Brackish and freshwater shallow lakes — different systems or variations on the same theme?

Abstract

Saline lakes may be divided into two categories — those primarily saline because they are endorheic and those secondarily saline (brackish) because of natural or anthropomorphic inputs of sea water. Endorheic lakes have greatly varying composition and salinity, occur in arid regions and are not as abundant for that reason as exorheic lakes. The area of land surface over which they are the characteristic types, is, however, very considerable. Many endorheic lakes have communities of low diversity but the reason for this is not necessarily high salinity — the world’s largest endorheic lake, the ocean, has a very high diversity — but the temporariness of many of these habitats which has led to wide and unpredictable fluctuations in salinity and to periodic drying out.Secondarily saline lakes are rather different. They generally have an ionic composition which is that of diluted sea water, and although they do not have the high diversity of marine communities, they often have comparable, if slightly lesser, diversities with those of freshwaters. There are no firm salinity boundaries between fresh and brackish waters, only ones drawn (e.g. at 5 per mille total salts) by convention.Brackish waters have the particular interest that estuarine organisms including mysid shrimps and essentially marine members of the algal division, the Haptophyta, may colonize and establish themselves in food webs otherwise characteristic of freshwaters. A comparison is made between the systems of brackish and freshwater lakes in the Norfolk Broadland of eastern England, most of which have undergone eutrophication. Although at first sight appearing very different, both fall into a similar pattern when processes of change are considered. Increases in salinity serve as indirect switch mechanisms which, like other but different mechanisms in freshwaters, may cause the lake community to change from submerged plant dominance to phytoplankton dominance at intermediate states of nutrient loading. The current drought in eastern England is inducing small salinity changes that support this assertion.