What initiates cyanobacterial blooms in the Baltic Sea?

Massive summer blooms of cyanobacteria are a regular feature of the Baltic Sea, with impacts on primary production, nitrogen cycling, food chain structure and bottom layer oxygen deficiency. While many attempts have been made to associate the initiation of those blooms with certain forcing factors, the resulting models have had very limited predictive power. Here we analyse the statistical relationships between the timing of cyanobacteria blooms in the Baltic Sea and various forcing factors. Our analysis is based on a 25-year time series (2000-2024) of satellite-derived frequency of cyanobacteria accumulations (FCA) in 36 characteristic areas. We find that sea surface temperature (SST) does not control the initiation of a bloom but, on the contrary, SST during bloom initiation is a function of the bloom timing. Bloom initiation is associated with increasing SST caused by surface irradiance, coupled with low wind speed that results in reduced vertical mixing. However, while this combination of factors is required, it is not always sufficient to start a bloom, suggesting that additional factors or interactions are required.