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Cyanobacteria, the algae that produce pond scum, routinely turn the Baltic Sea into something resembling a giant pond. On a regular basis, they coat a large part of the surface of the 377,000-square-kilometer (145,000-square-mile) body of water with greenish slime visible from space. That makes a slick almost the size of the United Kingdom.

Now researchers have found that the onslaught is indeed very regular. Mati Kahru, a research oceanographer at Scripps Institution of Oceanography at the University of California San Diego, led an international team of scientists in an analysis of 40 years of satellite data. They found that the algae were detected in very high concentrations every three years followed by one or two years of substantially lower concentrations.

What the researchers cannot do at the moment is understand why.

“I think that we have discovered a mysterious oscillation that we cannot explain now,” said Kahru. “It is caused by some sort of interaction between a set of factors, including different developmental stages of cyanobacteria themselves, zooplankton, bacteria, or viruses.”

The team detailed its findings April 23 in the journal Scientific Reports.

Natural resource managers from Lake Tahoe to the Mediterranean Sea have attempted to understand and control cyanobacteria blooms because of the damage they do to ecosystems and occasionally to human seafood consumers. The blooms starve seawater of oxygen.

In the Baltic, blooms are mostly caused by a species of Nodularia. This cyanobacterium produces a toxin called nodularin that attacks the liver and can sicken marine mammals or even humans when water containing this alga is swallowed.

Kahru and his colleagues ruled out physical factors such as seawater temperature, salinity, amount of daily sunlight, winds, and circulation as factors causing regular blooms in a sea bounded by Sweden to the west and Finland, Estonia, Latvia, and Lithuania to the east.  To grazers such as zooplankton, they are a food of last resort, so consumption of cyanobacteria does not appear to be a factor controlling their population density.

Though climate change is often cited as a cause of toxic algal blooms elsewhere in the world, it is not the culprit in the Baltic Sea. Sediment samples show evidence that cyanobacteria blooms have been a regular feature there for at least 5,000 years.

That’s not to say that human activities haven’t helped the cyanobacteria to flourish there. Agricultural runoff and sewage feed blooms of all phytoplankton. Attempts to mitigate them have actually made things worse. Officials from countries around the Baltic have attempted to limit the use of nitrogen in the runoff that drains to the sea but that instead has allowed the cyanobacteria to become dominant thanks to the algae’s ability to produce their own nitrogen when needed. That ability disadvantages other organisms.

Co-authors of the study, funded mostly by Stockholm University, suspect that most likely the cyanobacteria control their own destiny. At any given time, a certain population of the algae lies dormant on the seafloor while another subset is active in the water column. It is possible, the researchers said, that regular blooms are the result of some sort of interaction between these two populations.

In addition to Kahru, report co-authors include Scripps Oceanography alumnus Emanuele di Lorenzo, now of Georgia Tech University, and Ragnar Elmgren and Oleg Savchuk of Stockholm University.