Phytoplankton assemblage structure in and around a massive under-ice bloom in the Chukchi Sea

Standard and imaging flow cytometry were used to examine the composition of phytoplankton assemblages in and around a massive under-ice bloom in the Chukchi Sea in 2011. In the core of this bloom, roughly 100 km northwest of Hanna Shoal, diatoms represented roughly 87% of the water column carbon-specific biomass of phytoplankton, while nanophytoplankton contributed ~9%. Picoeukaryotes were also observed in this bloom, as were phycoerythrin-containing cells consistent with Synechococcus spp., but picophytoplankton, dinoflagellates, and prymnesiophytes each represented only ~1% of the bloom׳s phytoplankton biomass. More broadly along this part of the Chukchi shelf, nanophytoplankton typically comprised a larger fraction of phytoplankton biomass in the water column, 22% on average but up to 82% at certain locations. Dinoflagellates and prymnesiophytes contributed at most 2% of water column biomass at any location and were most abundant in the deeper slope stations northeast of Hanna Shoal, east of the bloom. Picophytoplankton were most abundant in these deeper slope stations as well, and also in recently ice-free areas to the south around Hanna Shoal. These cell-derived estimates of phytoplankton carbon biomass, which were computed from imaging and standard cytometric observations of phytoplankton cell sizes and from published carbon:volume relationships, agree well with independent measurements of particulate organic carbon concentration from traditional biochemical assays.