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Clean Coastal Waters: Understanding and Reducing the Effects of Nutrient Pollution
ents (nutrient over-enrichment) can lead to excessive, and sometimes toxic, production of algal biomass (including harmful red and brown tides), loss of important habitat such as seagrass beds and corals, changes in marine biodiversity and distribution of species (with impacts on commercial fisheries), and depletion of dissolved oxygen (hypoxia and anoxia) and associated die-offs of marine life. Each of these impacts carries associated costs. A single harmful algal bloom, taking place in a sensitive area during the right season, might cost the region millions of dollars in lost tourism or lost seafood revenues.
Nutrient over-enrichment is a significant problem for the coastal regions of the United States. Because rivers transport the vast majority of nutrients reaching coastal waters, the concentration of land-borne nutrients tends to be high near the mouths of rivers. These areas of mixed fresh and marine water, referred to as estuaries, tend to be relatively slow moving and biologically rich water bodies that are particularly susceptible to the effects of nutrient over-enrichment. Of 139 coastal sites (138 estuaries plus a portion of the Gulf of Mexico) examined in the only comprehensive examination of the extent of eutrophic coastal conditions conducted to date (Bricker et al. 1999), 44 were identified as experiencing conditions symptomatic of high overall levels of nutrient over-enrichment (e.g., showing symptoms such as low dissolved oxygen, nuisance and toxic algal blooms, loss of submerged aquatic vegetation). Problem areas occur on all coasts, including those of California, Florida, Louisiana, Maryland, Massachusetts, New York, North Carolina, Texas, and Washington, but problems are particularly severe along the mid-Atlantic coast and the Gulf of Mexico. Unless actions are taken to reduce inputs, conditions are predicted to worsen over the next 20 years at many of these sites.
Estuaries and coastal zones are among the most productive ecosystems on earth. There is strong concern that the natural resources they represent are in danger from eutrophication and other problems caused by excess input of nutrients. The major nutrients that cause eutrophication and other adverse impacts are nitrogen and phosphorus. In coastal
nutrient over-enrichment) such as nitrogen and phosphorus. The term eutrophication is sometimes loosely used to describe any result attributable to anthropogenic nutrient loading to a system, but eutrophication per se is not necessarily caused by human action. It is, however, one of the processes that can be triggered by nutrient over-enrichment. The distinction in this report between nutrient over-enrichment and eutrophication is an important one, since nutrient over-enrichment can lead to a number of problems other than just eutrophication of coastal waters (such as coral reef decline), and the excessive primary production associated with eutrophication often leads to a secondary set of problems (such as hypoxia). Confusing cause and effect can impede mitigation, as remediation efforts may not bring about desired effects if those efforts are improperly targeted.