16S rRNA amplicon sequencing of surface seawater samples collected at Helgoland during the spring bloom in the year 2018. Samples were successively filtered through 10, 3, and 0.2 µm pore-sized polycarbonate filters to separate the bulk of free-living (0.2 µm) from the bulk of particle-attached bacteria (3 and 10 µm). This sample refers to the 0.2 - 3 μm fraction. Sequencing was done on the Illumina HiSeq2500 sequencing platform in rapid mode with 2x 250 bp paired-end reads using primers targeting the V3-V4 region.