Estuaries

An estuary is a coastal water body where freshwater from rivers and streams mixes with seawater from the ocean. This mixing creates a brackish environment that varies with tides, river discharge, and weather. Estuaries are typically semi-enclosed and connected to the open sea, forming a transition zone between riverine and marine ecosystems.

Estuaries are dynamic systems driven by tides, currents, sediment supply, and freshwater inflow. They often exhibit strong vertical and horizontal salinity gradients, with fresher water near the river and more saline water toward the sea. Turbidity is common due to suspended sediments, and nutrient inputs from rivers support high biological productivity. Circulation patterns promote the transport and transformation of organic matter and nutrients, fueling complex food webs.

Ecologically, estuaries are among the most productive ecosystems, providing critical nursery grounds for many fish and shellfish species, as well as feeding habitat for birds and diving mammals. The habitat mosaic includes mudflats, salt marshes, mangroves, and seagrass or kelp beds, each supporting unique communities. Estuaries also act as natural filters, trapping sediments and pollutants and modulating nutrient loads before they reach the sea.

Estuaries can be classified by geomorphology into several types, such as drowned river valleys (coastal plain estuaries), bar-built estuaries, tectonic estuaries, and fjord-like estuaries. Examples include the Chesapeake Bay, the Thames Estuary, and the Yangtze Estuary, each with distinct physical and ecological characteristics.

Human impacts include pollution, overfishing, invasive species, dredging, water withdrawal, and watershed development, which can degrade water quality, alter hydrology, and reduce habitat availability. Management approaches emphasize integrated watershed planning, habitat restoration (notably tidal wetlands), sustainable fisheries, pollution control, and climate adaptation to protect estuarine ecosystems.