Ecological Impacts of Extreme Climatic Events on Mountain Lakes and Ponds

Abstract

Global warming is contributing to extreme climatic events, especially at higher elevations and latitudes. Phytoplankton assemblages are highly sensitive to these climate-related environmental changes, which include heatwaves and drought events. Knowledge gaps exist concerning the cumulative impact of environmental warming and drought on aquatic alpine communities, and whether it depends on the order of exposure to each stressor. In this thesis, I investigated climate-related predictors of alpine phytoplankton assemblages across 72 lakes in national mountain parks, and the effects of an experimental heatwave and drought event on periphyton, phytoplankton, macroinvertebrate, and rotifer assemblages in fishless alpine pond mesocosms. Mean annual precipitation, lake depth, and lake surface area were all identified as significant predictors of phytoplankton biomass. Mean annual precipitation and air temperature explained among-site variance in taxonomically diagnostic algal pigment concentration, but not genera biomass. In the mesocosm experiment, simultaneous exposure to a heatwave and drought resulted in antagonistic impacts as drought overrode the effect of heating on most taxonomic groups, especially in the case of the phytoplankton and benthic invertebrate assemblages. However, sequential exposure to the two stressors resulted in a multiplicative effect on the phytoplankton assemblage. Altering the order of exposure to drought and warming had a significant effect on periphyton biomass and assemblage. My findings highlight the dominant role that increasingly frequent drought events will likely play in structuring shallow mountain lake and pond communities under a rapidly warming climate.