Summer storms boost algal blooms that change long-term lake ecosystem dynamics. Summer storms boost algal blooms

An important consequence of climate change is the increasing intensity and frequency of extreme weather events that have the potential to change ecosystem structure and function in the short and long term. Exceptional summer storms can alter the physical structure, biogeochemical conditions and biodiversity; yet, our knowledge of their impacts on underlying mechanisms of lake functioning and whole-system responses mainly stem from unreplicated or observational case studies, with no control over site-specific factors. Due to these methodological limitations, observational studies fall short of isolating effects of physical forcing from secondary impacts commonly accompanying storms, e.g. external inputs of suspended solids, colored DOM and nutrients. To overcome these limitations, we performed an ecosystem-level experiment in a unique large-scale enclosure facility ensuring full replication under realistic field conditions. We demonstrate that physical forcing by an extreme storm in deep clear-water lakes entails disruptive changes in chemical and biological variables persisting for weeks. Specifically, diazotrophic cyanobacteria are boosted by mixing both nutrients and seed communities from deeper water layers into the well-lit upper mixed layer. This strongly increases the likelihood for toxic cyanobacteria blooms and changes in biogeochemistry. Experimental results are consistent with a minimal, dynamical model indicating that extreme summer storms induce cascade effects altering ecosystem structure and functioning of lakes, even those that have been least affected by anthropogenic pressures in the past. As these are lakes of particular conservation value, this represents an additional challenge for future lake management.