Physicochemical and biological controls on primary and net community production across northeast Pacific seascapes

Abstract

The subarctic–subtropical transition zone in the North Pacific represents the second largest sink of atmospheric carbon dioxide in the world ocean, yet the relative importance of physical and biological processes in this uptake is debated. In a step toward understanding the spatiotemporal variability of environmental, physiological, and ecological factors that contribute to the efficacy of the biological pump, near‐continuous measurements of net primary production (NPP), net community production (NCP), export efficiency (NCP : NPP), and several physiological and ecological variables were collected across subarctic, transition, and subtropical seascapes of the Northeast Pacific during August and September of 2008. Whereas hydrographic variability (e.g., temperature, salinity, and mixed layer) dominated at basin scales, the effects were balanced or subsumed by biomass or taxa effects within individual seascapes. Fluorescence diagnostics suggested that the transition seascape was neither iron nor macronutrient limited. NPP and NCP were strongly spatially coupled in both the transition (r = 0.70; p < 0.0001) and subtropics (r = 0.68, p < 0.0001); however, the strength of individual drivers as determined through multiple linear regression (MLR) varied across seascapes. NPP in the transition seascape was driven primarily by nano‐ and microphytoplankton biomass, whereas NCP appeared to be driven by changes in salinity, temperature, and to a lesser degree, diatom‐specific biomass. Although NPP was low in the subtropics, mesoscale changes in hydrographical factors and shifts in community structure from pico‐ to microphytoplankton contributed to moderate NCP and high export efficiency. Spatial variability in the relative importance of hydrography, phytoplankton community structure, and NPP in driving NCP illuminates regional sensitivity of the biological pump to future climate conditions.