Phytoplankton-zooplankton interactions at Duas Bocas Reservoir, Espírito Santo State, Brazil: Growth responses in the absence of grazing

Abstract

The aim of this study was to analyze phytoplankton development in the absence of grazing pressure in the Duas Bocas Reservoir (DBR). The DBR is located in the Duas Bocas Biological Reserve and supplies some of the water demand of Great Vitoria, Espirito Santo State. On September 25, 2008, 40 L of water were collected below the water’s surface, from the lacustrine region of the reservoir. In the laboratory, six microcosms were established (glass boxes), each with a six-liter volume of water, and were maintained for seven days. Three microcosms served as controls (C). In the other three microcosms, zooplankton was removed by water filtration in a plankton net with a 60-μm mesh size. A positive trend in cyanobacteria and chlorophyte growth was found in the absence of grazing.  Cyanobacteria benefited under herbivore feeding pressure (Controls) to the detriment of chlorophytes. However, in the absence of grazing pressure, the competition between these groups was sometimes higher, resulting in an alternative dominance. The chlorophyte Monoraphidium griffithii was dominant in both microcosms. Chlorella vulgaris was abundant, and the cyanobacteria Limnothrix bicudoi and Synechocystis sp. were abundant in both microcosms. No significant phytoplankton growth was found in the absence of grazing pressure. Then, other environmental factors might act as important regulators of phytoplankton growth in the DBR. Key words: herbivory, microcosms, plankton, tropical reservoir.