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It is important to predict how phytoplankton will respond to global warming, as changes in their composition can affect ecosystem functions. We evaluated the effect of water warming on the taxonomic and functional composition of phytoplankton and on chemical characteristics that affect their occurrence, such as dissolved oxygen, pH and conductivity. Microcosms were constructed outdoors and monitored over time. The temperature was manipulated to simulate different scenarios predicted for the future. Warming caused a reduction in dissolved oxygen, while the pH and conductivity remained unchanged. We found a joint effect of temperature and time on chlorophyll-a as well as on the species and functional groups. The substitution of species and groups occurred in a similar way between treatments. However, a greater number of Cyanophyceae individuals were found at higher temperatures, while Bacillariophyceae and Euglenophyceae species were found more commonly in the lower warming treatments. These results indicate that warming altered the taxonomic and functional composition of phytoplankton, causing species substitution as well as a change in their functional characteristics, which led to the predominance of small organisms. Thus, contribute to predicting how an increase in temperature might alter the patterns of dominance, homogenization and community dynamics in future warming scenarios.