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Salinity


Salinity is the concentration of salt dissolved in water. It is normally expressed in parts per thousand or the grams of salt per 1000 grams of water sample (ppt, o/oo). In a water body influenced by the ocean, such as the Indian River Lagoon, most of the dissolved salt in the water is due to only a handful of sea salt components.

Freshwater contains few salts and thus has low salinity. Drinking water has a Florida DEP limit of sodium chloride salinity of less that 0.5 ppt (410 ppm) Sea water, on the other hand, has an average salinity of 35 ppt (equivalent to about 5 oz. of salt in 1 gallon of water). The Indian River Lagoon is a site of mixing for freshwater and sea water. Since sea water enters the Indian River Lagoon at the inlets, the salinity is highest at those points and decreases as one moves away from the inlets toward the freshwater inputs. The salinity of the lagoon will thus normally vary between 0 and 35 ppt. Since sea water has an average salinity of 35 ppt, on a given day in the lagoon, it is possible to measure a salinity greater than 35. This is because, in some areas, evaporation is a major controlling force for salinity. There is also a trend of increasing salinity at times of the year when little or no rainfall is occurring. One important aspect of salinity is the density difference it causes. Sea water has a higher density than fresh water due to the dissolved salt. Near the mouth of tributaries, such as the Sebastian River, the lighter fresh water of the river will remain at the surface and flow right over the top of the denser saline water, which tends to remain near the river bottom. These waters will ultimately mix, but where that mixing occurs will depend on tides, winds and the volume of freshwater. Immediately after heavy rainstorms, the head of freshwater will push further out into the lagoon and can cause decreases in the overall salinity in the lagoon.

Probably the most important aspect of salinity with regards to water quality is its affect on aquatic organisms inhabiting the Indian River Lagoon. Salinity changes can affect the well being and distribution of biological populations. Large freshwater inputs, particularly after rainstorms, can severely hurt clam populations, causing clams to die as well as having adverse affects on growth and spawning. This can cause a severe impact on the clam industry in the Indian River Lagoon.



© 2003 Marine Resources Council of East Florida