Long-term retrieval (1998-2023) of Colored Dissolved Organic Matter absorption coefficient and Dissolved Organic Carbon in the Mackenzie River-Beaufort Sea Region using the Copernicus GlobColour product

Abstract

In the regime-shifting Arctic, organic carbon export from river watersheds is expected to rise due to changes in hydrological regimes and permafrost thawing, affecting coastal and shelf biogeochemistry. Ocean color remote sensing enables monitoring under-sampled inaccessible areas like the Beaufort Sea, improving our knowledge of coastal dynamics and land-to-ocean transport of Chromophoric Dissolved Organic Matter (CDOM) and Dissolved Organic Carbon (DOC) during the ice-free season. While multi-sensor datasets provide consistent monitoring over extended periods, little work has been done into leveraging merged remote sensing reflectance (Rrs) to derive carbon-based productsthe viability of using merged L3 remote sensing reflectance (Rrs) to derive carbon-based regional products remains unexplored. This study uses the Copernicus-GlobColour multi-sensor merged (Rrs) to generate a long-term dataset of CDOM absorption at 443 nm, aCDOM(443), and DOC concentrations in the Mackenzie River–Beaufort Sea region. We employ a modified GIOP (Generalized Inherent Optical Properties) inversion algorithm to retrieve aCDOM(443); then, derive DOC using a region-specific aCDOM(443)- DOC relationship. Validation against in situ observations shows a good performance of the methodperformance comparable to previous studies, with a MdAPD (r2) of 56% (0.55) for aCDOM(443) and 29% (0.70) for DOC. This novel dataset allows for a detailed analysis of riverine plume dynamics, interannual variability, and trends. Average summer plume extent ranges from 43068 to 92388 km2 and is significantly correlated with annual discharge (r = 0.6, S´anchez-Urrea Mar´ıa et al. Running Title p < 0.001). When compared to independent in-situ DOC estimates (1999-2017) at the river mouth, our satellite product shows consistent variability patterns. Contrary to initial expectations, a significant decline of −0.017 m−1 yr−1 and (−3.40 µM yr−1) is observed over a 26-year period for aCDOM(443) and DOC , respectively, at the Mackenzie delta outflow, calling for further assessment of interannual variability in remotely-sensed aCDOM and DOC and their uncertaintiesresulting from a slight increase (1998–2015) followed by a decrease (2016–2023). These results suggest that trends in DOC fluxes need to be assessed using longer time series while considering significant uncertainties in both retrieval accuracy and variable spatiotemporal coverage over multidecadal periods.