Changes in Mississippi River Water Quality this CenturyImplications for coastal food webs

Water quality in streams, rivers, lakes, and coastal waters may change when watersheds are modified by alterations in vegetation, sediment balance, or fertilizer use after, for example, industrialization or the conversion of forests and grasslands to farms and cities (e.g., Aber et al. 1989, Borman and Likens 1979, Schlosser and Karr 1981, White et al. 1981). Quantification of these watershed/water-quality relationships is important in understanding the consequences of existing or potential watershed changes as well as the possibilities for preventive or remedial action. There are two recent examples of improved understanding of these relationships contributing to public policy discussions. Knowledge of the role of phosphorus in structuring freshwater phytoplankton communities led to the regulation of phosphorus in detergents as well as to improved sewage-treatment strategies (e.g., Schindler 1974, 1977, 1978, Vollenweider and Kerekes 1980). Recognition that acid precipitation results from coal-fired power plants and is harmful to forests and lakes led to improved smokestack emission standards (e.g., Borman 1982, Ruess and Johnson 1986). Water-quality changes are coincident with