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The variations of environmental conditions (T °C, pH, δ13C, δ18O, Mg/Ca, and Sr/Ca) for two ostracod species (Psychrodromus olivaceus, Potamocypris similis) were analyzed and compared to identify the controls of environmental parameters on the chemical and isotopic composition of their valves. The specimens were collected from Karapınar Spring (Bolu, Turkey) between October 2009 and November 2010. The species were observed throughout the year. Psychrodromus olivaceus showed relatively constant Mg/Ca ratios during the study period, whereas those of P. similis showed more variability. The ratio was also significantly different (P < 0.05) between the two species. We did not find significant correlation between water temperature and either of the species' Mg/Ca ratios. The stable isotope record for P. similis was very patchy, showing a small range of values consistent with the groundwater source, similar to the behavior of Sr/Ca. The Sr/Ca ratio displayed little variation throughout the year, suggesting hydrochemical stability over time, most likely in response to the groundwater source. The stable isotope data displayed pronounced variability for P. similis but little for P. olivaceus, suggesting that the former may be responding to seasonality, whereas the latter responds to groundwater. [ABSTRACT FROM AUTHOR]