inputs are well known and described in Appendix A. In moderation, nutrient inputs to estuaries and coastal seas can be considered beneficial. They result in increased production of phytoplankton (the microscopic algae floating in water), which in turn can lead to increased production of fish and shellfish. However, excess nutrients can be highly damaging, leading to effects such as anoxia and hypoxia from eutrophication, nuisance algal blooms, dieback of seagrasses and corals, and reduced populations of fish and shellfish. 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). Coastal waters receive large inputs of nutrients from both point and nonpoint sources. This is particularly true for estuaries, many of which receive nutrient inputs at rates up to 10,000 times higher per unit area than heavily fertilized agricultural fields (Nixon et al. 1986). As a result of these inputs, many estuaries and coastal seas throughout the world are increasingly experiencing such problems as anoxia and nuisance algal blooms (see Appendix A).
The degree of risk posed by nutrients varies among regions and among different types of ecosystems. In general, the more enclosed the water body and the less water available for dilution, the greater the threat. However, even large areas on the continental shelf can sometimes become anoxic from excess algal production as was demonstrated in the New York Bight in 1976 (Mearns and Word 1982). The degree of density stratification and mixing in the water body are also critical in assessing its sensitivity to low-oxygen events. The degree of risk from nutrients is further affected by the dominant organisms present; for instance, coral reefs and sea grass beds are particularly sensitive to nutrient inputs.
In most estuaries and coastal seas of the temperate zone, nitrogen is the primary element of concern which controls eutrophication. This phenomenon is in sharp contrast to eutrophication in lakes where phosphorus is often the limiting element. Phosphorus is also limiting in tropical lagoons and may be limiting in some temperate estuaries during at least some times during the year. Exactly which element is more critical is a result of differences in the ratio of nitrogen to phosphorus in external inputs, to differences in rates and controls of nitrogen fixation, and to differences in recycling of elements from bottom sediments (Howarth 1988). The relative abundances of other elements can also be important in controlling eutrophication. For instance, some evidence suggests that toxic algal blooms frequently become prevalent only after the ratio of silicon to nitrogen or phosphorus becomes low, that is when silicon is in relatively short supply (Smayda 1989, Officer and Ryther 1980). Silicon is required by diatoms but not by other phytoplankton species; thus as long as silicon is available, diatoms can outcompete other species are suppress blooms of toxic algae. High