From organismal physiology to ecological processes: Effects of nutrient enrichment and salinity variation in a freshwater ecosystem

Environmental stressors have profound implications for species, communities, and ecosystems by altering fundamental processes. With increasing human impacts on aquatic ecosystems, two main scenarios have been reported: (1) the spatiotemporal superposition of multiple stressors, leading to interactions among them and (2) intensifying environmental gradients, leading to threshold responses. However, studies designed to assess the effects of multiple stressors (e.g., pulse stressors) along environmental gradients (e.g., press stressors) are uncommon, and interactions between pulse and press stressors may cause abrupt changes in biological responses. We conducted a laboratory experiment to investigate the effects of osmotic stress along a nutrient enrichment gradient on a freshwater community composed of periphyton, microorganisms, and zebra mus- sels (Dreissena polymorpha). Our objectives were to (1) quantify the individual and combined effects of stressors, (2) delineate thresholds along the nutrient gradient (press) in the absence and presence of osmotic stress (pulse), and (3) test for interactions between the two stressors. We evaluated effects on metabolic rates in D. polymorpha and on microbial activity, as well as phototrophic periphyton biomass and physiological sta- tus. We observed interactions between the two stressors for metabolic rates in D. polymorpha and periphytic phaeopigments. In contrast, we found an individual effect of osmotic stress on microbial activity and chloro- phyll a content. Thresholds were only identified in the presence of osmotic stress for metabolic rates in D. polymorpha. Our work highlights the importance of combining multiple stressors with environmental gra- dients and the need to consider multiple biological compartments when evaluating the impacts of stressors on ecosystems.