Mixing of high-density salt water and freshwater results in a region of brackish water, the 'salt wedge', which tends to underlie the freshwater and to extend upstream due to its higher density. This salt wedge gives a layered structure to the estuarine water body, which shows some similarity to the lower thermal layer (hypolimnion) of freshwater lakes. The salt wedge is highly mobile, moving upstream with the tide, and varies in extent during the year in relation to the inflow of stream water.
The region of mixing of salt water and freshwater provides an important habitat for organisms that are specialized for living in brackish environments. Organisms more suited to the marine environment may be introduced into freshwater conditions. This includes, for example, the toxic dinoflagellate Pfies-teria piscicida which has recently been implicated in extensive fish kills in the Pocomoke and Chica-maconico Rivers of the Chesapeake Bay estuarine system, USA (Section 10.1.4, Silbergeld et al., 2000). The region of saline and freshwater mixing is also a region of high biotic diversity, occurring as a result of the high productivity for both phyto-plankton and benthos. This arises due to 'salting
RIVER INFLOW: WATER MIXING, ESTUARINE PRODUCTIVITY, AND EUTROPHICATION OF COASTAL AREAS
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