Biological interactions in the plankton community of a tropical eutrophic reservoir: is the phytoplankton controlled by zooplankton?

Phytoplankton regulation by zooplankton might be weaker in tropical systems than is generally found in temperate regions. In this study, we evaluated the potential effect of grazing by three microcrustaceans, Daphnia laevis, Moina micrura and Thermocyclops decipiens, on the phytoplankton community of Pampulha reservoir, a small tropical eutrophic reservoir. Four experiments were carried out over a season under laboratory conditions with the natural phytoplankton community. Selectivity coefficient and filtration rates were measured for each zooplankton species, and phytoplankton abundance was estimated. The three crustaceans grazed selectively upon the same phytoplankton species, although the species selected were seasonally different between the experiments. Nevertheless, most algal species did not show a significant decrease due to grazing. Only Daphnia laevis, in one of the experiments, produced evident changes in the structure of the dominant groups of the phytoplankton community. The abundance of total phytoplankton available for the microcrustaceans (1.00-4.49 mg C L(-1)) was always above the threshold food concentration values reported in the literature. Therefore, grazer production could be supported almost exclusively by phytoplankton during most of the year. Food abundance was probably the reason for the almost undetectable top-down effect by the microcrustaceans and the lack of differences among them. These observations provide evidence of the inability of zooplankton to shape and control the phytoplankton community in this tropical system. It is possible that other herbivores such as fish play the role of main consumers of the algal community.