Estuaries are among the world's most productive ecosystems. Under pristine conditions these embayments, where sea water is measurably diluted with freshwater from the surrounding uplands, are naturally enriched with nutrients from the land and sea. They function as nutrient and particle traps, recycling nitrogen and phosphorus through repeated processes of growth and decay. Each estuary is different, with characteristic basin shape and depth, freshwater input, and tidal flushing rate. Estuaries are essential to the success of numerous fish and shellfish species of great commercial, ecological, and recreational importance. Estuaries are also the focal points for pollutants entering the sea from the land, and are now the most nutrient-enriched ecosystems on earth. The overabundance of nutrients, especially nitrogen, has caused profound changes in estuarine ecosystems. These include the proliferation of nuisance and noxious algal blooms, the reduction of oxygen availability, and the loss of essential fish and shellfish habitat. The severity of the effects of excess nutrients on estuarine ecosystem structure and function depends on the specific characteristics of each watershed and receiving estuary. The ability to support large populations and high diversity of important species is in jeopardy in many estuaries, and has been lost altogether in others.