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Laguna del Sauce, one of the main drinking water sources in Uruguay, is an eutrophic shallow lake with high temporal variation of inorganic turbidity caused by extreme wind events. During low turbidity periods, high phytoplankton biomass can be reached, frequently associated to cyanobacteria blooms, which can cause interferences in the water supply. In this study, we assessed the environmental drivers of cyanobacteria dominance, composition, and nitrogen-fixation behavior. For this, we analyzed the spatial and temporal phytoplankton composition, physical and chemical variables performing weekly samplings during two summers: 2015-2016 and 2016-2017. When inorganic turbidity was high (above 30 NTU), phytoplankton biomass was controlled, below this threshold, temperature, secchi depth and nutrients played key factors controlling cyanobacteria biomass and composition. Blooms of N 2 -fixing cyanobacteria (Dolichospermum crassum, Aphanizomenon gracile, and Cuspidothrix issatschenkoi) were promoted by low N:P ratios (average 11.5) and wide TN range (286-1300 lg l -1 ). Non-heterocystous cyanobacteria blooms occurred above TN

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