Earlier spring may explain dominance of the slow‐growing dinoflagellate Naiadinium polonicum over the cyanobacterium Microcystis during a heatwave

Global warming and extreme weather events have been linked to a rise in cyanobacterial blooms, which threaten water quality and ecosystem services. However, during an extreme hot and dry summer, Microcystis was outcompeted by a dinoflagellate, Naiadinium polonicum, in a drinking water lake in southern Sweden. Surface water temperatures were particularly high in May and July, peaking at 18.7 and 24.3°C, respectively. A 3-year field study of the lake further revealed surface water temperature to be a strong predictor of N. polonicum biomass, but not Microcystis biomass. To investigate the mechanisms underlying the success of N. polonicum over Microcystis in 2018, we examined the direct effects of temperature on the recruitment and growth rates of both species. Recruitment experiments were conducted using lake sediments containing resting stages, while growth rate experiments used strains of both species isolated from the lake. Recruitment of N. polonicum was temperature dependent, declining at temperatures above 12°C, whereas its growth rate peaked at 20°C. In contrast, Microcystis recruitment was unaffected by temperature, with optimal growth at the highest temperature used in this study, 28°C. We conclude that the extreme heatwave acted as an "early spring," advancing the recruitment window for N. polonicum but not for Microcystis. This shift in timing gave the slower-growing dinoflagellate a crucial head start, allowing it to accumulate biomass and dominate the community before peak summer temperatures set in. These findings highlight how climate-induced shifts in temperature regimes can restructure phytoplankton communities, altering bloom dynamics in unpredictable ways.