roughly as follows. Increased input of nutrients, especially phosphorus, leads to an increased incidence of nuisance blooms of algae (especially blue-green algae), leading to a loss of water clarity, a buildup of organic and nutrient-rich sediments, loss of oxygen from the bottom waters of the lake (which in turn, accelerates nutrient recycling processes), and changes in the lake's food web structure. Secondary nutrient limitation by silica or nitrogen that results when phosphorus levels are elevated also leads to changes in the phytoplankton community and to the development of nuisance species of algae (e.g., blue-green forms). Proliferation of macrophytes is also associated with eutrophication, especially in shallow lakes, but these problems are not tied directly to excessive rates of nutrient loading (see "Exotic Species," below). Although increases in nutrient levels enhance fish production, the loss of habitat (e.g., by sediment buildup, deoxygenation, undesirable proliferation of macrophytes) and food sources (by food web simplification) causes a shift from more desirable game fish to less desirable species, especially in more extreme cases of eutrophication. Stocking of exotics and overfishing exacerbate this problem. From a human use perspective these changes create numerous problems, including the following: fouling of boats and structures (by algal growths), loss of aesthetic appeal, accessibility problems for swimmers and boaters (because of macrophyte proliferation), economic damage to resort and property owners, and increased costs and technical difficulties of treating water for drinking purposes (because of taste and odor problems and increased potential for trihalomethane production).
The causes of eutrophication resulting from human activity are reasonably well understood. Once an oligotrophic lake has been made eutrophic, processes develop that may delay recovery after nutrient loadings have been decreased. If the hypolimnion becomes anoxic, recycling of phosphorus from the sediments is enhanced, in effect increasing the efficiency of use of the phosphorus input. During the eutrophic phase many changes may occur that will not be automatically reversed by a reduction in nutrient supply, such as loss of desirable macrophyte, invertebrate, and fish species. Nutrient reduction is a necessary, but not always a sufficient, condition for reversal of eutrophy.
Point sources of nutrients are the primary cause of excessive loadings in some lakes, but nonpoint sources (urban and agricultural runoff) contribute most of the nutrient input to the majority of U.S. lakes. Based on a modeling exercise with loading data on phosphorus for 255 lakes in the eastern United States, Gakstatter et al. (1978) concluded that only 18 to 22 percent of the lakes would show a