In this study we combine abundance estimates of the potentially toxic P. polylepis, the total abundance of Prymnesiales cells and data on the number of nesting eiders from 2007 to 2010 at selected monitoring and nesting sites in the Baltic Sea. We especially analyse the temporal and spatial relationship between spring cell concentrations of P. polylepis and other Prymnesiales and the number of nesting female eiders. We discuss possible alternative causes for the observed dramatic and synchronous decline of nesting eiders in the central and southern part of the Baltic Proper in the spring 2008. We suggest that the intensive spring bloom of P. polylepis affected the quality of the main benthic food for eiders, i.e. blue mussels, and, subsequently, the body condition of female eiders and their breeding propensity.