A tidal zone is a complex ecosystem made up of tidal channels, mud flats, shoals, salt marshes and ponds. Each of these niches has its own characteristic temperature and salinity, subject to daily and seasonal Each supports its own forms of life. Hundreds of species of algae, plankton, and water plants thrive on the mix of carbon from the sea and nitrogen and phosphorus from the rivers. These simpler life forms nourish the profusion of fish and shellfish that make the estuary their home for all or part of the year, and the fish and shellfish in turn draw many thousands of birds to the area.
A tidal zone may contain an estuary, where a freshwater drainage meets the ocean. An estuary is a deepwater tidal habitat and its adjacent tidal wetlands. It is usually semi-enclosed by land but with open, partly obstructed or sporadic access to the open ocean. The ocean water is at least occasionally diluted from freshwater runoff from the land and the estuarine waters form an interface between fresh and salt waters. Estuarine areas sustain a rich abundance of fish, bird, and invertebrate species, and habitats such as mangroves, seagrass beds, and saltmarshes. Estuaries provide important nursery grounds for marine fish, sustain fisheries, control flood events, provide a rich feeding ground for birds, and are a necessary component for the life-cycle of many invertebrate species.
Three regional pacts that aim to improve water quality in the Chesapeake Bay and its tributaries were signed late in 2001 to encourage regional planning, storm water management and low impact development in the Chesapeake Bay watershed, draining into USA's largest estuary.