Marine Biology (2+1)

Salinity

The dominant feature of the estuarine environment is the fluctuation in salinity. By definition, a salinity gradient exists at some time in an estuary, but the pattern of that gradient varies with the seasons, the topography of the estuary, the tides and the amount of freshwater. However, there are other factors that often salinity. Where the tidal range is significant, high tide drives the salt water further up the estuary, displacing the isohalines upstream, low tides, by contrast, displace the isohaline downstream. As a result, a certain area of the estuary is subjected to a salinity regime that changes with each tide. This area of an estuary has the maximum salinity fluctuations is such and the range of salinity over a 6 to 12 hour period equals or exceeds the entire annual range of salinities for some areas, even within an estuary.

A second force is the Coriolis effect. The rotation of the earth deflects flowing water. In the Northern Hemisphere, this effect deflects outfloming freshwater to the right as one looks down the estuary toward the sea. Salt water flowing into the estuary from the ocean is also displaced to the right looking from the sea toward the estuary. The opposite is true in the Southern Hemisphere. As a result, two points, each on opposite sides of the estuary equidistant from the mouth, may have predictably different salinities.

Seasonal changes in salinity in the estuary are usually the result of seasonal changes in evaporation, freshwater flow, or both. In areas where freshwater discharge is reduced or absent for part of the year, higher salinities (salt wedge) may be found further upstream. With the onset of increased freshwater flow, the salinity gradients are moved down stream toward the month. At different seasons of the year, therefore, a given point in the estuary may experience different salinities.

Thus far, we have considered only the salinity changes in the water column only in the substrate, a different situation may prevail. Since, estuarine substrates are sandy and muddy, water is held in the interstices between the particles. The salinity of  interstitial water changes much more slowly than the overlying water because of the flow interchange between the two. The interstitial water and the surrounding mud and sand are, therefore, buffered with respect to the overlying water. Organisms dwelling within the substrate are subject to less drastic salinity changes than the organisms  in thewater column at the same point. This buffering effect is probably most effective in the lower intertidal. Variability in interstitial soil water at higher elevations has more opportunity for dilution during rains and is subject to more evaporation in dry weather.