Satellite measurements of cyanobacterial bloom frequency in the Baltic Sea: interannual and spatial variability

Owing to the potentially harmful character of nitrogen-fixing filamentous cyanobacterial blooms in the Baltic Sea, a capacity to predict their occurrence is of interest. We quantified the surface accumulations of cyanobacteria, mainly Nodularia spumigena, using ocean colour satellite data. The spatial and temporal frequency of the accumulations was mapped with an automated detection algorithm, using their high reflectance in the 670 or 555 nm bands. Coastal Zone Color Scanner (CZCS) data were used for a first period (1979 to 1984), and combined SeaWiFS and MODIS-Aqua data for a second period (1998 to 2006). The frequency of cyanobacterial accumulations (FCA) for each 1 km2 pixel was calculated as the ratio of scenes with detected cyanobacteria to the number of valid cloud-free scenes in July-August. FCA varied greatly among years and the basins of the Baltic Sea. Mean FCA was 39% higher in the second period, but the increase was not statistically significant. The mean FCA for the whole Baltic was positively correlated with the residual phosphate (RP) concentration after the spring bloom in May-June. RP was a useful predictor of FCA in the coming summer, but could not predict the spatial pattern of the bloom. This was better explained by prevailing winds, with high FCA in the southwestern Baltic after northerly winds in July. It seems that, at the time of writing, useful FCA predictions can be made after the spring bloom, but only for the whole Baltic.