NONPOINT POLLUTION OF SURFACE WATERS WITH PHOSPHORUS AND NITROGEN

Agriculture and urban activities are major sources of phosphorus and n itrogen to aquatic ecosystems. Atmospheric deposition further contribu tes as a source of N. These nonpoint inputs of nutrients are difficult to measure and regulate because they derive from activities dispersed over wide areas of land and are variable in time due to effects of we ather. In aquatic ecosystems, these nutrients cause diverse problems s uch as toxic algal blooms, loss of oxygen, fish kills, loss of biodive rsity (including species important for commerce and recreation), loss of aquatic plant beds and coral reefs, and other problems. Nutrient en richment seriously degrades aquatic ecosystems and impairs the use of water for drinking, industry, agriculture, recreation, and other purpo ses. Based on our review of the scientific literature, we are certain that (1) eutrophication is a widespread problem in rivers, lakes, estu aries, and coastal oceans, caused by over-enrichment with P and N; (2) nonpoint pollution, a major source of P and N to surface waters of th e United States, results primarily from agriculture and urban activity , including industry; (3) inputs of P and N to agriculture in the form of fertilizers exceed outputs in produce in the United States and man y other nations; (4) nutrient flows to aquatic ecosystems are directly related to animal stocking densities, and under high livestock densit ies, manure production exceeds the needs of crops to which the manure is applied; (5) excess fertilization and manure production cause a P s urplus to accumulate in soil, some of which is transported to aquatic ecosystems; and (6) excess fertilization and manure production on agri cultural lands create surplus N, which is mobile in many soils and oft en leaches to downstream aquatic ecosystems, and which can also volati lize to the atmosphere, redepositing elsewhere and eventually reaching aquatic ecosystems. If current practices continue, nonpoint pollution of surface waters is virtually certain to increase in the future. Suc h an outcome is not inevitable, however, because a number of technolog ies, land use practices, and conservation measures are capable of decr easing the flow of nonpoint P and N into surface waters. From our revi ew of the available scientific information, we are confident that: (1) nonpoint pollution of surface waters with P and N could be reduced by reducing surplus nutrient flows in agricultural systems and processes , reducing agricultural and urban runoff by diverse methods, and reduc ing N emissions from fossil fuel burning; and (2) eutrophication can b e reversed by decreasing input rates of P and N to aquatic ecosystems, but rates of recovery are highly variable among water bodies. Often, the eutrophic state is persistent, and recovery is slow.