| Summary: | Despite intense public and scientific interest in cyanobacterial blooms in the Baltic Sea, their origin and generation has not been well investigated. The aim of this work was to elucidate the overwintering strategies of Aphanizomenon, Dolichospermum and Nodularia. The presence of undetectably small start populations of these three genera in March 2009 and partly also in May 2006 was concluded from their growth in mesocosms. The results suggested that Aphanizomenon originates primarily from the surface-water population, whereas Dolichospermum from benthic akinetes. Nodularia was of particular interest as the potential of its extremely small winter/spring population to form large summer blooms was still under debate. The results verified that Nodularia does not develop in waters taken from below the halocline, but most likely overwinters in the upper water layers. This small planktonic start population alone, and not the benthic akinetes, may give rise to the summer blooms.
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