Is all salinity the same? I. The effect of ionic compositions on the salinity tolerance of five species of freshwater invertebrates

Salts of marine origin, predominantly consisting of Na+ and Cl- ions, are dominant in most Australian inland saline waters. The proportions of other ions, Ca2+, Mg2+, SO42-, HCO3- and CO32-, in the water may influence salinity tolerance of freshwater organisms and thus the effect of increasing salinity may vary with difference in ionic proportions. We exposed freshwater invertebrates to different concentrations of four ionic compositions and compared them with commercial sea salt ( Ocean Nature). They were: synthetic Ocean Nature (ONS) and three saline water types (ONS but without: SO42-, HCO3- and CO32- (S1); Ca2+, HCO3- and CO32- (S2); and Ca2+ and Mg2+ (S3)), which are considered to be the predominant saline water types in south-eastern Australia and the Western Australian wheatbelt. The 96-h LC50 values for the five media were determined for six invertebrate species and sub-lethal responses were observed for two species. There were no differences between responses of invertebrates to various ionic compositions in acute toxicity tests. However, in prolonged sub-lethal tests, animals reacted differently to the various ionic compositions. The greatest effect was observed in water types lacking Ca, for which plausible physiological mechanisms exist. Variation in ionic proportions should be taken into account when considering sub-lethal effects of salinity on freshwater invertebrates.