Organic carbon pathways across the fluvial-marine transition zone of the Mackenzie River Delta – Beaufort Sea region and implications on ocean color remote sensing

Abstract. Arctic warming and hydrological intensification are accelerating permafrost thaw and increasing the export of terrestrial organic carbon (OC) and sediments from land via rivers and shallow coastal waters into marine waters, yet the fate of these materials in deltaic and coastal transition zones remains poorly understood. Here, we synthesize multiyear in situ biogeochemical, optical, and radiometric observations (2009–2024) across the Mackenzie River Delta–southern Beaufort Sea land–ocean continuum. By using a compartmental approach (river, delta, coastal, marine) we quantify spatial and seasonal variability in dissolved organic carbon (DOC), particulate organic carbon (POC), and suspended particulate matter (SPM) and refine bio-optical relationships that support satellite retrievals in optically complex Arctic waters. Our results show that DOC concentrations declined from river to marine waters (mean 4.8 to 1.9 mg L⁻¹), while POC and SPM showed more variability with marked reductions across the transition, consistent with retention and transformation processes in deltaic and nearshore zones. Across all compartments, DOC exhibited a strong non-linear relationship with CDOM absorption at 443 nm (aCDOM(443); r²=0.81), whereas POC related linearly to particulate absorption at 443 nm (aP(443); r²=0.73), with substantial compartment dependent differences in slope and fit strength that indicate shifting OC composition and optical regimes along the salinity gradient. Optical Water Type (OWT) classification derived from remote sensing reflectance (Rrs) resolved transitions from turbid, particle-dominated waters to clearer coastal and marine regimes, providing a framework for guiding algorithm selection and improving retrieval performance. These results provide the first concurrent, Arctic fluvial-marine assessment of DOC, POC, SPM, and optical properties and demonstrate how land–sea connectivity governs both organic carbon processing and optical structure in Arctic coastal waters.