Rudstam 1990; Rosenberg et al. 1990; Parker and O'Reilly 1991; Lein and Ivanov 1992; Smayda 1992). Eutrophication also may change the plankton-based food web from one based on diatoms toward one based on flagellates or other phytoplankton, which are less desirable as food to organisms at higher trophic levels (Doering et al. 1989).

Whether or not nutrient inputs should be considered excessive depends in part upon the physics and ecological sensitivity of the receiving water body. In many parts of the world, estuaries and coastal seas clearly are receiving an excess of nutrients, and the resulting eutrophication is one of the major causes of decline of coastal waters. Nutrients should be considered a major ecological concern along with sewage disposal in many coastal urban areas and be regulated accordingly.

This section first briefly discusses the negative effects of eutrophication and nuisance algal blooms in coastal marine ecosystems. It then reviews in more detail the controls on eutrophication; discusses the issue of whether nitrogen or phosphorus is more limiting to eutrophication; presents dose-response information, which relates nitrogen to algal biomass and production; and reviews the information on controls of nuisance algal blooms.

ADVERSE CONSEQUENCES OF EUTROPHICATION AND NUISANCE ALGAE

Anoxia and Hypoxia

Anoxia is the complete removal of dissolved oxygen from the water column, an event which obviously causes widespread damage to aquatic plants and animals. Even mobile animals that can escape from anoxic waters can suffer population declines from the loss of habitat area. For example, in parts of the Baltic Sea, cod eggs laid in oxic surface waters sink into anoxic bottom waters where they die (Rosenberg et al. 1990). Oxygen concentrations in the bottom waters of the deep basins of the Baltic between 1969 and 1983 correlate negatively with codfish populations (Hansson and Rudstam 1990).

Oxygen need not be completely absent for damage to occur, and a lowering of oxygen to concentrations as low as 3 to 4.3 mg per liter can cause ecological harm in some estuaries and coastal seas (EPA 1990). Such a depletion of oxygen is termed hypoxia. Examples of ecological damage from hypoxia include lowered survival of larval fish, mortality of some species of benthic invertebrates, and loss of habitat for some mobile species of fish and shellfish that require higher concentrations of oxygen, such as lobster and codfish (Baden et al. 1990, EPA 1990). Significant mortalities of lobsters and population declines of both lobster and codfish have been



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