15-year shift in protist communities and a decline of heterotrophic dinoflagellates in the Oslofjord and Skagerrak

Climate change induced warming of temperate coastal waters is expected to affect the biodiversity, community structure, seasonality and production in planktonic protist communities, and benefit heterotrophic taxa. We investigated long-term changes in the plankton protist community of the Outer Oslofjord by comparing two distinct periods across 15 years (2009–2011 vs. 2023–2024). Monthly 18S rRNA gene metabarcoding (V4 region) revealed dinoflagellates and diatoms as the dominating major taxonomic groups, with a 5.6-fold decrease in the dinoflagellate:diatom ratio and a 3.1-fold decrease in the heterotrophic dinoflagellate:phototrophic dinoflagellate ratio. These changes were primarily driven by a decrease in relative read abundance of the heterotrophic dinoflagellate genus Gyrodinium, and a widespread increase in the relative read abundances of diatoms (24 out of 27 ASVs with significant increase) between the time periods. Our results indicate that projected trends of heterotrophic protists profiting from changing oceanic conditions due to climate change, e.g. in temperate coastal waters, may not be the case for all coastal ecosystems.