In-depth analysis of an alternate-stage Prymnesium polylepis (Haptophyta) bloom and long-term trends in abundance of Prymnesiales species in the Baltic Sea

In late autumn/winter 2007, high abundances of the alternate-stage Prymnesium polylepis were observed in many Baltic Sea areas, attaining bloom concentrations in spring 2008. To understand long-term variability in Prymnesiales density, we analysed changes in the abundance of different size classes of Prymnesiales using a long-term time series (1985-2008) for an inshore-offshore gradient in the northern Baltic proper. Further, to understand environmental conditions contributing to the P. polylepis bloom, we examined environmental factors associated with the increase in Prymnesiales abundances by placing the bloom dynamics within the context of temporal and spatial environmental variability over the last decades in the Baltic Sea. Significantly increasing abundances were found for larger size classes of Prymnesiales (6-10 mu m and >10 mu m) but not for the smaller species (Prymnesiales 10 mu m) that tolerate low annual NO2+NO3 concentrations. Thus, long-term trends in increase in abundance, as well as changes in the relative contributions of species within these potentially toxic haptophytes, are likely to be the result of interacting changes in temperature and nutrient conditions since the late 1980s.