As with terrestrial ecosystems, there are regions of the ocean that teem with fish and their predators while others are marine deserts, harboring little diversity of life. The latter occur in areas of tropical oceans, where there is little mixing of the marine layers and where the warm surface waters retain less oxygen; of course, the deep waters of these marine deserts remain relatively unexplored and, as we have discovered along mid-oceanic ridges (spreading zones), they may harbor unimagined forms of life.
By contrast, colder oceans in the Temperate and Polar regions are relatively rich in oxygen and, where ocean currents encounter island ridges or continental shelves, a diverse web of marine life is supported. Upwelling of deep ocean water brings a steady supply of nutrients to the surface, nourishing krill, the primary consumers of the sea. Fish and baleen whales feed on the krill and they, in turn, are consumed by marine mammals, larger fish, sharks, killer whales and a wide variety of sea birds.
When polar waters freeze over during the winter months, this food chain shuts down and most of the secondary and tertiary consumers migrate to warmer areas. While baleen whales fast during this period, the other migrants head for coastal areas, island chains and reefs where upwelling currents sustain schools of prey. All of this explains why rich, marine fisheries develop where cold ocean currents sweep along the continental shelves; unfortunately, many of these areas are threatened by over-fishing and pollution, potentially resulting in man-induced ocean deserts.