A whole-lake food web manipulation suggested that planktivorous fish can play an important role in regulating the pelagic food web structure of mesotrophic lakes. In this study, we examined the impact of golden shiners (Notemigonus crysoleucas) on zooplankton, ciliates, phytoplankton and nutrients. We conducted a mesocosm experiment using treatments with and without golden shiners with three replicates per treatment in summer. We monitored plankton and nutrient dynamics in these mesocosms for 6 weeks. Total macrozooplankton biomass and the proportion of large crustaceans decreased dramatically in the golden shiner treatment, while rotifier biomass decreased only in the second half of the experiment. In the mesocosms with golden shiners, total ciliate biovolume increased. However, the impact of golden shiners on ciliates was species specific. Chlorophyll a concentrations increased and dissolved nutrients (inorganic nitrogen and phosphorus) were statistically unaffected in the golden shiner treatment. This experiment showed that golden shiners had a strong negative impact on macrozooplankton, a variable impact on rotifers, weak positive impacts on ciliates and phytoplankton, and no discernible impact on dissolved inorganic nutrient concentrations. The results of this study help integrate aspects of previous research in mesotrophic lakes and provide evidence for cascading trophic interactions from fish to protozoans in a mesotrophic lake.