We 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 macro- and micronutrient limitation, and sub-mesoscale hydrographic frontal zones. Throughout the majority of our survey region, carbon to chlorophyll ratios (Cphyto:Chl) ranged between 50-100. 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 50-80 in high nutrient low Chl (HNLC) waters in June, and from 80-120 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. photo-acclimation), 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 macro-nutrient concentrations. In contrast, iron-limited waters exhibited Cphyto:Chl ratios lower than predicted from the photo-acclimation 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.

Supplement to: Burt, William J; Westberry, Toby K; Behrenfeld, Michael J; Zeng, Chen; Izett, Robert W; Tortell, Philippe Daniel (2018): Carbon : Chlorophyll ratios and net primary productivity of Subarctic Pacific surface waters derived from autonomous shipboard sensors. Global Biogeochemical Cycles