Fig. 1

Major phases of the spring phytoplankton bloom at Helgoland Roads during March to May 2020. A Biovolumes of abundant phytoplankton taxa and chlorophyll a concentrations as measured by HPLC (black line). B Biovolumes of less abundant algal taxa. C DAPI-based bacterial total cell counts (TCC; black line), flagellate counts (red line), and counts of copepods (excluding Copepoda nauplii; green line). Estimated phytoplankton biovolumes are plotted as a gray area in the background. D Abundances of the 18 most prominent bacterial clades as assessed by metagenomic 16S rRNA gene frequencies. The sum of bacteroidetal, alpha-, and gammaproteobacterial clades among these top 18 clades is depicted by lines. Bacterial total cell counts are represented as gray area in the background