Interactive Effects of Copper Contamination and Salinization Across Multiple Genotypes of Daphnia magna

Understanding how organisms respond to multiple environmental stressors is essential for predicting ecosystem impacts in the face of increasing anthropogenic pressures. However, few studies have explicitly examined how genotypes of the same species respond to combined stressors, with the specific objective of disentangling variation both within and across geographic locations. In this study, we examined the individual and combined effects of copper contamination and elevated salinity on multiple genotypes of Daphnia magna from US and French populations. Our findings revealed that copper exposure consistently increased mortality across all genotypes, with US genotypes displaying greater sensitivity than French counterparts. Salinity stress primarily reduced fecundity, and again, US genotypes exhibited lower resilience. Under combined copper and salinity stress, however, US genotypes showed survival benefits, suggesting potential cross-tolerance mechanisms between these stressors. Moreover, there was substantial variation in the response to both stressors within both locations. This genotype-specific variation underscores the necessity of considering genetic factors and genotype-specific sensitivity/tolerance in ecosystem management and conservation strategies, particularly under multiple-stressor scenarios. Further exploration of the genetic pathways and adaptation potential driving these responses will enhance our ability to support biodiversity and ecosystem resilience amid global environmental change.