The proportion of all Prymnesiales cells of the total phytoplankton volume was high in 2008, generally low in 2007 and 2009, and close to zero in 2010 (Fig. 4B). In April and May 2008, the Prymnesiales cells, most of which were larger than 6 μm, constituted between 30 and 80% of the total phytoplankton biovolume. In 2007 and 2009 some high values was observed in May at the southern monitoring sites (Fig. 5b). No other harmful algal species have been observed in high abundances in spring during the study period.