The effects of climatic fluctuations and extreme events on running water ecosystems

Most research on the effects of environmental change in freshwaters has focused on incremental changes in average conditions, rather than fluctuations or extreme events such as heatwaves, cold snaps, droughts, floods, or wildfires, which may have even more profound consequences. Such events are commonly predicted to increase in frequency, intensity, and duration with global climate change, with many systems being exposed to conditions with no recent historical precedent. We propose a mechanistic framework for predicting potential impacts of environmental fluctuations on running water ecosystems by scaling up effects of fluctuations from individuals to entire ecosystems. It requires the integration of five key components: effects of the environment on individual metabolism, metabolic and biomechanical constraints on species interactions, effects of trait mismatches between interacting species, assembly dynamics of open local food-webs, and mapping the dynamics of the meta-community onto ecosystem function. We illustrate the framework by developing a mathematical model of perturbations on dynamically assembling food webs, and highlighting where the (currently limited) empirical evidence supports these ideas across a range of spatial and temporal scales and organizational levels. For example, widely supported predictions are that rare, large species at the top of the food web, and especially those with high per capita metabolic demands, will be especially vulnerable to fluctuations, which also lead to simplifications of network structure, impaired energetic transfer efficiency, reduced resilience and top-down relative to bottom-up regulation of the food web and ecosystem processes. We conclude by identifying key questions that need to be addressed for more accurate and predictive bioassessment, and the implications for management practices in an increasingly uncertain world.