The coastal ecosystem where freshwater meets saltwater and provides a nursery for marine species is an estuary.
An estuary is a partly enclosed coastal body of brackish water where rivers or streams mix with the sea. Tides push seawater inland while river flow carries freshwater, sediment, and nutrients seaward. This creates changing salinity, oxygen, and water-depth conditions that support organisms from freshwater, marine, and terrestrial environments.
Estuaries are highly productive because nutrients collect in their shallow waters, mudflats, marshes, seagrass beds, and tidal channels. Young fish, shrimp, crabs, and shellfish often find food and shelter there before moving into open coastal waters. Estuaries also provide migration routes for fish such as salmon and habitat for migratory birds.
A salt marsh or mangrove forest may be part of an estuary, but neither term describes every place where freshwater mixes with seawater. A coral reef is a different marine ecosystem, built largely by reef-forming corals in warm, shallow seas. The word “estuary” comes from the Latin aestuarium, associated with a tidal inlet or tide.