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

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, whereas increased nutrient supply might, depending on the extent, increase food availability. In this study, we performed a mesocosm experiment, including benthic and pelagic communities, together with young-of-the-year three-spined stickleback (Gasterosteus aculeatus) as predators. The three-spined stickleback is not only a common model organism but also an increasingly common and important mesopredator in the Baltic Sea. We examined the extent to which browning and nutrient enrichment, alone and in combination, influenced the density, biomass, and composition of stickleback prey, and diet choice, body growth and condition of the stickleback. Stickleback body growth was positively affected by nutrient-enrichment, probably because of a positive bottom-up effect with increased primary production, as evident in the much higher chlorophyll -a concentrations in the pelagic habitat, and increased food availability. In contrast, there was a marginal negative effect of browning on stickleback body growth and condition, most likely due to negative effects of reduced visibility on feeding rates. We also found that prey availability increased with nutrient-enrichment but not with browning. Interestingly, nutrient-enrichment counteracted the negative effects of browning when combined. Our 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