An estuary is a partially enclosed body of water that is connected with the ocean and filled with a combination of seawater from the ocean and freshwater from land run-off.
Most estuaries are found where rivers finally reach the sea and may extend inland for hundreds of kilometres. The salinity often varies day by day with the rise and fall of the tides, and seasonally, with fluctuations in the amount of freshwater flowing down the river.
Despite changes in salinity and the harsh environment that such conditions create, estuaries are full of life and are very productive habitats.
The nutrients collected by a river from land run-off eventually end up, and are deposited in the muddy substrate of estuaries. This happens because the flow of water in estuaries heads both ways, rather than just one way, creating circulation patterns which allow deposition.
This nutrient rich substrate is the basis for a vast array of organisms to thrive, in turn feeding larger animals. Many tropical fish come from estuarine areas, including many mangrove swamp species. These fish are termed as brackish fish and require some salt to be added to the aquarium.