Estuaries are nothing but a semi enclosed coastal body of water which has a free connection with an open sea. These are affected by tidal action and accordingly fresh water from land drainage gets mixed with sea water. Examples of estuaries are coastal bay, tidal marshes, water body behind barrier beaches and river months.
Estuaries are classified on the basis of geomorphology. These are (a) drowned river valley (b) ford type estuaries (c) completely mixed or vertically homogeneous (d) partially mixed stratified estuaries (e) hypersaline estuaries. However, on the basis of ecosystem energetic, the estuaries are classified as under:
1. Physically stressed systems of wide latitudinal range.
2. Natural arctic ecosystems with stress.
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3. Natural temperate coastal ecosystems.
4. Natural tropical coastal ecosystems.