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Estuaries are transition zones between the land and the sea which form aquatic ecosystems, often referred to as marginal filters. Numerous classifications of estuaries are based on such criteria as: geomorphological structure, the character of water circulation and stratification, the ratio between freshwater and marine water and the type of energy flow. However, not all estuaries fall into one of the above categories. Some of them represent an intermediate type, while the type of others may change depending on the season and related changes in water inflow rate. Fisheries, including bivalve mollusks, depend on the high productivity of estuarine areas, which can be also used as ports or sites of recreation and industrial activities. Estuaries play a fundamental role in maintaining an ecological balance in the biosphere.
Key words: estuary, classification of estuaries, economic role of estuaries