Terrestrial browning from Colored Dissolved Organic Matter (CDOM) changes the seasonal phenology of the coastal Arctic carbon cycle

Abstract. Arctic warming affects land-to-ocean fluxes of organic matter, with significant impacts on coastal ecosystems and air-sea CO₂ fluxes. In this study, we modify a regional ECCO-Darwin ocean biogeochemistry simulation of the Mackenzie River region to include riverine export of colored dissolved organic matter (CDOM) and its effect on light attenuation, marine carbon cycling, and water-column heating from UV-A to visible light absorption. We find that CDOM light attenuation triggers both a two-week delay in the seasonal phytoplankton bloom and an increase in sea-surface temperature (SST) by 1.7 °C. While the change in phytoplankton phenology has limited effect on air-sea CO₂ fluxes, the local increase in SST due to terrestrial browning switches the coastal zone from an annual sink of atmospheric CO₂ to a source (7.35 Gg C yr^-1). Our work suggests that the projected increase in terrestrial CDOM has strong implications for phytoplankton phenology and coastal air-sea carbon exchange in the Arctic.