Accumulation of organic carbon in a large canyon reservoir in Karstic area, Southwest China

Artificial reservoirs play an important role in intercepting organic carbon (OC), which affects the carbon cycle of inland waters. However, limited by the methods of field observations, studies on the potential carbon sink of sediment in reservoirs are not accurate enough. In this study, we investigated OC accumulation in the Wujiangdu Reservoir (WJDR), a canyon reservoir located in a karst area of Southwest China, by coupling seismic survey data with physicochemical data of sediment cores. The results showed that irregular underwater topography resulted in spatial heterogeneity of the sediment deposition. The mean OC accumulation rate in the WJDR was 579.2 g C m⁻² year⁻¹, and the OC accumulation flux of the WJDR was 1.4 × 10¹⁰ g C year⁻¹. δ¹³C values coupled with a mixing model indicated that the autochthonous OC ranged from 14.2 to 97.0% of the total OC in the sediment, with an average of 48.4%. The significantly positive relationship between autochthonous OC and TN indicated that the autochthonous OC generated in the reservoir was associated with nutrient input. The autochthonous OC accumulation flux was 6.6 × 10⁹ g C year⁻¹, which was equal to 2.3 times the carbon emission from the reservoir surface to the atmosphere as CO₂-C, indicating that the autochthonous OC produced within the reservoir is a large carbon sink.