The impact of fish and cyclopoid copepod predation on zooplankton communities was evaluated using large-volume mesocosms (depth 9.5 m; volume 13 m3) in the Římov reservoir (Czech Republic). Two yearling roach and perch individuals introduced into mesocosms represented the fish treatment, which was compared to cyclopoid copepods (initial abundance of 2 ind.L−1) and a control with no initial addition of predators. Our results clearly support the hypothesis that planktivorous fish feeding leads to the suppression of large-bodied cladocerans. In the presence of fish, the cladoceran community changed from a dominance of large-bodied Daphnia spp. at the beginning to dominance by the smaller Bosmina longirostris at the end of the experiment. Chlorophyll-a concentration and rotifer abundances increased in the absence of daphnids. In the absence of fish, the presence of large-bodied cladocerans resulted in decreasing chlorophyll-a concentration. Although no significant differences were observed between cyclopoid abundances in treatments stocked with cyclopoids and the control, the proportion of large cladocerans clearly showed the effect of the manipulation. The similar trends in both of these treatments did not confirm the importance of cyclopoid predation in our experiment. The overall strong effect of fish over cyclopoid predation suggests the main role of fish predation in the forming of zooplankton communities and in turn impacting phytoplankton biomass in mesocosms.

Nutrient manipulation experiments were conducted on a natural planktonic community in outdoor mesocosms. Inorganic nitrogen (N) and silicon (Si) were added to achieve N[ratio ]Si ratios of 1[ratio ]1 and 4[ratio ]1. Total particulate carbon (PC) biomass of the microbial assemblage was determined by elemental analysis. Cell volume measurements by microscope on individual components of the community (bacteria, diatoms, photosynthetic nanoflagellates, heterotrophic nanoflagellates, dinoflagellates and ciliates) were also made. We applied published C[ratio ]volume relationships to determine the volume estimated C content (CBV) of these microbial groups and hence of the total assemblage. The total CBV and total PC were compared to test the applicability of C[ratio ]volume relationships under different nutrient regimes both before and after nutrient exhaustion. For initial N[ratio ]Si ratios of 1[ratio ]1, prior to nutrient exhaustion, the relationship between CBV and PC was linear with a gradient of approximately 1, (0.99±0.06), indicating that the published C[ratio ]volume relationships accurately predicted the C content of the microbial assemblage. For N[ratio ]Si ratios of 4[ratio ]1, a linear relationship was again evident between CBV and PC (slope: 1.36±0.08). However, statistical comparison using a general linear model indicated that the gradient of this relationship differed significantly from that when the N[ratio ]Si ratio was 1[ratio ]1, and hence CBV overestimated elemental C. For both N[ratio ]Si ratios, subsequent to nutrient exhaustion (N or Si), and hence when the diatom fraction of the microbial assemblage was in yield-limited post-exponential phase, the two measures of biomass were not well correlated. This indicated that measured cytoplasmic cell volume was a poor indicator of C biomass within the microbial assemblage in nutrient-deplete conditions.