Upwellings

Upwellings are highly productive areas along the edges of continents or continental shelves where waters are drawn up from the ocean depths to the surface. Rich in nutrients, these waters nourish algae, which in turn support an abundance of fish and other aquatic life. The most common locations for upwellings are the western edge of continents, such as Peru and southern California in the Pacific, northern and southwestern Africa in the Atlantic, and around Antarctica .

In order for upwellings to occur, there must be deep currents flowing close to the continental margin. There must also be prevailing winds that push the surface waters away from the coast—as the surface waters move offshore, the cold, nutrient-rich bottom waters move up to replace them. The Pacific upwelling near Peru is the result of the northward flow of the cold Humboldt current and the prevailing offshore wind pattern. Every seven to ten years, there is a shift in the prevailing wind pattern off the shore of Peru, a condition known as El Ni ño . The prevailing wind in the eastern Pacific shifts direction, causing the cold Humboldt current to be replaced by warm equatorial waters. This prevents upwelling, and the ecological productivity of the ocean in this area falls dramatically.

In oceans, the majority of nutrients sink to the sea bed, leaving the surface waters poor in nutrients. In marine ecosystems, the primary producers are microscopic planktonic algae known as phytoplankton , and these are only found in surface waters, where there is enough sunlight for photosynthesis . The lack of nutrients limits their growth and thus the productivity of the entire system, and most open oceans have the same low levels of ecological productivity as tundra or desert ecosystems. In upwellings, the water is cold and oxygen poor but it carries a tremendous amount of nitrates and phosphates , which fertilize the phytoplankton in the surface waters. These producers increase dramatically, showing high levels of net primary productivity , and they become the foundation of a biologically rich and varied oceanic food web, teeming with fish and bird life.

The high productivity of the upwelling area off Peru supports one of the richest sardine and anchovy fisheries in the world. In addition, the large population of seabirds associated with the Peruvian upwelling deposit equally large quantities of phosphate and nitrogen rich droppings called guano on rocky islands and the mainland. Until oil-based fertilizers were developed, Peruvian guano was the principle source for agricultural fertilizers.

In the Antarctic upwelling zone, the nutrient-rich waters support food webs at the base of which are massive populations of large marine shrimp, called krill . These invertebrates support a wide array of squid, whales , seals , penguins, cormorants, boobies, and sea ducks. Commercial fisheries in the Antarctic upwelling zone are now exploiting the krill harvest, an act which endangers all of the members of this unique and complex cold-water food web.

See also Commercial fishing

[Neil Cumberlidge Ph.D. ]

RESOURCES

BOOKS

Ainley, D. G., and R. J. Boekelheide. Seabirds of the Farallon Islands: Ecology, Dynamics, and Structure of an Upwelling-System Community. Stanford: Stanford University Press, 1990.

Glantz, M. H., and J. D. Thompson. Resource Management and Environmental Uncertainty: Lessons from Coastal Upwelling Fisheries. New York: Wiley, 1981.