Modeling the Responses of Phytoplankton Assemblage and Biological Pump Efficiency to Environmental Changes in the Chukchi Borderland, Western Arctic Ocean

Abstract The Chukchi Borderland, connecting the Chukchi continental shelf and the Canada Basin, has become a hotspot for studying how ecosystems respond to rapid environmental changes in the Arctic Ocean. Based on a long‐term hindcast simulation during 1998–2015 using a coupled ocean‐sea ice‐biogeochemical model, this study investigates the responses of phytoplankton assemblage and the biological carbon pump efficiency within the upper layers (0–100 m) of the Chukchi Borderland. The nitrate concentration is found to be a crucial factor controlling the total phytoplankton biomass and determining the spatiotemporal variations in the evolution pattern of phytoplankton assemblage. In the shelf break adjacent region, nitrate concentration increased after 2009, boosting phytoplankton biomass with diatoms persistently dominating. In the Canada Basin adjacent region, the westward expansion of the Beaufort Gyre after 2009 extended the influence of oligotrophic water, leading to phytoplankton miniaturization and a shift in phytoplankton assemblage evolution, from a pre‐2009 pattern that non‐diatoms at start were succeeded by diatoms, to a post‐2009 scenario that non‐diatoms dominated throughout the growing season. The biological pump efficiency evidently increased in the shelf break region after 2009, due to heightened biomass and intensified horizontal advection‐induced particulate organic carbon (POC) supply. The western Canada Basin adjacent region presented the reduced primary production and vertical POC flux. However, the deeper nitracline deepened the phytoplankton habitat, shortening POC residence time in the upper layers and enhancing the biological pump efficiency.