Aquatic ecology and Biodiversity

Unit 10- Coastal ecosystems

10.3. Estuaries
Estuaries are partially enclosed bodies of water along coastlines where fresh water and salt water meet and mix. Most scientists accept the definition of D.W. Pritchard (1967): "An estuary is a semi-enclosed coastal body of water which has a free connection with the open sea and within which sea water is measurably diluted with fresh water derived from land drainage."

Estuary act as a transition zone between oceans and continents. An estuary has a free connection with the ocean. Fresh water input from land sources (usually rivers) dilutes the estuary's salt content. They must feature the mixing of fresh and salt water to be a true estuary.

Estuaries, however, have large amounts of organic matter, large numbers of organisms, and high secondary productivity. The detritus forms a substrate for a rich bacterial and algal growth, which is an important food source for various suspension and detritus-feeding animals. This accumulation of detritus forms an important source of food for many estuarine organisms.
Estuaries are vital habitats for many marine and fresh water species. They are called the "nurseries of the sea" because the protected environment and abundant food provide an ideal location for fish and shellfish to reproduce. Besides fish, many species of birds depend on estuaries for food and nesting areas. Marine mammals also use estuaries as feeding grounds and nurseries. Estuaries filter sediment and pollutants from the water before it flows into the oceans. Excess nutrients are removed in bordering salt marshes, resulting in cleaner water for people and marine organisms.