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| | Tides define estuaries. Estuaries are either daily or twice daily washed with seawater. At high tide the salinity of the estuary will rise as sea water (20-35 parts per thousand of salt dissolved in the water) enters the estuary mixing with freshwater (0-0.5 parts per thousand) coming downstream. Estuary salinity can thus vary from 0-35 ppt (parts per thousand) depending on the tide and amount of freshwater input. | | | | This range of salinity means that animals living in estuaries must be capable of surviving in both high and low salinities. The meeting of two water masses ie sea and freshwater, means that a range of different temperatures, water levels, currents and levels of oxygen are also possible and must be tolerated for survival. This is obviously a stressful environment and few species can actually tolerate the continuous fluctuations. However estuaries are incredibly productive. The shallow nutrient rich waters mean an abundance of photosynthesising phytoplankton, aquatic plants and algae. All these primary producers nourish an enormous biomass of life in a relatively small area. Many inshore fisheries rely on the shrimp, crabs and molluscs that inhabit estuaries and much recreational fishing and boating occurs within estuary waters. And offshore fisheries rely heavily on the recruitment of juveniles that use estuaries as safe havens from predation and as rich hunting grounds. There are also abundant resident and migratory bird species inhabiting many estuaries. | | |  ACE Basin National Estuarine Research Reserve. An impoundment on Bear Island with many great egrets and common egrets. Photo courtesy of NOAA. Estuary habitats include salt marshes, mangroves, seagrasses, coastal forests and beaches. And destruction of estuary habitat has occurred ever more frequently as human settlements by the coast have expanded and grown. Overfishing, habitat clearing, and increased pollution are the principal problems in the rapidly diminishing estuarine areas of the world. | | | | |
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