Resilience of littoral food webs to oligotrophication and neozoa invasion

Abstract

Lakes are complex ecosystems that are essential to many organism groups. Over the last centuries, lakes have been faced with multiple anthropogenic pressures that have caused abiotic and biotic changes in the different lake habitats. For instance, anthropogenically derived increases in external nutrient loading caused eutrophication that was later reversed and caused lakes to return to their original oligotrophic status. Additionally, the introduction of non-native species has become a global threat also affecting various lake communities. While the effects of nutrient change (i.e. eu- and oligotrophication) and the introduction of non-native species on open-water (i.e. pelagic) communities and dynamics are often studied, the impact of trophic change and non-native species on near-shore (i.e. littoral) communities is much less analysed. However, the littoral zone is a highly diverse and structured habitat that provides crucial resources for various communities and therefore influences the whole lake ecosystem. The deep peri-alpine European Lake Constance is a prominent example of how anthropogenically caused nutrient shifts were reversed leading to re-oligotrophic conditions in the beginning of the 21st century. Here, I investigate the long-term (1997 to 2015) response of the littoral macroinvertebrate and fish communities of Lake Constance to declined nutrient concentrations from 18 to 6 µg l-1 and the introduction of non-native species. I analysed changes in the trophic structure of the macroinvertebrate community using the isotope values of native and invasive taxa exhibiting different feeding strategies (i.e. filter feeder, grazer, and predator) to determine the pattern of changing δ13C and δ15N values in Lake Constance. Observed changes in δ13C and δ15N of littoral invertebrates suggest a season-dependent shift in their feeding pattern and changes in carbon and nitrogen signature due to oligotrophication. Results also show that biodeposed material of the invasive mussel Dreissena polymorpha was used as additional food resources in the early 2000s, but not during the advanced oligotrophication process. This, in combination with a higher dependence on littoral carbon sources suggests that the littoral productivity became more relevant during oligotrophication in Lake Constance. To study the numerical and trait response of the littoral fish community, I analysed long-term monitoring data of catch per unit effort (CPUE) and length of 29 littoral fish species. The total CPUE was stable and, therefore, resilient towards environmental changes, whereas the community composition strongly changed during this period. The decrease of some species (i.e. bream, burbot, and ruffe) was compensated by increases of other species (i.e. dace and perch) resulting in significantly asynchronous long-term dynamics of species. Furthermore, despite large changes in the relative importance of species, species diversity and size diversity of fish were also rather stable during the investigated period. However, long-term dynamics of the species and size diversity relationship suggest decreased size-overlap between species resulting in lower functional redundancy and higher vulnerability of the community towards environmental stressors. Results of this study show that environmental changes such as oligotrophication and the introduction of non-native species influenced different fish and invertebrate species in the littoral community of Lake Constance. Nevertheless, on the community level fish showed resilient dynamics towards oligotrophication. In the invertebrate community, littoral productivity increasingly contributed to the energy flow, whereas the importance of additional food resources provided by invasive species decreased.