Abstract

Hypoxia and salinization increasingly co-occur in freshwater and estuarine ecosystems, yet their combined effects under fluctuating oxygen regimes remain poorly resolved. We experimentally quantified survival responses of four planktonic crustaceans (Daphnia magna, Daphnia longispina, Eurytemora velox, and Thermocyclops crassus) along a salinity gradient expressed as conductivity (0.6–7.7 mS cm−1) under two oxygen regimes: sustained hypoxia and repeated short-term hypoxia over 120 h, under standardized food-deprived conditions to isolate abiotic stress effects. Under sustained hypoxia, survival declined monotonically with decreasing oxygen in all species, with marked interspecific differences at ≤2 mg L−1. Under fluctuating oxygen conditions, survival peaked at low to intermediate salinity and declined sharply at elevated salinity, indicating a strong combined effect of oxygen limitation and salinity-related stress. Multivariate analysis identified a significant salinity × oxygen interaction, indicating that the effect of oxygen depletion on survival depended on salinity. Daphnia magna showed the highest tolerance, whereas D. longispina and E. velox were highly sensitive to elevated salinity; T. crassus showed intermediate tolerance. These results indicate that elevated salinity is associated with negative effects under both sustained and episodic hypoxia and that fluctuating oxygen regimes elicit strong combined stress responses. Our findings highlight the importance of combined abiotic stress effects when predicting zooplankton responses in oxygen-depleted and salinizing aquatic ecosystems.

IPC Classification

Keywords