To set the stage for any discussion of estuarine ecosystems, a clear working definition is needed. One of the simplest and most utilized definitions of an estuarine ecosystem is:
the zone where freshwater from land runoff mixes with seawater.
Another common definition is:
An estuary is a semi-enclosed coastal body of water that has free connection with the sea where seawater is diluted by freshwater derived from land drainage.
The preceding definitions focus on the geomorpholo-gical and hydrological aspects of estuaries with no mention of the abiotic or physical driving sources of energy, that is, tides and solar insolation. Nor are any biotic components or processes utilized. Thus, the following definition is proposed:
An estuarine ecosystem is a system composed of relatively heterogeneous biologically diverse subsystems, i.e., water column, mud and sand flats, bivalve reefs and beds, and seagrass meadows as well as salt marshes. These subsystems are connected by mobile animals and tidal water flows that are embedded in the geomorphological structure of creeks as well as channels, and together form one of the most productive natural systems in the biosphere.
Recent quantitative studies indicate that estuaries are ecoclines that are composed of gradients containing
relatively heterogeneous subsystems that are environmentally more stable than ecotones (Figure 1). Ecoclines are boundaries with more gradual, progressive change between freshwater and the sea. In this view, the organisms in the estuary are either from freshwater or from marine environments; there are no brackish water species. Each estuarine system will respond to at least a freshwater and a marine gradient as well as have its own particular combination of biological and physical components and processes. Thus, every estuarine ecosystem is unique.
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