Effects of re-oligotrophication and invasive species on fish-zooplankton interactions

Lake ecosystems perform essential environmental services for humans and other organisms. They provide drinking water, habitat for aquatic animals and plants, recycle nutrients and are vital for fisheries, leisure activities, and economics. With the increasing world population and development, the negative human impact on lakes is becoming a problem of high priority. To improve management and prevent further deterioration of aquatic ecosystems, substantial effort is invested in research to better understand their biological processes and environmental stressors. Change of lake trophic stage and invasive species are among the most substantial stressors for lakes. In pelagial, they lead to rewiring the food web, change of productivity, and energy transfer, which can adversely affect fisheries and native organisms. Lake Constance, located on the borders between Germany, Switzerland, and Austria, has been intensively researched regarding biotic response to nutrient concentrations and invasion of new species. Still, with recent pelagic food web changes, biotic responses have to be re-evaluated. During the re-oligotrophication of Lake Constance (phosphorus value dropped from 80 μg/L in the 1970s to below 10 μg/L nowadays), primary production declined. This affected growth and yield of whitefish (Coregonus wartmanni), which is the most important fish species for the local fishery in Lake Constance and presumably exerts intense predation pressure on the zooplankton community. An additional threat for whitefish is the invasion of three-spined stickleback (Gasterosteus aculeatus), which firstly appeared in the pelagic zone in 2012/2013 and quickly outnumbered the native whitefish population and presumably rewired the pelagic food web. This thesis investigated whitefish and invasive stickleback diet and their top-down effects on lower trophic levels to understand fish-zooplankton interactions related to stickleback invasion and re-oligotrophication changes. To better understand interspecific feeding differences, we compared native whitefish and invasive stickleback prey selection and feeding rates on different zooplankton species in aquaria experiments using small fish ranging from 2 to 8 cm. The experiments did not reveal any difference in both fish species diet breadth and only small differences in feeding rates on zooplankton species. However, sticklebacks were selecting larger prey, and there were important changes in feeding rate with fish size. In a mesocosm experiment, we compared both fish species top-down effects on lower trophic levels. The results show comparable effects of both fish species, with intense predation on large daphniids. Large daphniids had a substantial impact on the phytoplankton community and were therefore identified as a keystone species for linking fish with primary producers in a trophic cascade. 2 To investigate the seasonal diet of whitefish and stickleback, fishing was performed monthly. There was a high diet overlap between both fish species for most of the year. The same prey species had the highest relative importance index in the fish diet and were therefore also positively selected. The most noticeable difference was in winter, when whitefish mostly stopped feeding, whereas sticklebacks intensively predated on copepods. To disentangle stickleback effects on the zooplankton community from other (a)biotic changes of the lake, we used long-term data where we compared two periods, before (2006–2011) and after stickleback invasion (2012–2017). We have shown that phosphorus concentrations, water temperature, and chlorophyll a did not differ between these two periods. However, herbivorous cladocerans size decreased, and the abundance of small zooplankton species increased as a consequence of stickleback invasion. This thesis shows that native whitefish and invasive sticklebacks both prefer large zooplankton prey. They do not differ strongly in their feeding abilities and top-down effects on zooplankton when comparing the same sizes. However, the stickleback invasion had a more substantial impact on fish-zooplankton interactions than re-oligotrophication and other (a)biotic changes, which is presumably a consequence of the changed size structure of the pelagic fish community and increased predation pressure on zooplankton.publishe