Unexpected shift from cyanobacterial to dinoflagellate dominance due to a summer drought

The ruling paradigm is that future climate change scenarios will lead to an increase in the frequency, intensity, and duration of cyanobacterial blooms across the globe. It was therefore unexpected when, during an unusually warm and dry summer in southern Sweden in 2018, cyanobacteria did not dominate the phytoplankton community of the temperate Lake Vombsjön known for toxic cyanobacterial blooms. Instead, a potentially toxic dinoflagellate, Naiadinium polonicum, formed a large bloom and was dominant for the entire season. Dinoflagellates share important functional traits with cyanobacteria, but few studies have considered competition between dinoflagellates and cyanobacteria under future climate change scenarios. In order to understand the underlying factors leading to the N. polonicum bloom in 2018, we designed a study of Lake Vombsjön for an additional two years. When compared with subsequent years, 2018 was defined by an extended period of thermal stratification and hypoxia near the sediment surface. A significant positive relationship was also identified between N. polonicum biomass and both surface water temperature and soluble reactive phosphorus. We therefore suggest that the combination of higher temperatures and higher water column stability benefitted N. polonicum more so than cyanobacteria. Given that these factors are generally associated with cyanobacterial success, we conclude that freshwater phytoplankton community composition in temperate lakes can take different pathways in a warmer world.