An estuary is a partially enclosed body of water, and its surrounding coastal habitats, where salt water from the ocean typically mixes with fresh water from rivers or streams. They are classified by the geology that defines them or the way in which water circulates throughout them. The habitats that surround an estuary as well as the populations of plants and animals that inhabit them are specially adapted to their environment.
Because species living in most estuaries have to deal with constant changes in salinity, these environments harbor many unique populations of organisms. Salt marshes are covered with salt-tolerant plants, such as smooth cordgrass. Mangrove trees that grow in oxygen-poor soils populate mangrove forests. Oysters, such as those found in the Chesapeake Bay, are often found in estuaries because they are able to adapt their behavior to the changing environment.
A rich array of habitats surrounds estuaries. The type of habitat is usually determined by the local geology and climate. Habitats associated with estuaries include salt marshes, mangrove forests, mudflats, tidal streams, rocky intertidal shores, and barrier beaches.
While each estuary is unique, they are classified into a few broad types based on their geology and patterns of water circulation. The features of an estuary are determined by a region’s geology and its physical, chemical, and climatic conditions. Water movements in estuaries transport organisms, circulate and renew nutrients and oxygen, and remove wastes. The daily mixing of fresh water and salt water in estuaries leads to variable and dynamic chemical conditions (especially salinity).