The basic aim of the present investigation was related to the question: How important is the phytoplankton as a food resource for zooplankton in a tropical hypereutrophic reservoir? For answering this question, two different approaches were adopted. The first one included a regular monitoring program aimed to follow the composition and biomass of zooplankton as well as the major food resources, expressed in terms of C-biomass of sestonic particles. A second approach included the experimental determination of the balance between production and consumption of organic carbon in the planktonic community of Pampulha Reservoir. This was done through simultaneous determination of maximum potential primary production and assimilation rates of mesozooplankton estimated from respiration rates. The algal production was evaluated considering two different size fractions: a) nanno- (< 50 µm) and b) net phytoplankton (50-160 µm). The respiration of zooplankton community was restricted to larger organisms (mesozooplankton). These rates were determined at the laboratory under optimal conditions of illumination and temperature. Field data from the monitoring program showed that the limnetic zone of the reservoir supported a relatively low algal biomass for most parts of the seasonal cycle (< 30 µg.L-1). Except the short term algal (Microcystis spp.) blooming periods, chlorophyll-a concentrations in the water column remained at relatively low levels. The annual range of chlorophyll-a was 0-126 µg.L-1. Furthermore, field data also indicated that the bulk of the sestonic carbon was concentrated on non-pigmented particles. Evidences from laboratory suggested that the carbon demand of zooplankton could not be supplied by primary producers from the limnetic zone alone. The potential primary production ranged from low to moderate values (2.0-37.3 mgC. m-3h-1) and, as a consequence, assimilation rates of zooplankton remained higher than algal production (< 50 µm) for most parts of the year. Considering these evidences from field and laboratory, this study concludes that the energetic demand of (meso)zooplankton was, to a large extent, supplied by the detrital food chain.
primary production; secondary production; particulate organic carbon; zooplankton
