Haptophyta encompasses more than 300 species of mostly marine pico‐ and nanoplanktonic flagellates. Our aims were to investigate the Oslofjorden haptophyte diversity and vertical distribution by metabarcoding, and to improve the approach to study haptophyte community composition, richness and proportional abundance by comparing tworRNAmarkers and scanning electron microscopy (SEM). Samples were collected in August 2013 at the Outer Oslofjorden, Norway. TotalRNA/cDNAwas amplified by haptophyte‐specific primers targeting the V4 region of the 18S, and the D1‐D2 region of the 28SrRNA. Taxonomy was assigned using curated haptophyte reference databases and phylogenetic analyses. Both marker genes showed Chrysochromulinaceae and Prymnesiaceae to be the families with highest number of Operational Taxonomic Units (OTUs), as well as proportional abundance. The 18SrRNAdata set also containedOTUs assigned to eight supported and defined clades consisting of environmental sequences only, possibly representing novel lineages from family to class. We also recorded new species for the area. Comparing coccolithophores bySEMwith metabarcoding shows a good correspondence with the 18SrRNAgene proportional abundances. Our results contribute to link morphological and molecular data and 28S to 18SrRNAgene sequences of haptophytes without cultured representatives, and to improve metabarcoding methodology.

Blooms of ichthyotoxic microalgae pose a great challenge to the aquaculture industry world-wide, and there is a need for fast and specific methods for their detection and quantification in monitoring programs. In this study, quantitative real-time PCR (qPCR) assays for the detection and enumeration of three ichthyotoxic flagellates: the dinoflagellate Karenia mikimotoi (Miyake & Kominami ex Oda) Hansen & Moestrup and the two raphidophytes Heterosigma akashiwo (Hada) Hada ex Hara & Chihara and Fibrocapsa japonica Toriumi & Takano were developed. Further, a previously published qPCR assay for the dinoflagellate Karlodinium veneficum (Ballantine) Larsen was used. Monthly samples collected for three years (Aug 2009-Jun 2012) in outer Oslofjorden, Norway were analysed, and the results compared with light microscopy cell counts. The results indicate a higher sensitivity and a lower detection limit (down to 1 cell L-1) for both qPCR assays. Qualitative and semi-quantitative results were further compared with those obtained by environmental 454 high throughput sequencing (HTS, metabarcoding) and scanning electron microscopy (SEM) examination from the same samplings. All four species were detected by qPCR and HTS and/or SEM in outer Oslofjorden (Aug 2009-Jun 2012); Karlodinium veneficum was present year-round, whereas Karenia mikimotoi, Heterosigma akashiwo and Fibrocapsa japonica appeared mainly during the autumn in all three years. This is the first observation of Fibrocapsa japonica in Norwegian coastal waters. This species has previously been recorded off the Swedish west coast and German Bight, which may suggest a northward dispersal.

Viruses are a highly abundant, dynamic, and diverse component of planktonic communities that have key roles in marine ecosystems. We aimed to reveal the diversity and dynamics of marine large dsDNA viruses infecting algae in the Northern Skagerrak, South Norway through the year by metabarcoding, targeting the major capsid protein (MCP) and its correlation to protist diversity and dynamics. Metabarcoding results demonstrated a high diversity of algal viruses compared to previous metabarcoding surveys in Norwegian coastal waters. We obtained 313 putative algal virus operational taxonomic units (vOTUs), all classified by phylogenetic analyses to either the Phycodnaviridae or Mimiviridae families, most of them in clades without any cultured or environmental reference sequences. The viral community showed a clear temporal variation, with some vOTUs persisting for several months. The results indicate co-occurrences between abundant viruses and potential hosts during long periods. This study gives new insights into the virus-algal host dynamics and provides a baseline for future studies of algal virus diversity and temporal dynamics.

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.