Little is known about the effects of microclimate on the phytoplankton community in Amazonian reservoirs. The reservoir of the Curuá-Una hydroelectric dam was built 39 years ago and is characterized by being flooded with standing vegetation, a fact that probably caused system eutrophication due to the decomposition of organic matter. This study aimed to evaluate the influence of microclimate in the composition and abundance of phytoplankton. Thus, 26 samples were collected on a weekly basis, from 24 July 2015 to 29 January 2016. To calculate the densities, the phytoplankton samples were counted under inverted light microscope. The following variables were measured: wind direction, wind speed, atmospheric pressure, humidity, air temperature, precipitation, pH, transparency, dissolved oxygen, water temperature, conductivity and turbidity. The taxa found belonged to eight divisions, and Bacillariophyta, Chlorophyta and Cyanophyta, were the groups with the highest densities. There was a trend in increasing the algal density as the water level of the reservoir decreased. The analysis revealed that 35.7% of the variation was explained by liminologic variables, 16.5% by climate variables and 42.6% by unknown factors that were not measured. Wind speed was, amongst the microclimatic variables, the most strongly correlated with the density variation observed. Thus, we can say that the microclimate had some influence on the phytoplankton density in the study period.
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