The Arctic Ocean is the smallest and shallowest of the world's five major oceans. It spans an area of approximately 14,060,000 km2 and is known as the coldest of all the oceans. It is also seen as the northernmost part of the all-encompassing World Ocean.
Due to the pronounced seasonality of 2–6 months of midnight sun and polar night in the Arctic Ocean, the primary production of photosynthesizing organisms such as ice algae and phytoplankton is limited to the spring and summer months, Important consumers of primary producers in the central Arctic Ocean and the adjacent shelf seas include zooplankton, especially copepods and euphausiids, as well as ice-associated fauna, These primary consumers form an important link between the primary producers and higher trophic levels. The composition of higher trophic levels in the Arctic Ocean varies with region and with the sea-ice cover. Secondary consumers in the Barents Sea, an Atlantic-influenced Arctic shelf sea, are mainly sub-Arctic species including herring, young cod, and capelin. In ice-covered regions of the central Arctic Ocean, polar cod is a central predator of primary consumers. The apex predators in the Arctic Ocean—marine mammals such as seals, whales, and polar bears—prey upon fish.
Endangered marine species in the Arctic Ocean include walruses and whales. The area has a fragile ecosystem, and it is especially exposed to climate change, because it warms faster than the rest of the world. Lion's mane jellyfish are abundant in the waters of the Arctic, and the banded gunnel is the only species of gunnel that lives in the ocean.