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The consequences of climate warming may be dramatic in lakes because changes in surface heating can affect physical and biological processes. We examined sedimentary remains of Cladocera and Chironomidae from a climatically sensitive lake in the Austrian Alps to test the hypothesis that changes in surface water temperature cause direct and indirect shifts in species composition. Contrary to the expectation that oxygen availability in lakes generally decreases under climate warming, our records showed that the assemblages experienced a succession towards a state with increased oxygen availability. Among the benthic fauna, an ecological threshold was crossed around the year 1850, when taxa tolerant of low oxygen concentrations dis- appeared and oxyphilous taxa began to dominate. In the zooplankton community, the keystone grazer Daphnia was replaced by Bosmina. These ecological changes, especially during the past 20 yr, were most likely caused by improved oxygen conditions following the decrease in depth of the thermocline. As the faunal succession was strongly correlated with alpine air temperature trends, the change in thermocline depth has likely been driven by the increasing air temperatures, which have caused warming of the epilimnion and consequently, in combination with wind- induced mixing, deepening of the thermocline. These recent changes occur alongside further increasing alpine temperatures and may be reflected in further deepening of the thermocline. We conclude that, in addition to direct climate influences, changes in summer stratification and mixing depth may cause significant changes in aquatic community compositions through changes in oxygen availability under the present climate warming conditions.

Climate-driven limnological changes determine ecological thresholds in an alpine lake