The effects of eutrophication and browning on prey availability and performance of the three-spined stickleback (Gasterosteus aculeatus)

Sammanfattning: Shallow coastal areas often have high productivity and diversity, in part due to the high availability of light and nutrients. At the same time, they are exposed to multiple environmental pressures, such as browning and eutrophication. Browning is mainly caused by runoff bringing coloured dissolved organic matter (CDOM), reducing light availability in waters, whereas eutrophication is caused by high nutrient loading, leading to eutrophication symptoms such as algal blooms. Existing variation and further change in light and nutrients of coastal areas could have large implications for aquatic food webs, including fish. For instance, reduced light might alter food availability and reduce foraging abilities. Increased nutrient supply might, depending on the extent, instead increase food availability. In this study, I ask how browning and nutrient-enrichment, alone and in combination, affect benthic and pelagic invertebrate communities and how those changes, in turn, affect the performance of the three-spined stickleback (Gasterosteus aculeatus). The three-spined stickleback is not only a common model organism but also an increasingly common and important mesopredator in the Baltic Sea. Here, I performed a mesocosm experiment, including benthic and pelagic habitats and invertebrate prey, together with young-of-the-year three-spined stickleback as predators. In doing so, I examined the extent to which browning and nutrient-enrichment influenced the density, biomass and composition of stickleback prey, in addition to diet choice, body growth and condition of the stickleback. I found that prey availability increased with nutrient-enrichment but not with browning. The reason for the increase with added nutrients is most likely due to increased primary production, as evident in the much higher chlorophyll a concentrations. Accordingly, stickleback body growth was positively affected by nutrient-enrichment, probably because of increased food availability. In contrast, stickleback body growth and condition were negatively affected by browning, most likely due to negative effects of reduced visibility on feeding rates. Interestingly, nutrient-enrichment counteracted the negative effects of browning when combined. My findings add novel understandings about the potential for both eutrophication and browning to affect coastal food webs and fish body growth in the Baltic Sea.