These are ecosystems in which fresh water from rivers meets the ocean water, the two being mixed by action of tides.
The degree of mixing of salt and fresh water depend on part of the morphology of the estuary basin, the rate and volume of fresh water flow and the amount of tides in the flow.
The biota of estuaries is in turn regulated by the degree of mixing, however the major components are species restricted to the estuarine situation such as oysters and crabs and those that come from the sea such as shrimp.
The high productivity of estuaries than adjacent sea or river results from being nutrient traps for both physical and biological reasons also estuarine producers which includes sea weeds, marsh grasses as well as benthic algae and phytoplankton are capable of nearly year round photosynthesis.
Further more,r higher the fluctuations of the water level higher is the productivity, tide serving to remove waste and to transport food and nutrients