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Water body eutrophication process has been a serious problem around the world, especially in semiarid and arid region where main approach to storage water is reservoir. The aim of this work was to investigate the spatial and seasonal variability in water quality of a reservoir in a semi-arid tropical region with regards to its trophic state, and to identify the sources of nutrient input to the reservoir. Essa pesquisa foi realizada no reservatorio Oros, segundo maior do estado do Ceara-Brasil. This research was developed in the Oros reservoir, the second one most important reservoir in Ceara State, Brazil. Samples were collected bimonthly during the period April 2008 to April 2010, from sex sampling points representing the outlet of tributaries, and one point down of the dam. The study included the following variables: total phosphorus, transparency and chlorophyll-a. Trophic state was evaluated by application of the Carlson modified Trophic State Index ( TSI). Maps of the spatial and seasonal dynamics of the water quality in the reservoir were constructed using GIS soft. According to the average values of total phosphorus (> 0.050 mg L-1), the waters can be classified as eutrophic. The highest concentrations of chlorophyll-a were found during the dry season, with values ranging from 3.8 to 26.5 mu g L-1, showing high temporal and spatial variations. The waters presented as eutrophic regarding average TSI, at all points sampled. The results indicate a deterioration in water quality and the need for intervention to reduce the release of waste, and thus improve the trophic state of the waters of the Oros reservoir.