An extreme drought event homogenises the diatom composition of two shallow lakes in southwest China

In the context of global warming, extreme drought events have occurred more frequently and this is projected to continue in the future in many parts of the world. However, their ecological consequences on shallow lakes are not well studied. Here we compare the succession of diatom assemblages of two medium-sized shallow lakes in southwest China during the past ~70 years, comprising an extreme drought event between ~2009 and ~2013. The diatom community of Lake Chahei showed a clear response to eutrophication from ~1980. The pioneer species Achnanthes minutissima increased strongly with elevated sedimentary total nitrogen content and at the expense of Epithemia sorex. In contrast, the diatom community of Lake Yuxian was stable and constantly dominated by the Fragilaria construens/pinnata complex. For both lakes, the most striking change occurred with the extreme drought event when environmental conditions and diatom compositions were homogenised. With strongly shrinking lake areas, peaks in the proportion of silt were observed in the sediments, suggesting increased particle burial rates and decreased light availability. These conditions selected for motile and presumably heterotrophic species mainly of the genera Navicula and Nitzschia which exhibited abrupt increases in their relative abundances at both lakes. Meanwhile, the degree of community homogeneity (inferred by Jaccard's similarity index) of the formerly distinct diatom assemblages increased strongly between both lakes. Hence, the extreme drought event has overridden and homogenised previous abiotic conditions and led to a decrease in beta diversity between the two study sites. Navicula and Nitzschia species may be reliable (paleo)indicators for extreme drought events in comparable shallow lake systems.