Changes in hydrological regime regulate POC export across permafrost-dominated Arctic River basins

Warming-driven acceleration of hydrological processes is altering the carbon cycle in permafrost-dominated Arctic regions, yet the underlying drivers remain unclear. This study analyzes ArcticGRO data (2003–2021) from six major Arctic rivers (Ob, Yenisei, Lena, Kolyma, Yukon, and Mackenzie) to investigate trends and spatial–temporal variations in riverine particulate organic carbon (POC). The annual POC flux from these six rivers, estimated using the Load Estimator (LOADEST), averaged 2.78 Tg. Only the Lena River showed a notable annual decrease in POC flux (−3.9%/yr, p < 0.001) and concentration (−12%/yr, p < 0.001), while the Yukon River exhibited increasing streamflow (+0.98%/yr, p < 0.001) and POC flux (+3.2%/yr, p < 0.001). POC flux variations were primarily governed by streamflow and POC concentration, with higher concentrations in spring floods period and lower during winter. Spatial differences were linked to drainage density (Dd) and forest coverage (Fc). The Yukon River basin, with a higher Dd of 0.2 km/km² and lower Fc approximately 24%, exhibits the highest POC concentrations (2.3 mg/L). In contrast, the Yenisei River basin has the lowest POC concentration (∼0.4 mg/L), along with a relatively low drainage density (Dd = 0.18 km/km²) and a high forest cover (Fc = 67%). Permafrost conditions constrained riverine POC export, with isotopic evidence indicating a shift from a carbon sink to a source, as POC carbon age increased by ∼ 200 to 1700 years (4%–68%) annually, peaking in winter (700–2500 years) after 2012. Rivers with lower permafrost coverage (e.g., Ob, Yenisei), exhibit higher winter POC fluxes contributions (10%–20%), while others contributed < 5%, suggesting the role of permafrost degradation in winter carbon export. This study emphasizes the need to assess climate-driven hydrological shifts and permafrost thaw in shaping Arctic land-to-ocean carbon fluxes.