Variations in the Summer Phytoplankton Community Structure in Atlantic sub-Arctic and Arctic Waters

Shifts in phytoplankton community structure serve not only as indicators of environmental change but also have implications for food-web interactions and biogeochemical cycles. The community structure of marine phytoplankton in sub-Arctic and Arctic waters was examined using 159 samples collected in the summer of 2013 along a latitudinal gradient spanning from 61.1 to 83.1 degrees N along the east coast of Greenland. Accessory pigment concentrations were used to infer information about the phytoplankton taxa present using CHEMTAX (CHEMical TAXonomy), an iterative MATLAB subroutine. The main algal classes found within the study region were diatoms, dinoflagellates, haptophytes, chlorophytes, cryptophytes and prasinophytes. Diatoms were present at nearly all stations and depths and were large contributors to the total pigment biomass for both ice and open water stations. Deeper samples were mainly dominated by diatoms and haptophytes. Surface sample communities were characterised by mixed assemblages, including dinoflagellates and chlorophytes although diatoms and haptophytes still comprised a significant portion of the pigment biomass. The differences in community structure were investigated in relation to the environmental conditions through multivariate statistical analysis (cluster and principle component analyses) in order to understand the factors influencing the spatial distribution of the various algal classes. Diagnostic pigment indices were also used to calculate the concentration of Chl-a attributed to three size classes (picophytoplankton 0.2-2µm, nanophytoplankton 2-20µm and microphytoplankton >20µm). These data were compared to a similar dataset from the same cruise where size fractionated Chl-a was separated by sequential filtration and quantified by fluorometric analysis. Size-fractionated Chl-a as measured directly by sequential filtration suggested a primarily mixed community across the study region. In contrast pigment based analysis suggested a strong dominance of larger cells and also resulted in the complete absence of picoplankton in some samples. These results suggest that diagnostic pigment indices may not be an accurate method of determining size classes in this region.