Colored dissolved organic matter (CDOM) alters the seasonal physics and biogeochemistry of the Arctic Mackenzie River plume

Summary: We adjusted a model of the Mackenzie River region to account for the riverine export of organic matter that affects light in the water. We show that such export causes a delay in the phytoplankton growth by two weeks and raises the water surface temperature by 1.7 °C. We found that temperature increase turns this coastal region from a sink of carbon dioxide to an emitter. Our findings suggest that rising exports of organic matter can significantly affect the carbon cycle in Arctic coastal areas.

Abstract: Arctic warming affects land-to-ocean fluxes of organic matter through increased permafrost thaw, coastal erosion or river discharge, with significant impacts on coastal ecosystems and air-sea CO₂ fluxes. In this study, we modify a regional version of the Estimating the Circulation and Climate of the Ocean model coupled to the Darwin ocean biogeochemistry module (ECCO-Darwin) to simulate Mackenzie River 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 organic matter input 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.