Vertical and temporal variability of protist and prokaryotic summer plankton communities in a North Sea fjord

Microbial plankton communities play key roles in biogeochemical cycling and primary production in marine coastal ecosystems. Given the impacts of global change on Norwegian coastal waters, there is a need to understand the drivers of microbial plankton community structure and diversity. Environmental drivers such as temperature, salinity, and light influence the dynamics of microbial community structure and abundance in temperate oceans. Here we characterize the summer diversity of protist and prokaryotic plankton communities, using DNA metabarcoding and light microscopy, in the surface, upper mixed layer, and deep waters of Spindsfjorden, a South Norwegian fjord facing the North Sea. The sampling site was vertically stratified throughout the summer, with compositionally variable communities dominated by phototrophs and mixotrophs in the surface and upper mixed layers, and stable communities dominated by heterotrophs and parasites in the deep layer. Late summer blooms were dominated by the diatom Cerataulina pelagica in the surface and dinoflagellates of Tripos spp. in the upper mixed layer. Positive co-occurrences between certain diatom taxa and flavobacteria, and between diatoms and Alphaproteobacteria suggest potential symbiotic relationships or overlapping environmental preferences. Negative associations were observed between certain dinoflagellate groups and Syndiniales, possibly due to parasitic interactions. Temperature emerged as a key environmental driver of community change, underscoring its role in shaping the dynamics of microbial communities in temperate, coastal ecosystems.