Spatial Diversity of Marine Protists Along the Northern Norway Fjords During the Sub-Arctic Summer.

Polar coastal waters are highly complex and productive ecosystems that experience increasingly higher average water temperatures, shallower and stronger stratification, and higher glacier-melting rates. Yet, the environmental complexity and high microbial diversity within fjords pose challenges to understanding the impacts of these changes and the ecological processes driving them. Here, we studied protists and prokaryote communities, their distribution, and functional ecology along the northern Norwegian coast in summer 2019 using SSU rRNA gene metabarcoding. Physicochemical data provided insights into the main drivers for differences in plankton communities. We found significant differences in alpha and beta diversity among the different fjords and between the western (Vesterålen, Balsfjorden, and Lyngenfjord) and northern areas (Porsangerfjord, Laksefjord, and Tanafjord). Temperature, salinity, and phosphate are the main drivers for community composition differences, positively correlating with the western fjords and negatively with the northern ones. Metabarcoding analyses revealed dinoflagellates, cryptophytes, and ochrophytes as the most abundant groups on the north Norwegian coast and confirmed the bloom of the ichthyotoxic haptophyte Chrysochromulina leadbeateri in Balsfjorden. Our comprehensive spatial study of communities and key contributors across different fjords set a baseline for future environmental monitoring of these unique and productive ecosystems, which are subject to rapid environmental changes.