AbstractWe present optically derived estimates of phytoplankton carbon (Cphyto) and chlorophyll a concentration (Chl) across a wide range of productivity and hydrographic regimes in the Subarctic Pacific Ocean. Our high‐frequency measurements capture changes in Cphyto and Chl across regional gradients in macronutrient and micronutrient limitations and submesoscale hydrographic frontal zones. Throughout the majority of our survey region, carbon to chlorophyll ratios (Cphyto:Chl) ranged between 50 and 100. Lower values (10–20) were constrained to the highly productive coastal upwelling system along Vancouver Island, whereas higher estimated values (>200) were found directly off the southern British Columbia continental shelf. Further offshore, Cphyto:Chl was less variable, ranging from 50 to 80 in high nutrient low Chl waters in June and from 80 to 120 in the Gulf of Alaska in July. Much of the variability in Cphyto:Chl throughout the study region could be explained by mixed‐layer light levels (i.e., photoacclimation), with additional variability attributed to nutrient‐controlled changes in phytoplankton growth rates in some regions. Elevated Cphyto:Chl ratios resulting from apparent nutrient stress were found in areas of low macronutrient concentrations. In contrast, iron‐limited waters exhibited Cphyto:Chl ratios lower than predicted from the photoacclimation model. Applying the carbon‐based production model, we derived Cphyto and Chl‐based estimates of net primary productivity, which showed good coherence with independent 14C uptake measurements. Our results highlight the utility of ship‐board optical data to examine phytoplankton physiological ecology and productivity in surface marine waters.