An experimental study of the effects of fish zooplanktivory on the phytoplankton of a Brazilian reservoir

Two short-term (4–5 week) sets of enclosure experiments were conducted during winter periods (1982, 1983) in Americana Reservoir near Sao Paulo to test effects of fish predation reducing grazing pressure by large cladoceran zooplankton (mainlyDaphnia gessneri) on phytoplankton density, cell size distribution, biomass, species composition and richness. Two enclosures were stocked with the zooplanktivorous characin fishAstyanax whereas two others remained fishless during each set of experiments. Within two weeks or less, phytoplankton cell density and biomass were significantly reduced in the fishless enclosures, particularly in the small cell size (< 15 µm) fraction. Changes also occurred in species composition and richness. These shifts generally were maintained or intensified during the course of the experiments.