Estuaries represent one of the most biologically productive aquatic ecosystems on Earth. An estuary is a coastal body of water where chemical and physical conditions modulate in an intermediate range between the freshwater rivers that feed into them and the salt water of the ocean beyond them. It is the point of mixture for these two very different aquatic ecosystems. The freshwater of the rivers mix with the salt water pushed by the incoming tides to provide a brackish water habitat ideally suited to a tremendous diversity of coastal marine life.

Estuaries are nursery grounds for the developing young of commercially important fish and shellfish. The young of any species are less tolerant of physical extremes in their environment than adults. Many species of marine life cannot tolerate the concentrations of salt in ocean water as they develop from egg to subadult, and by providing a mixture of...