Evaluating the response of Cladocera to recent environmental changes in lakes from the central Canadian Arctic treeline region

Arctic and subarctic ecosystems have undergone considerable environmental changes in recent years as the result of climate warming. Fossil records of freshwater diatoms in lakes throughout the circumpolar Arctic, including the central Canadian Arctic treeline region, have revealed marked directional shifts in diatom assemblages in recent lake sediments. These algal changes have been linked to longer growing seasons, decreased duration of ice cover, and/or increased thermal stability. The effects of these recent environmental changes on higher trophic levels, such as the Cladocera, are still unclear. Using cladoceran remains preserved in the sediments of 50 lakes, which were previously examined for changes in diatoms, we show that significant changes in cladoceran species composition have occurred from pre-industrial times to the present. However, these changes are considerably muted compared to the more substantial changes observed in the diatom record. We found no consistent patterns of change in planktonic cladocerans (i.e. Daphnia, Bosmina) within our study lakes, and the response of the Cladocera to environmental changes does not appear to be strongly coupled to recent changes in diatom communities, thus further confirming that the previously observed diatom changes were related to bottom-up limnological controls. These results highlight the complex response of Arctic freshwater food webs to climate change, and the need for incorporating multi-trophic studies into climate change investigations.