An Evaluation of Optically-Derived Estimates of Phytoplankton Productivity and Carbon Flux in the North Pacific and North Atlantic Oceans

The vast quantity of organic carbon fixed by phytoplankton in the sunlit layer of the ocean provides the primary source of energy for marine food webs. Approximately 85-90% of the carbon fixed through photosynthesis will remain within the upper ocean, including the twilight zone, where it is remineralized by zooplankton and prokaryotes. To understand global carbon dynamics and trophic energy flow, it is therefore essential to accurately quantify phytoplankton productivity and further develop our predictive understanding of the fate of fixed carbon through the water column. Here, optics-based observations from two large NASA field campaigns (NAAMES & EXPORTS) were used to investigate spatio-temporal variability in net primary production (NPP) and carbon export in the North Atlantic and North Pacific. Previously developed photoacclimation and carbon-based productivity models were used to derive depth-resolved estimates of phytoplankton division rate and NPP throughout the euphotic zone. This approach gave strong agreement with discrete measurements of ¹⁴C-based NPP (r² = 0.82) and ranged from 2 to 70 mg C m^-3. The modeled NPP estimates, together with Underwater Vertical Profilers (UVP) used in both campaigns, provide important insight into the key factors driving subsurface carbon flux attenuation. Further work will investigate the influence of surface properties, such as phytoplankton community composition, loss terms, and particle size distribution on flux attenuation. Our results revealed strong spatial and temporal shifts in phytoplankton productivity over the annual phytoplankton bloom cycle and give key details that help explain the interactions between physical and biological processes controlling phytoplankton growth and physiology in the subarctic basins of the Atlantic and Pacific.