Context . The increasing intensity and extent of anthropogenically mediated salinisation in freshwater systems has the potential to affect freshwater species through physiological and ecological processes. Determining responses to salinisation is critical to predicting impacts on fauna.

Aims . We aimed to quantify the response of wild-caught turtles from freshwater lakes that had become saline in the lower Murray River catchment.

Methods . Plasma electrolytes of all three species of freshwater turtle from South Australia were compared among two freshwater sites (Horseshoe Lagoon and Swan Reach), a brackish lake (Lake Bonney) and a saline lake (Lake Alexandrina).

Key results . Chelodina longicollis, C. expansa and Emydura macquarii from a brackish lake had higher concentrations of plasma sodium and chloride than those from freshwater habitats. However, osmolytes known to increase under severe osmotic stress (urea and uric acid) were not elevated in brackish sites. Turtles from the highly saline lake were colonised by an invasive marine worm which encased the carapace and inhibited limb movement.

Conclusions . Freshwater turtles in brackish backwaters had little response to salinity, whereas the C. longicollis in a saline lake had a significant physiological response caused by salt and further impacts from colonisation of marine worms.

Implications . Short periods of high salinity are unlikely to adversely affect freshwater turtles. However, secondary ecological processes, such as immobilisation from a marine worm may cause unexpected impacts on freshwater fauna.