Effects of multiple abiotic stressors on the species and genetic biodiversity of littoral Cladocera in two types of acidic habitats in Germany

Ecological research on acidic lakes has historically focused on soft-waters, whereas biota in natural and man-made acidic hard-water lakes remains poorly studied. My thesis deals with the effects of multiple abiotic stressors on species and genetic biodiversity of littoral Cladocera in two types of acidic waters in Germany: hard-water mining and soft-water bog lakes. I found that these two types of acidic lakes supported highly distinct cladoceran communities, both with respect to their species diversity and composition, what led to the rejection of my null hypothesis that pH was the only abiotic factor, determining species diversity patterns. Further analyses revealed that, apart from pH, TDS-related parameters and trophy significantly affected species composition of littoral Cladocera in the studied lakes. Particularly the role of TDS and its constituents for colonization of acidic lakes was virtually neglected by previous studies, based on soft-water lakes’ data sets. My study on the genetic diversity of a generalist cladoceran Chydorus sphaericus provided evidence for genetic erosion, i. e. negative effects of abiotic stress in the acidic hard-water mining lakes, but not in the acidic soft-water bog lakes. I conclude that the observed genetic erosion in acidic mining lakes may have negative long-term consequences for population persistence even in broadly tolerant species. In general, biodiversity at both species and genetic levels was much more impaired in the acidic hard-water mining lakes, compared to the naturally acidic soft-water bog lakes. This was possibly due to both ecological (higher abiotic stress) and evolutionary (insufficient time for long-term adaptations to evolve) mechanisms. My results suggest that colonization of acidic hard- and soft-water lakes in each case should require specific physiological adaptations at both species and population levels. The practical relevance of my studies is that they provide the scientific basis for monitoring of acidic hard-water mining lakes based on littoral Cladocera. Especially genetic diversity proved a valuable indicator of water quality in acidic mining lakes, as it provided unique information on population-level responses, being complementary to species diversity assessment. Overall, my results demonstrate that inclusion of acidic hard-water lakes into the research framework as well as comparisons among communities in chemically diverse acidic lakes can result in important new insights, concerning the community organization and adaptations in acidic environments.