An invasive benthic fish magnifies trophic cascades and alters pelagic communities in an experimental freshwater system

Most studies of the consequences of nonnative fish introductions address only direct effects rather than cascading effects on lower trophic levels or other ecosystem compartments. These trophic cascades have not been studied in comparison to the effects of a functionally similar native species nor with consideration of bottom-up forces (e.g., nutrient supply). The Round Goby (Neogobius melanostomus) is an invasive benthic predator that can deplete local populations of grazers, potentially generating substantive increases in benthic algal biomass. Here, we tested, with a factorial experimental design, the influence of Round Gobies on the relative strength of top-down and bottom-up forces in benthic communities by manipulating consumer type (Round Goby or native Logperch (Percina caprodes), snails only, or controls where snails and fish were absent), nutrient addition, and the presence of an invasive suspension feeder (Quagga Mussel [Dreissena bugensis]) in freshwater mesocosms. We also investigated how these perturbations affect phytoplankton and zooplankton communities. We observed stronger top-down forces in the presence of Round Gobies vs native Logperch, and these forces resulted in reduced grazer abundance and a concomitant increase in benthic algal abundance. However, when Quagga Mussels were present, bottom-up forces dominated. Round Gobies had a significantly greater positive effect on phytoplankton than did native Logperch. Our results highlight the importance of measuring top-down and bottom-up forces in comparison with a trophically similar native species to elucidate the ramifying effects of a nonnative predator on food webs.