Phytoflagellate diversity in Roskilde Fjord (Denmark), including the description of Pyramimonas octopora sp. nov. (Pyramimonadales, Chlorophyta)

Identification of planktonic flagellates is challenging, and multiple species are often lumped together, limiting our understanding their diversity ecology. Our aim was to investigate changes in the phytoplankton community composition over annual cycle Roskilde Fjord, Denmark, focusing on identification organisms smaller than 20 µm. Weekly microscope pigment data indicated a low contribution diatoms dinoflagellates importance cryptophytes, euglenophytes green algae, with high contributions alloxanthin chlorophyll b. Community using Serial Dilution Cultures (SDC) confirmed prasinophytes community. In contrast, only played minor role. Seasonal were observed within Teleaulax dominance during cold months shifting Hemiselmis summer. Small pico-eukaryotes always present but had relative total biomass. Cultured enabled cf. cryptochromatica, recorded for first time European waters, an undescribed Pyramimonas species. We describe new as octopora sp. nov., placed subg. Vestigiferae, based morphological molecular (SSU rDNA) data. The distinctive features eight irregularly rounded perforations arranged square box corners around centre base plate scales, complex crown not previously described