Changes in copper toxicity towards diatom communities with experimental warming.

Biological communities in aquatic environments most commonly face multiple stress, where natural and anthropogenic stressors often act jointly. Their interactions are most easily assessed using short cycle organisms such as periphytic diatoms. In this experiment, we analyzed the combined effects of copper exposure and warming on diatom successions over 6 weeks. Natural biofilm collected in winter was left to grow in mesocosms exposed or unexposed to realistic Cu concentrations at four different temperatures. Separate and joint impacts of the two stressors were determined through structural and functional endpoints. Both temperature and copper influenced the biological responses; their interaction, when significant, was always antagonistic. Diatom communities gradually changed with rising temperature. Under copper exposure, the dominant Planothidium lanceolatum was superseded by Achnanthidium exiguum, which accounted for about 70% relative abundance in the warmest conditions (18-23°C). Tolerance to copper was derived from dose-response curves based on photosynthesis inhibition. Cu-induced community tolerance was always found, but it decreased with warming and time. Biodiversity loss associated with lower Cu tolerance under combined Cu exposure and increasing temperatures evidences the major influence of cumulative stressors on aquatic health. These results highlight the crucial interplay between environmental stressors, which are expected to intensify with climate change.