An estuary is a partially enclosed coastal water body where freshwater runoff from the land mixes with the salty ocean. Entire watersheds contribute freshwater to the estuary through gradual ground water seepage, streams, and rivers, and thus events and activities far inland can influence an estuary. The estuary is one of many environments participating in the larger hydrological cycle.
Freshwater run-off is rich in nutrients from eroded top soil and other plant and animal sources. These nutrients coupled with other food sources and protective habitats offered by sea grasses and mangroves create an ideal environment for many species of wildlife. Estuaries are vital nurseries for salt water fishes and over 90 percent of all oceanic creatures spend part of their life cycle in estuarine waters. The estuaries are an equally important habitat for birds that fish the shallow waters and take roost along the mangrove shoreline. Sub-tropical estuaries are the most biologically productive environments in the world.
Freshwater run-off is rich in nutrients from eroded top soil and other plant and animal sources. These nutrients coupled with other food sources and protective habitats offered by sea grasses and mangroves create an ideal environment for many species of wildlife. Estuaries are vital nurseries for salt water fishes and over 90 percent of all oceanic creatures spend part of their life cycle in estuarine waters. The estuaries are an equally important habitat for birds that fish the shallow waters and take roost along the mangrove shoreline. Sub-tropical estuaries are the most biologically productive environments in the world.
Water salinity within an estuary varies widely due to the mixing of fresh and salt water. Generally, the water is saltier near the ocean side, brackish in the middle, and fresher near river outflows. This variation, coupled with water depth gives rise to diverse micro-environments within the estuary, which in turn influence the distribution of wildlife and, in particular, the distribution of relatively immobile shellfish.
Change is constant in estuary ecosystems. Cycles of day and night, high and low tides, and annual rainy and dry seasons all influence the size and distribution of micro-environments within the estuary. In addition, tropical storms and hurricanes can dramatically reshape micro-environments within an estuary. For example, the opening and closing of barrier island passes can influence greatly local salinity characteristics.
