Meeting Documents

The Impact of Mackenzie River Colored Dissolved Organic Matter (CDOM) on Coastal Arctic Ocean Carbon Cycling

Abstract

Rivers flowing into the Arctic Ocean (AO) drain an area exceeding 12 million km². As the Arctic is warming four time faster than the global average, freshwater export from Arctic rivers is increasing and their associated land-to-ocean flux of dissolved organic matter is changing in both quantity and chemical composition. The response of the coastal AO to climate-driven variability in biogeochemical river runoff remains highly uncertain, as observations in these remote, high-latitude regions remain sparse. Here we aim to better understand how terrestrial dissolved organic matter and its associated optical properties impact coastal-ocean biogeochemistry and air-sea CO₂ flux. We use a regional set-up of the ECCO-Darwin ocean/sea-ice/biogeochemistry model to explore and quantify the effect of terrestrial colored dissolved organic matter (CDOM) on coastal Beaufort Sea carbon cycling. We develop a novel parameterization for riverine CDOM light absorption (from UV to visible light wavelengths) as this signal transitions across the Land-Ocean-Aquatic-Continuum (LOAC). Our model captures the complex dynamics of terrestrial CDOM via mass fluxes with labile and refractory dissolved organic carbon pools and includes warming effects on seawater. Using these model improvements, we first quantify the impact of terrestrial CDOM on the Mackenzie River plume hydrography over seasonal timescales. We find that CDOM increase near-surface ocean temperatures by up to 1.5°C, with a decrease in local sea-ice cover of 5%. The combined effects of light attenuation by sea-ice cover and CDOM during freshet triggers a more vigorous, but more-rapidly-decaying phytoplankton bloom, which is delayed by two weeks compared to simulations without CDOM. As a result of this shift in bloom phenology, the seasonality of air-sea CO₂ flux is altered, promoting a substantial decrease in the plume’s CO₂ sink during summer. In summary, our numerical study and new CDOM parameterization is particularly relevant and timely as intensified land-to-sea coupling strongly responds to climate change and drives new carbon cycling states along the periphery of the AO.