Limnetica 38

Herbivory in aquatic environments reflects the organization of trophic webs as a structuring factor in the communities of primary producers. We experimentally evaluated the influence of grazing by zooplankton and by different predator groups on the abundance and biomass of phytoplankton and morphologically-based functional groups (MBFGs), in a subtropical floodplain shallow lake (Brazil). We tested if different grazing levels modify the structure of the phytoplankton, considering that higher zooplankton abundance reduces the abundance and biomass of phytoplankton and the MBFGs. We used five treatments (5 L) with different zooplankton abundance and five replicas to represent an increasing grazing gradient. Sampling of phytoplankton and zooplankton was carried out after 24 hours, in each treatment. The zooplankton abundance presented significant differences among treatments (F = 11.79; p < 0.01), showing the evident grazing gradient in the experiment. Rotifers dominated all the treatments, followed by copepods, although their abundance decreased at the end of the experiment. In contrast, cladoceran abundance increased. An inverse and significant relationship was observed between zooplankton and rotifer abundances and MBFG III abundance (β = -0.85, p < 0.01; r2 = 0.32; β = -0.54; p < 0.01; r2 = 0.29, respectively), showing an effective predator-prey interaction. Many positive and significant relationships were also observed among the grazers and the prey. The increase of the abundance and biovolume of phytoplankton was related to the bottom-up mechanisms, the rapid population growth of prey, and the short time of the experiment. The change in the MBFGs’ contribution was related to the increase in the zooplankton and group abundance, grazer competition and exploitative competition among the MBFGs. The experiment contributes to the knowledge on an important part of the trophic dynamics among planktonic communities in subtropical floodplain shallow lakes.