III.

Environmental Factors

Life in the oceans is not uniformly abundant. Because of the low ratio of surface water to deep water and the lack of seasonal nutrient enrichment, much of the open ocean is a watery desert, especially the tropical seas. The most productive areas are the coastal regions, areas of upwellings (see below), and the Arctic and Antarctic oceans.

The intense cold that makes the Antarctic landmass so forbidding also influences the great productivity of the Antarctic waters. Cold water, made denser by the freezing of ice (which removes fresh water from salt), sinks to the bottom and moves northward from the continental shelf, as does the surface water. The northward-moving water is replaced by a deep mass of water flowing southward between the surface and bottom layers. That water, rich in nutrients, rises to the surface in an upwelling that stimulates a heavy growth of phytoplankton in the form of diatoms and dinoflagellates. The phytoplankton is consumed by zooplankton and other grazers, including the enormous populations of shrimplike krill, which in turn serve as food for many fish species and whales.

The rich Antarctic waters are pulled away from the shore and become part of the Antarctic Circumpolar Current, also known as West Wind Drift. This current, the strongest ocean current on earth, is partially diverted by the southern tip of South America, forming the Humboldt Current off the coast of Peru. As the surface water is pulled out by winds, the nutrient-rich deep water replaces it, aided by the absence of a continental shelf. This is an important upwelling region, and it supports an enormous amount of life. Copepods and opossum shrimp replace the krill of the Antarctic. Feeding on these crustaceans are immense schools of small fish that, in turn, are consumed by seabirds, replacing whales at the top of the food web.