The euryhaline ostracod Cyprideis torosa lives in Akyatan Lagoon, Turkey, which is exposed to large spatial and seasonal variations in water salinity, delta O-18, and temperature. Hydrogen and oxygen isotope measurements of waters reveal that the large range of salinity (15-80 g L-1) in the lagoon results from a combination of evaporation and mixing between Mediterranean seawater and Seyhan River input. Round sieve-pore relative abundance in C. torosa provides a robust proxy for water salinity (S) from 15 to 80 g L-1, according to the equation: S = 161.41 (+/- 4.52) * log(10)(% rounded pores) - 94.04 (+/- 3.44) (R-2 = 0.937; p = 10(-31)). Seasonal sampling and isotope analysis of C. torosa in waters of known delta O-18 values (-4.7 to +6.9 aEuro degrees V-PDB) and temperatures (15-35 A degrees C) yielded a weak positive correlation (r = 0.71) between 1000 ln alpha((calcite-water)) aEuro degrees V-SMOW) and 10(3) * T-1. Specimens of C. torosa collected during the mild and warm seasons have oxygen isotope compositions close to those of inorganic calcite precipitated in equilibrium with ambient water. The large oxygen-isotope variability observed during any season of the year most likely results from shell calcification in water bodies of highly variable salinity, alkalinity, Mg/Ca and water saturation relative to calcite. Indeed, distinct water bodies in the Akyatan Lagoon are generated by mixing of fresh and marine waters, which are exposed to different evaporation rates at the seasonal scale.