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Relationships between sea surface temperature (SST, > 10 m) and vertical density stratification, nutrient concentrations, and phytoplankton biomass, composition, and chlorophyll a (Chl a) specific absorption were assessed in spring and summer from latitudes 29 to 63° N in the northeast Atlantic Ocean. The goal of this study was to identify relationships between phytoplankton and abiotic factors in an existing SST and stratification gradient. Furthermore, a bio-optical model was used to estimate productivity for five phytoplankton groups. Nutrient concentration (integrated from 0 to 125 m) was inversely correlated with SST in spring and summer. SST was also inversely correlated with near-surface (0–50 m) Chl a and productivity for stratified stations. Near-surface Chl a and productivity showed exponential relationships with SST. Chl a specific absorption and excess light experiments indicated photoacclimation to lower irradiance in spring as compared to summer. In addition, Chl a specific absorption suggested that phytoplankton size decreased in summer. The contribution of cyanobacteria to water column productivity of stratified stations correlated positively with SST and inversely with nutrient concentration. This suggests that a rise in SST (over a 13–23 °C range) stimulates productivity by cyanobacteria at the expense of haptophytes, which showed an inverse relationship to SST. At higher latitudes, where rising SST may prolong the stratified season, haptophyte productivity may expand at the expense of diatom productivity. Depth-integrated Chl a (0–410 m) was greatest in the spring at higher latitudes, where stratification in the upper 200 m …