Salinity is the salt content of natural
waters. It is normally measured in the sea in parts per thousand
(written ‰). This is the weight of salt in grams which is dissolved in
a kilogramme of water (what weight of salt you would be left with if you
evaporated off all the water). Sea water is generally very consistent,
with a salinity of around 35‰. Freshwater in rivers and lakes, unless
very polluted has a very low salt content compared with seawater (below
1‰). Water bodies like Zandvlei are estuaries C
very special >brackish'
places where freshwater mixes with seawater. Not only is the salinity in
between that of seawater and freshwater, but it also changes rapidly and
often depending on how much it rains (river flow into the estuary) and
how much seawater gets in (is the mouth open?; when are the very high
Salinity in an estuary is therefore much more variable than in the sea or in a river, and plants and animals in estuaries have to be able to cope with this. The living things in an estuary are usually very different from those living in either freshwater or the sea. Some species live part of their life in estuaries and part in the sea (for example, some sea fish need estuaries as 'nurseries' for their young).
Other species, including problem aquatic weeds like the water hyacinth and many wetland plants, will not grow in estuaries –they cannot stand the salt.
The salinity of Zandvlei has been dropping over the last 20 years. In central parts of the main Vlei, where the surface salinity was15‰ in autumn in the past, this year at the same place it measured 2‰. The reason for this is that not enough seawater has been getting in during winter, when the mouth is open. If this process continues the Vlei will not be an estuary any more, and may eventually not even be a vlei, if the silt coming down the rivers is no longer flushed out. Freshwater is lighter than salt water, and floats on it. Recently, after the mouth had been open for a couple of months, the salinity had gone up from 2‰ before the opening to 6‰ at the surface and 20‰ at the bottom. This is better, but is not enough. It is one important reason why the SPM are looking at options for a new weir – if something isn't done our Vlei, the only estuary of any size entering False Bay – will be changed, perhaps forever.