Effects of oligotrophication and Dreissena littoral-pelagic coupling on littoral invertebrate and fish communities: insights from stable isotopes of archived samples

Changing nutrient concentrations and introduction of non-native species affect the energy and nutrient fluxes in aquatic ecosystems. As a response, invertebrate communities could be altered, and 'novel' trophic communities could develop. Lake Constance (Europe) experienced pronounced nutrient variation caused by cultural eutrophication (prior 1980s) and oligotrophication (post-1980) and experienced many neobiota invasions. We used archived samples to investigate the impact of oligotrophication and invasive species on carbon and nitrogen isotope signatures of littoral macroinvertebrates during spring and autumn periods from 2000 to 2015 and of littoral fish species in 2018. Isotopic signatures of invertebrate and fish species were in the same range and showed similar seasonal differences and clear patterns in respect to littoral vs pelagic food sources and trophic level. Oligotrophication did not strongly alter isotopic values and only delta N-15 values of invertebrates declined weakly with oligotrophication. In contrast, interannual variability of stable isotope signatures was related to abundances of the invasive Dreissena polymorpha suggesting interannual differences in the importance of benthic-pelagic coupling due to Dreissena pseudofaeces for the nutrition of littoral invertebrates. This study illustrates how stored samples can be used to detect the importance of oligotrophication and benthic-pelagic coupling on the stable isotope signatures of a benthic community.