Effects of pH on salinity tolerance of selected freshwater invertebrates

Salinization of freshwaters often co-occurs with other changes in the environment, including pH. We investigate the effect of pH on salinity tolerance in selected macroinvertebrates (Notalina fulva, Centroptilum sp. and Physa acuta--lethal effects only) and microinvertebrates (Paramecium caudatum and Hydra oligactis--lethal and sublethal effects). Despite seemingly plausible physiological arguments, no difference in salinity tolerance over 96-h period was detected between low (5 or 6 nominal) pH and circumneutral (7-8.2 nominal) pH. P. caudatum was more salt sensitive in pH 11 than in pH 5, 7, and 10 in terms of mortality, and in terms of a sublethal endpoint, number of individuals produced and survived over 72 h, more sensitive to salinity in pH 10 than in pH 5 and 7. No other effects of pH on salinity tolerance were detected. Acidification will likely have effects on freshwater organisms on its own, however, when combined with salinization (from saline waters approximating seawater) acidification level tested did not modify the direct effects of salinity on the sample of freshwater invertebrates tested from a range of taxonomic groups. Thus the risk of low (5 or 6) pH modifying the effect of salinity on freshwater invertebrates is not high. Logically, lower pH values might have modified the effect of salinity, but there is a limited scope for lower pH values that would keep the species studied alive. In contrast, alkaline pH may increase the effect of salinity in some freshwater invertebrates. It is possible that the effect of pH on salinity tolerance may, however, be increased in saline waters with low calcium concentrations.