Small phytoplankton community composition cycles annually with a coastal bloom

Small photosynthetic eukaryotes are a productive and dynamic component of marine planktonic communities. Here, we investigate how seasonal changes in abundance of these primary producers relate to changes in their community composition at a coastal site on the Northeast U.S. Shelf. We present a 9-yr time series of 18S rRNA sequencing data and identify gradual transitions within the pico- and nanoplankton community that occur repeatedly over the annual cycle. We then compare these compositional changes to concurrent high-resolution in situ flow cytometry measurements of eukaryotic phytoplankton abundance and division rate. We find that the Chlorophyta contribute a large proportion of the sequences in our samples and drive much of the seasonal variability within the small phytoplankton community. Across the time series, Bathycoccus, Micromonas , and Picochlorum are the dominant genera, with the first being present year round, while Micromonas bravo and Picochlorum are representative of the summer community. We also find a strong winter Phaeocystis signal which might be leading to flow cytometry measurements of relatively large cells in the early spring. Our results provide fundamental knowledge of the taxonomy of the phytoplankton community on the Northeast U.S. Shelf, improving our understanding of the region’s diversity and compositional variability over time.