The Arctic is a region undergoing unprecedented and unequivocal climate change. The seas of this extreme region form a major component of the oceanic thermohaline conveyor and natural carbon cycle. Using a combination of recent and historical datasets this study examines the distribution, diversity, photophysiology and primary productivity of phytoplankton in the Atlantic sector of the Arctic Ocean. CHEMTAX analysis reveals a diverse phytoplankton community structure in the Greenland Sea comprising six main phytoplankton groups. The influence of sea-ice and water column stratification are key factors in the presence or absence of groups such as haptophytes and prasinophytes. Group-specific differences are observed in spectral absorption and photophysiological parameters. However, the influence of environmental factors has a stronger influence than taxonomic composition on photophysiology. A clear division between the photoacclimatory response of algal communities beneath sea-ice and those of open-ocean stations is predominantly due to ‘E_k independent’ photoacclimation beneath sea-ice. This occurs due to the combined effect of sea-ice decreasing irradiance entering the water column and a positive correlation between P_m ^B and temperature. This variation in photophysiology is important for primary production models as a sensitivity analysis shows that errors in these parameters propagate to give the largest final errors in primary production values. The importance of other model parameters varies with the level of biomass in the water column and the presence or absence of sea ice. Accelerated ice-melt and an increase in open water due to climate change are likely to increase primary production in the Atlantic Arctic alongside an altered distribution of phytoplankton groups, with an increase in the importance of prasinophytes or haptophytes.

Phytoplankton play a key role in the geochemical cycles of the Earth, are responsible for 50% of global carbon fixation, and through this, provide almost all of the energy for the entire marine trophic system. Understanding the dynamics of phytoplankton, and the species composition in relation to environmental factors is therefore of great importance. In this thesis a range of techniques to identify phytoplankton groups that use accessory pigment data obtained from high performance liquid chromatography are compared. While fixed indicator pigment:chl-a ratio approaches provide a quick and simple way of estimating phytoplankton distributions either on the basis of size-class or taxonomic group, the more sophisticated iterative approach of CHEMTAX divides the biomass into more categories and allows more flexibility to adapt to changes in indicator pigment:chl-a ratios caused by environmental variability. Combined with flow cytometric cell counts, depth-dependent trends in the intracellular concentration and composition of phytoplankton pigments can be identified. These data show an exponential decrease in the ratio of carbon-to-chlorophyll with depth, in response to decreasing light intensity. The relative as well as absolute concentrations of phytoplankton pigments are also seen to change with depth, particularly under stratified conditions, with the ratio of zeaxanthin to chlorophyll-a decreasing with increasing depth, and the ratio of chlorophyll-b to chlorophyll-a increasing. Cluster analysis is used to identify the main phytoplankton populations in the North West Atlantic, with communities of large, fucoxanthin-containing phytoplankton dominating in spring when mixing is strong, before being replaced by smaller cells upon the onset of stratification. The links between trends in phytoplankton photophysiology and abiotic conditions are also explored, with temperature being found to be the most important forcing factor. Size-class specific relationships between phytoplankton photosynthetic rates and temperature are identified, with the potential for use in remotely-sensed models of primary production.