2Universidade Federal Rural de Pernambuco, Departamento de Biologia,

Reis Lake is located in the municipality of Caracaraí, state of Roraima (Brazil) and is subject to fl uctuations in water level. The aim of this study was to analyze the structure of the phytoplankton community on the nictemeral and seasonal scales and determined the infl uence of limnological variables. Sampling was performed in the rainy season (June/2006) and dry season (November/2006), considering two nictemeral cycles. The phytoplankton community was assessed with regard to composition and density, abiotic variables were analyzed simultaneously. The lake had low concentrations of oxygen, clinograde profi le and water stratifi ed during the day and homogenous at night, with low concentrations of nutrients and waters ranging from slightly acidic to alkaline. The phytoplankton was represented by 43 taxa, 35 species in the dry season and 29 species in the rainy season. Low densities of phytoplankton occurred in both nictemeral cycles, with accentuated vertical gradient. The highest densities were recorded in the dry season. Reis Lake exhibits characteristics that classify it as a polymythic and oligotrophic environment. The variability in the data was more important seasonally than on the nictemeral scale, supporting the hypothesis of the infl uence of the hydrological cycle on the dynamics of phytoplankton communities in fl oodplain lakes