This study explores how differences in ionic composition of south-eastern Australian saline lake waters, caused by path differentiation according to the Eugster-Jones-Hardie models of solute evolution and halite recycling, influence species composition of ostracod faunas. Ostracod occurrences are reported as physiologically important ionic ratios set in a marine-meteoric framework, with chemical boundaries determined by mixing and evaporation models. The occurrence of halophilous ostracods coincides with changes in the ionic structure of lake waters. Chemical diversity is found to be biologically important, with most ostracods preferring a specific pathway of the Eugster-Jones-Hardie models. Path preference predominantly reflects the different tolerance ranges of species to a combination of Na+/H+, Na+/Ca2+ and alkalinity/Cl- activity ratios, which probably govern acid-base balance and Na+ and CA2+ regulation. An Alkalinity/Cl- activity ratio of ~-2.3 corresponds to the main division in the ostracod data and reflects the abrupt change in alkalinity/Cl- ratios that occurs when a seawater-like solute matrix is diluted with a large amount of meteoric water (95%). Most halobiont ostracods occur in waters enriched with Na-Cl as a result of halite recycling. Evidence is presented that the same geochemical processes are relevant to other aquatic organisms (e.g. zooplankton, diatoms, insects) found in salt water.
