Diversity and biogeography of planktonic diatoms in Svalbard fjords: The role of dispersal and Arctic endemism in phytoplankton community structuring

Understanding the processes that shape the community structure of Arctic phytoplankton is crucial for predicting responses of Arctic ecosystems to the ongoing ocean warming. In particular, little is known about the importance of phytoplankton dispersal by the North Atlantic Current and the prevalence and maintenance of Arctic endemism. We investigated the diversity and biogeography of diatoms from five Svalbard fjords and the Hausgarten observatory (Fram Strait) by combining diatom cultivation and 18S rRNA gene metabarcoding. In total, 50 diatom strains were isolated from the area during the HE492 cruise in August 2017. The strains were identified taxonomically using molecular and morphological approaches, and their biogeographic distribution was mapped using the local metabarcoding dataset and a global compilation of published metabarcoding datasets. Biogeographic analysis was also conducted for the locally most abundant diatom metabarcoding amplicon sequence variants. The biogeographic analyses demonstrated that Arctic diatoms exhibit three general biogeographic distribution types: Arctic, Arctic- temperate, and cosmopolitan. At Hausgarten and in outer Isfjorden on the west coast of Svalbard, the communities were dominated by genotypes with Arctic-temperate and cosmopolitan distribution. Diatom communities in nearby Van Mijenfjorden, inner Isfjorden and Kongsfjorden were dominated by genotypes with Arctic-temperate distribution, and cosmopolitan species were less abundant. The genotypes endemic to the Arctic had lower abundance on the west coast of Svalbard. The two northernmost fjords (Woodfjorden and Wijdefjorden) had a higher abundance of genotypes endemic to the Arctic. Our results demonstrate that the diatom communities in the Svalbard area consist of genotypes endemic to the Arctic, and genotypes with broader biogeographic distribution, all of which are further structured by local environmental gradients. Finer biogeographic patterns observed within Arctic-temperate and cosmopolitan genotypes suggest that certain genotypes can be used as indicators of increasing influence of Atlantic waters on the phytoplankton community structure in the Svalbard area.