Increased brownification of freshwaters has been noticed during recent decades and is predicted to continue with climate change and increased precipitation. Cyanobacteria have been observed to benefit from increased brownification, posing a possible future threat to drinking water sources by increased toxic microcystin production. A mesocosm study was conducted during the summer of 2018 in Lake Bolmen, one of southern Sweden’s most important drinking water reservoirs, to test the effect of predicted future brownification (in 50 and 100 years) on extracellular (free) microcystin concentrations. The results of this study show that future brownification alone of oligotrophic freshwaters seem not to result in increased extracellular microcystin concentrations. Despite variability of temperature during the experiment, temperature and extracellular microcystin concentrations were not correlated in this study. Although intracellular (cell bound) microcystin should be measured before any certain assumptions of the risk are drawn, the concentrations of extracellular microcystin were in general very low and significantly under the guideline value of 1 µg/L provided from the WHO (World Health Organisation). These results are very positive for Lake Bolmen from a drinking water perspective. My results suggest that browning, even in combination with relatively high temperatures, poses a low threat to Lake Bolmen’s drinking water quality. However, an increase of both eutrophication and increased brownification may pose a bigger general threat regarding cyanobacterial toxin production than brownification alone will do.