Terrestrial browning from colored dissolved organic matter (CDOM) changes the seasonal phenology of the coastal arctic carbon cycle

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.