosystem could support.
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The Arctic Ocean Ecosystem
On land, plants turn sunlight into food, herbivores eat plants, and carnivores and omnivores eat herbivores often larger than themselves (and sometimes other carnivores). In open ocean ecosystems, especially in the middle parts of the food web, who eats whom is more a matter of size than species or trophic level: large living things eat smaller ones, and are in turn eaten by larger ones. This holds true for Arctic Ocean marine life as well.
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Resources and Image Credits
Arctic Ocean Diversity (also source of copepod and puffin images)
Polar Discovery at Woods Hole Oceanographic Institution (WHOI)
Oceanus at WHOI
Polar bear image by Alan Wilson, used under CC-A-SA-3.0 license