Cyanobacterial bloom in the Baltic Sea July 11, 2005. Satellite image from NASA’ s Terra satellite, MODIS instrument.
Cyanobacterial bloom in the Baltic Sea July 11, 2005. Satellite image from NASA’ s Terra satellite, MODIS instrument.

Using satellite images from 1979-2013, researchers from Stockholm University and Scripps Institution of Oceanography show that surface accumulations of cyanobacteria (formerly called blue-green algae) in the Baltic Sea have occurred progressively earlier.  At the start of the period, the blooms were centred about 8 August, but now as early as around July 19, almost 3 weeks earlier.

– The surface accumulations are dominated by the toxic cyanobacterium Nodularia spumigena. When they drift ashore, they can spoil bathing opportunities and at worst can kill animals, such as dogs. As a result, the main vacation period in Sweden is now more affected by these unpleasant surface scums, says Ragnar Elmgren at the Department of Ecology, Environment and Plant Sciences and the Baltic Sea Centre of Stockholm University.

The researchers had to pool observations from a series of satellites with different measurement sensitivity. By comparing results from periods when observations were made simultaneously by at least two different types, they were able to calibrate the measurements well enough to create a reliable 35-year time series

– The results agree well with observations from ships, but the surface accumulations occur so patchily over the surface of the Baltic, that ship observation cannot provide a reliable overall picture of their distribution. This requires remote sensing approaches, preferably by use of satellite observations, says Ragnar Elmgren.

The study also found that the surface accumulations were more prevalent during July-August in the second half of the study period, whether measured as the average frequency of accumulations or as the total area affected at any time in the season.  It was already known that the water in the Baltic now heats up more rapidly in spring, and has become a few degrees warmer in summer, and that the spring phytoplankton bloom is also occurring earlier.   It is, however, still too early to say conclusively why the summer accumulations have become earlier.

– That the accumulations are now earlier and larger can either be because the cyanobacterial species that dominates them has become more common, or because the weather is now calmer, so that they float to the surface more often. Now that the time series had been compiled, research has started to answer such question, as part of Stockholm University’s research program in support of ecosystem-based management of the Baltic Sea, says Ragnar Elmgren.

Public discussions of climate change often concern how the climate and hence nature will be affected in the long run.

– Such predictions of the far future are necessarily based on complicated climate models with large uncertainties. They are therefore often perceived as so uncertain that there is no need to bother. Our results underline that the effects of climate change are not only a worry for the distant future, they already affect our vacations, concludes Ragnar Elmgren.

Link to the article in Biogeosciences

For further information
Professor Ragnar Elmgren, Dept Ecology, Environment and Plant Sciences and Baltic Sea Centre, Stockholm University, Telephone 08-16 40 16, cell 0707- 47 79 33, e-mail

Mati Kahru, Researcher, Integrative Oceanography Division, Scripps Institution of Oceanography University of California, San Diego, (visiting scientist at Stockholm University), e-mail