Despite their importance for ecosystem functioning, little is known about the composition of active marine bacterioplankton communities. Hence, this study was focused on assessing the diversity of these communities in the southern North Sea and examining the impact of a phytoplankton spring bloom on the ambient bacterioplankton community. Community composition in and outside the bloom was assessed in 14 samples by pyrosequencing-based analysis of 16S rRNA amplicons generated from environmental RNA. The dataset comprised 211,769 16S rRNA sequences. Proteobacteria were the predominant phylogenetic group with Alphaproteobacteria and Gammaproteobacteria as the most abundant classes. Actinobacteria and Bacteroidetes were identified in minor abundances. Active bacterial communities were dominated by few lineages such as the Roseobacter RCA cluster and the SAR92 clade. Community structures of three selected samples were also assessed by direct sequencing of cDNA generated from rRNA-depleted environmental RNA. Generated datasets comprised 988,202 sequences. Taxonomic assignment of the reads confirmed the predominance of Proteobacteria. The phytoplankton spring bloom affected the bacterioplankton community structures significantly. Bacterial richness was reduced in the bloom area and the abundance of certain bacterial groups was affected by bloom presence. The SAR92 clade and the Roseobacter RCA cluster were significantly more abundant and active in the bloom. Functions affected by the bloom include photosynthesis, protein metabolism, and DNA metabolism. This article is protected by copyright. All rights reserved.