Widespread Aquatic Insect Responses to Recent Warming in Swiss Mountain Lakes

Mountain lakes are highly sensitive to climatic change, yet the extent to which alpine aquatic communities respond to recent warming remains uncertain. We used a palaeolimnological approach based on analysing the remains of non-biting midge (chironomid) larvae preserved in surface sediments from 24 Swiss mountain lakes to assess species-environment relationships of chironomids and compare our results to previous, detailed survey data from similar campaigns in 1993/2002. This allowed us to determine changes in chironomid assemblages relative to lake physicochemistry and increasing temperatures over the past decades. We show that high-elevation oligo- to mesotrophic lakes generally experienced a shift towards warmer chironomid assemblages, expansion of taxa with larger thermal range, and a simultaneous reduction in cold-stenothermic taxa, consistent with rising air and water temperatures. However, 9 out of 24 lakes, mainly in the lower-elevational range, exhibited stable assemblages or, in some cases, shifts towards colder chironomid communities. This is likely related to local catchment and lake conditions such as shading, changing human activities such as pasturing or hydrological inputs from snow and ice, indicating highly individual, lake-specific responses. Environmental parameters accounting for the highest variability in the modern distribution of chironomid assemblages include variables representing lakewater organic matter content and elevation, as well as oxygen, total nitrogen, and total phosphorus concentrations, demonstrating the sensitivity of chironomid assemblages to temperature and associated limnological variables. Overall, our findings highlight that both large-scale climatic drivers and local environmental heterogeneity shape chironomid assemblages in alpine environments, and that the majority of Swiss mountain lakes are showing responses in aquatic insect communities due to increasing temperatures. We conclude that shifts in chironomid populations are expected to further increase in amplitude under continued warming in the Alps, as rising temperatures increasingly affect alpine ecosystems and progressively cross critical thermal thresholds and tipping points.