Bacterioplankton dynamics driven by inter-annual variation in phytoplankton spring bloom communities in the Baltic Sea

Abstract

In the Baltic Sea, climate change has caused shifts in the phytoplankton spring bloom communities with co-occurrence of diatoms and dinoflagellates. Such changes likely affect the composition and function of associated bacterioplankton, key members of the carbon cycling, although the actual effects are unknown. To understand how changes in phytoplankton impact on bacterioplankton composition and function, we analysed bacterioplankton communities and their production during different phases of the spring bloom in four consecutive years across the Baltic Sea, and related them to environmental variables. Phytoplankton communities varied largely in composition, modifying the taxonomic structure and richness of the associated bacterioplankton assemblages. In presence of certain diatoms (Achnanthes taeniata, Skeletonema costatum and Chaetoceros spp.), bacterial production and diversity were high and with more relative abundance of Flavobacteriia, Gammaproteobacteria and Betaproteobacteria. This bacterial community structure correlated positively with high diatom biomass and with high bacterial production rates. In contrast, during dinoflagellate-dominated blooms or when the diatom Thalassiosira baltica was abundant, both bacterial production rates and diversity were low, with bacterial communities dominated by SAR11 and Rhodobacteraceae. Our results demonstrate that, changes in the phytoplankton spring bloom will have profound consequences on bacterial community structure and their role in carbon cycling.