In the early spring 2008, the plankton monitoring programme detected very high abundances of the alternate form of the potentially toxic haptophyte P. polylepis in the southern, central and northwestern Baltic Proper. Only low abundances and absences were recorded in the northeastern Baltic Proper, Gulf of Finland and Gulf of Riga. In 2007, 2009 and 2010 only low concentrations of P. polylepis cells were recorded in the Baltic Sea. Hence, the temporal and the spatial distribution of the P. polylepis bloom in the Baltic Sea closely matched the observed distribution of extensive non-breeding in Baltic eider colonies. However, because a correlative match alone does not imply causation we discuss other possible alternative causes for the exceptional decline of nesting eiders in 2008 in parts of the Baltic Sea.