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The Seashore

Estuaries

Estuaries occur where freshwater flows into the sea and constitutes a very specialised environment. Essentially these are the meeting points for rivers and seawater. The size and number of rivers entering the estuary effects the problems animals will face and some estuaries can run for many miles inland. Humans invariably create barriers to the incoming tide so that seawater is prevented from entering the river (freshwater is usually seen as economically more important). The freshwater is less dense than the seawater and “floats” across the incoming tide. Mixing does gradually occur reducing the salinity and causing osmotic problems. Ragworm like most animals will burrow deep in the mud to avoid the lowering salinity. These animals, however, can tolerate the gradual movement of freshwater into its tissues. Hence, it can be found high up an estuary and may be one of the only species present. The Common Shore Crab is able to cope with the change in salinity but this is a regulator, actively pumping some of the freshwater out of the body. However, these measures are only temporary until the seawater returns and the level and duration varies between species. Stenohaline species are those unable to tolerate a wide salinity variation whilst Euryhaline species can tolerate a wide range.

A small section across the Clyde Estuary A small section across the Clyde Estuary, near Glasgow. Note the small, dredged channel amongst the mud.

Water flow varies considerably, especially over a 24 hour period. At times they may appear to be stagnant, depositing considerable amounts of silt and mud. The minimal movement of water results in a deoxygenation of the substrate. The high level of bacteria and organic matter will result in a dense, black layer of mud below the surface. This is called the sulphide layer and is very smelly, largely of hydrogen sulphide. Estuaries are not usually seen by the public at large as an important natural resource. Cities and towns, for strategic reasons are constructed on estuarine banks. A combination of factors has reduced the area of estuarine shoreline considerably and yet these are “nature’s hotels”, places where birds (especially waders) can stop on migration in spring and autumn. At these times there may be many thousands of waders present on the mud feeding. Humans replace upper shores with concrete or stone wharves whilst deep dredging will be done lower down the shore to increase depth for shipping. A steady increase in pleasure craft in recent years has increased the number of marinas located in wide estuaries (see below). Anti-fouling paints of small craft, until recently, contained Tributyl Tin (TBT). This leached from the moored boats, entered the ecosystem and reduced population densities of molluscs like the dogwhelk.

Marina in the estuary of the R.Itchen, Southampton

Marina in the estuary of the R.Itchen, Southampton

 


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