Topographically shaded lakes may provide refugia for cold-adapted aquatic fauna threatened by climate change

Abstract Understanding the structure and diversity of high-altitude lake communities and the patterns of their altitudinal changes is important for predicting their response to ongoing climate change. We analysed littoral benthic communities of 18 mountain lakes of glacial origin distributed along a 500-m altitudinal gradient and characterized by differing topographic shading levels: unshaded lakes and shaded lakes with a substantially lower (by 1151 h) mean annual duration of direct solar radiation. We hypothesized that local topographic shading modifies diversity–altitude relationships and affects the pattern of community turnover along the altitudinal gradient. We found a decreasing trend in diversity with increasing altitude and a significant deviation from that pattern in shaded lakes. Investigated lake groups supported distinct communities in lower altitudes. However, their community composition converged towards higher altitudes in communities typical for a greater abundance of cold stenotherms. The proportion of cold-stenothermal species increased with increasing altitude in shaded lakes and was notably greater than that in unshaded lakes along the studied altitudinal gradient. The lower temperatures of shaded lakes and different temperature variabilities of the two groups of lakes likely explain the observed patterns. We hypothesize that topographically shaded lakes may provide refugia for cold-stenothermal communities threatened by ongoing global warming.