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Here we examine the seasonal distribution of marine bacterioplankton species in the northern Baltic Sea. The population density of different bacteria was determined by whole-genome DNA hybridization to community DNA. During spring, concomitant with the phytoplankton spring bloom and its decay, the bacterial community was dominated by 5 bacterial species belonging to the Flexibacter-Cytophaga-Bacteroides phylum. Together they accounted for up to 43% of the intact cells as indicated by the number of nucleoid-containing cells at this time of year (20% of the total bacterial counts). In late June a number of new bacteria proliferated to form a summer community. The dominant bacteria during summer were largely members of the α-Proteobacteria, with a significant contribution of Sphingomonas and Caulobacter species. The capacity of these species to cope with oligotrophic growth conditions may explain …