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Managing Wastewater in Coastal Urban Areas
open coastal areas. Internal recirculation in these systems can lead to the entrapment of pollutants in the sediments and increased concentrations in the water column. Tidal fluxes can also have important influences.
Depending on the circulation patterns and other physical factors, as well as the different ecosystems present, different coastal systems respond differently to wastewater and stormwater inputs. Thus, it is important that wastewater and stormwater management practices be tailored to the characteristics of the particular receiving environment. Because of the wide variations encountered in coastal systems, it is not possible to prescribe a particular technology or approach at the national level that will address all water quality problems relating to wastewater and stormwater management satisfactorily. Any such approach would necessarily fail to protect some coastal regions and place excessive or ineffective requirements on others.
NUTRIENTS IN COASTAL WATERS
Perhaps the most pressing problem in many estuarine and marine systems today is that of nutrient enrichment. While not known to be a problem along most of the open Pacific coast, excess nutrient enrichment, or eutrophication, is a persistent problem in many estuaries, bays, and semi-enclosed water bodies and may be of concern over a large scale in some more open areas along the Atlantic and Gulf coasts. (A complete discussion of coastal nutrient enrichment issues is included in Appendix A; the following is a summary of the key points.)
Nutrients are essential for primary production, the plant growth that forms the base of the food web in all coastal systems. Nitrogen, phosphorus, and a host of other nutrients sustain the production of phytoplankton. In general, in productive freshwaters of the temperate zone, phosphorus is the most important of these elements. It controls the overall rate at which primary production takes place. In nontropical coastal marine waters, however, nitrogen is the most important factor in limiting primary production. At the interface between marine and freshwater, both of these elements are important.
In moderation nutrients can be beneficial, promoting increased production of phytoplankton and, in turn, fish and shellfish. In excess amounts, however, nutrients cause overproduction of phytoplankton, which results in oxygen depletion, which then can reduce the numbers of fish, shellfish, and other living organisms in a water body. Other problems caused by excess nutrient enrichment include nuisance algal blooms, which are aesthetically displeasing and can sometimes carry toxins harmful to fish populations or to humans through consumption of seafood. Nutrient enrichment may also shift the plankton-based food web from one based on diatoms toward one based on flagellates or other phytoplankton that are less desirable as food to