Great Slave Lake as a Key Modulator of Dissolved Organic Carbon Fluxes From Boreal Landscapes to the Mackenzie River

Abstract Understanding dissolved organic carbon (DOC) fluxes in boreal freshwater systems is critical for constraining high‐latitude carbon budgets and anticipating climate‐driven changes. Here, we present a retrospective assessment (2000–2024) of DOC fluxes in the Upper Mackenzie River watershed, focusing on Great Slave Lake (GSL), a key hydrological node in North America's largest Arctic‐draining river system. Using field observations, remote sensing, and hydrological modeling, we quantify tributary DOC inputs to GSL, estimate DOC export through its outflow—the Mackenzie River—and construct a lake‐wide DOC budget. DOC fluxes exhibited pronounced spatial heterogeneity across tributaries, driven primarily by discharge and watershed characteristics such as wetland extent, slope, and soil organic carbon content. Wetland‐rich, low‐relief basins delivered the highest DOC concentrations, whereas Canadian Shield rivers contributed the lowest. Mass balance estimates indicated that the Slave River contributes ∼70% of the total flux to GSL, followed by the Hay River (∼10%), and GSL removed ∼30% of incoming DOC, functioning as a substantial DOC sink and modulator of downstream fluxes. The major tributaries supplying GSL also exhibited DOC chemical compositions distinct from the Mackenzie River outflow, indicating in‐lake transformations. This regulatory role helps explain previously reported differences between the Mackenzie River and major Siberian rivers that lack large lake influences. By altering oceanward DOC fluxes, GSL could potentially affect Arctic Ocean carbon cycling and the long‐term fate of terrigenous DOC. As boreal warm seasons intensify, the modulatory influence of GSL and large lakes is expected to increase, underscoring the need to explicitly represent lake processes in northern carbon assessments.