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Phytoplankton composition in lotic environments has received less attention than that of lentic environments. The aim of the present study was to describe the micro-phytoplankton of the Contas River, which is one of the five most important water bodies in the state of Bahia, Brazil. Collections were carried out at 28 sampling sites at three-month intervals over a three-year period between December 2007 and September 2010. The micro-phytoplankton community was represented by 198 taxa distributed among Bacillariophyceae, Coscinodiscophyceae, Cyanophyceae, Fragilariophyceae, Chlorophyceae, Euglenophyceae, Dinophyceae, Cryptophyceae, Chrysophyceae and Xantophyceae. Geitlerinema amphibium (C. Agardh) Anagnostidis, Planktothrix agardhii (Gom.) Anagnostidis and Komárek, Pleurosira laevis (Ehrenberg) Compère and Ulnaria ulna (Nitzsch) P. Compère occurred in all samples. The majority of species (n = 111; 56.06% of the taxa) were classified as rare in the Contas River, as these organisms were only recorded during one or two months.

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This study aims to analyze the effects of nutrients and predation by zooplankton on phytoplankton biomass (chlorophyll <i>a</i>) in a eutrophic reservoir in Brazil (Apipucos Reservoir, State of Pernambuco), through experiments in microcosms. For this, samples of water were placed in 1 L Erlenmeyer flasks and kept for seven days. Treatments included the addition of nutrients (nitrogen combined with phosphorus and isolated additions of nitrogen and phosphorus), with presence and absence of zooplankton and a control which contained the reservoir water without any manipulation. The addition of nutrients did not stimulate phytoplankton growth. However, zooplankton significantly decreased phytoplankton biomass in the treatments it was added to (p < 0.05). The results of this study showed that for the reservoir studied, predation by zooplankton is the most significant factor in the regulation of phytoplankton, contradicting several studies which show that phytoplankton biomass is more strongly controlled by nutrients (bottom-up control) than by predation (top-down control)