High-resolution dissolved O2/Ar and optical scattering observations from the North Pacific subtropical/subpolar transition zone reveal contributions of small phytoplankton to oceanic net community production

While long-standing paradigms equate high rates of net primary production (NPP), net community production (NCP), and high sinking fluxes of particulate organic carbon into the ocean interior with large phytoplankton size classes, recent studies pairing high-resolution surface O₂/Ar observations with other proxies of community structure (e.g., pigments, and -omics based approaches) have indicated these paradigms do not always hold. Here, we report new results from three meridional surveys of the North Pacific subtropical/subpolar transition zone, a region where previous observations have identified high rates of biologically-mediated ocean CO₂ uptake. Our observations indicate high rates of O₂/Ar-based NCP in this region (up to 50 mmol O₂ m^-2 d^-1), with significant temporal variability between outbound and return surveys. We also find fine-scale features in O₂/Ar-based NCP are coherent with those of the “medium” (2-20 μm) phytoplankton size class observed via optical scattering over multiple cruises. Our results illustrate the power of combining high-resolution surface dissolved gas observations with approaches that yield information on community structure at similar scales. More broadly, this work helps to build the case for moving beyond a large/small size class approach to modeling biological pump function in the ocean.