In the Xingó reservoir, Aulacoseira granulata and Fragilaria crotonensis Kitton were considered very frequent, while the cyanobacteria Cylindrospermopsis raciborskii and Geitlerinema amphibium (C. Agardh) Anagnostidis, the diatoms Aulacoseira ambigua and Urosolenia longiseta (Zacharias) Bukhtiyarova and the green algae Planktosphaeria gelatinosa and Sphaerocystis schroeteri Chodat were frequent (Table 2). The very frequent and/or frequent species in both reservoirs, C. raciborskii, A. granulata,A. ambigua and P. gelatinosa, are cosmopolitan, widely distributed in freshwater ecosystems. C. raciborskii is widely studied since it forms dense blooms in water bodies and has the potential to produce toxins that are harmful to humans and other animals. It presents multiple adaptive strategies that enable it to tolerate wide ranges of environmental conditions, such as resistance to herbivory, tolerance to low light intensity, ability to migrate in the water column, ability to store and use intracellular stores, and the ability to fix atmospheric nitrogen (Ogawa and Carr, 1969; Bittencourt-Oliveira et al., 2011).