Summer Phytoplankton Communities Across Contrasting Water Masses in the Fram Strait

In the Fram Strait, Atlantic Water (AW) flows northward through the eastern part of the strait, while Polar Surface Water (PSW) moves southward from the Arctic Ocean through its western part. Mixing of these water masses produces mixed AW (mAW). As these water masses move, significant changes occur in salinity, temperature, density, and nutrient concentrations, restructuring the phytoplankton community. Based on surveys conducted in August 2021 during the 84th cruise of the R/V Akademik Mstislav Keldysh, the dominant species were identified for each water mass. Among the key species (10 diatoms, 11 dinoflagellates, two coccolithophores, and one species each of chrysophyte, haptophyte, and ciliates), three ecological assemblages were distinguished: a cold-water PSW indicator complex (Porosira glacialis and Rhizosolenia hebetata f. hebetata among diatoms, the dinoflagellate Protoperidinium islandicum, and the ciliate Mesodinium rubrum); a warm-water AW complex (Navicula planamembranacea, Stephanopyxis turris, Rhizosolenia styliformis, the dinoflagellates Protoperidinium depressum and Tripos spp., and the coccolithophore Coccolithus pelagicus); and an mAW complex (the diatom Thalassiosira rotula, the coccolithophore Gephyrocapsa huxleyi, the haptophyte Phaeocystis pouchetii, and the chrysophyte Monochrysis sp.). Moving from south to north into the Fram Strait and from east to west towards the Greenland shelf, warm-water Atlantic indicator species were first replaced by mAW species, and then by cold-water species in the PSW. Maximum phytoplankton biomasses (up to 670.8 mg/m3) were observed in the marginal ice zone, driven by the intensive growth of large diatoms at depths within the euphotic zone where AW and PSW interact. If this interaction depth lies outside the euphotic zone, the phytoplankton biomass is determined by ciliates. In open water, large dinoflagellates contributed most to the biomass, indicating the decisive role of mixoplankton (mixotrophic plankton) in summer at high Arctic latitudes.