Plankton richness in a eutrophic reservoir (Barra Bonita Reservoir, SP, Brazil)

Abstract

Species richness of plankton was studied in a eutrophic reservoir (Barra Bonita Reservoir) of the Middle Tiete River, Sao Paulo State, Brazil, during the period 1985–1986. This reservoir is formed by two rivers: the Tiete and the Piracicaba (the main tributaries), of which the Tiete is more eutrophic, having conductivity and nutrient concentration values twice those of the Piracicaba. In addition, the reservoir is fed by 114 smaller tributaries. Monthly sampling was carried out at three stations representing different environmental conditions: St1 on the Tiete, S2 on the Piracicaba, and St3 at the confluence of the two rivers. For the phytoplankton community, the Piracicaba River (St2) proved the richest site, with a listed 72 species with abundance of Chlorophyta, while St1 and St3 registered 59 and 50 species, respectively, with abundance of Cyanophyta. For the zooplankton community a great difference was not registered in species number at the three stations but the species composition and dominant species of rotifers and copepods were quite different. The occurrence of Mesocyclops meridianus and Metacyclops mendocinus, specific for St2; Mesocyclops ogunnus and Notodiaptomus iheringi, specific for St1, and that of these two species plus Mesocyclops meridianus at St3 shows that the conditions combining at this station were favorable to Mesocyclops meridianus but not to Metacyclops mendocinus. Both for phytoplankton and zooplankton, high values found of species richness were compared to species richness of natural lakes, e.g., Dom Helvecio, a monomictic stable lake in eastern Brazil, and another fifteen lakes in the same region.