Drought and heatwave mesocosm experiment on Canadian Rocky mountain periphyton communities from alpine ponds.

The accelerating rate of global climate change at higher elevations and latitudes is increasing the potential for extreme climatic events. Here, a knowledge gap exists in how the order of exposure to and duration of droughts and heatwaves affect their cumulative impact on aquatic communities. We tested experimentally for the effects of simultaneous versus sequential exposures to drought and heatwave on sediment-dwelling algal communities (epipelon) from small fishless alpine lakes. In both simultaneous and sequential exposure treatments involving drought followed by a heatwave the negative effect of drought offset the negative effect of warming on chlorophyll-inferred algal biomass and taxonomic composition. Reversal of order of exposure (i.e., heatwave followed by drought) lowered their cumulative effect on community structure. These findings highlight the potential for drought events to dominate over heatwaves in altering shallow littoral ecosystems at high elevations under a rapidly warming climate.