Limnetica 33

Reservoirs exhibit a marked degree of spatial heterogeneity, and environmental heterogeneity directly affects organism distribution and community composition. The objective of this study was to investigate the spatial horizontal heterogeneity of the planktonic communities along the longitudinal axis of the Guarapiranga reservoir (São Paulo, Brazil). We sampled nine stations along the Guarapiranga reservoir longitudinal axis for physical, chemical and biological variables. In addition, we analysed phytoplankton and zooplankton communities. To explore the data, we applied multivariate techniques, such as cluster hierarchical analysis and discriminant analysis. Our results showed a clear tendency towards compartmentalisation of the epilimnetic layers in this reservoir, especially when based on water quality. Three main compartments were observed: 1) the Embu-Graçu region, the upper part of the reservoir, more protected and less eutrophic; 2) the Parelheiros region, a very eutrophic branch of the reservoir; and 3) the lower part of the reservoir, with eutrophic and lacustrine characteristics. The groups formed by the cluster analysis based on the plankton data did not coincide with the groups based on water quality. These different outcomes indicate that the water quality, phytoplankton and zooplankton communities captured different features from the epilimnetic layer in the longitudinal axis of the Guarapiranga reservoir. Importantly, the phytoplankton and zooplankton communities in the dam region were altered, directly or indirectly, by copper sulphate treatment. Therefore, phytoplankton biomass is being driven by forces other than the forces intrinsic to the reservoir, such as human-induced losses. Consequently, zooplankton structure is being affected as well.