Taxonomy as an indicator of trophic diversity in microbial eukaryotic plankton in an Arctic outflow gateway.

Marine microbial eukaryotes are major contributors to oceanic biodiversity and biogeochemical cycling, in both surface and deep waters yet their functional diversity and distribution remain incompletely understood. The Arctic Ocean, with strong environmental gradients, long-term basin isolation, and pronounced water mass stratification provides a model system to test how environmental filtering and evolutionary history might add to current understanding. We examined microbial eukaryote assemblages in the Arctic from Nares Strait into Baffin Bay, testing the hypothesis that dominant trophic strategies track water mass origin. Using high-throughput sequencing of the V4 region of the 18S rRNA gene across two size fractions (0.22–3 μm and 3–50 μm), we characterized communities associated with distinct Arctic water masses during summer. We then used the data set to ascertain the characteristics of rare taxa that dominated in few of the samples. Assemblages at in the four vertically stacked water masses were dominated by dinoflagellates, and heterotrophic lineages. Mixotrophic and phototrophic lineages were mostly confined to the euphotic zone in the absence of likely advective intrusions or sinking earlier blooms.