Identification of terrigenous and autochthonous organic carbon in sediment cores from cascade reservoirs in the upper stream of Pearl River and Wujiang River, southwest China: lignin phenol as a tracer

Organic carbon (OC) source attribution for cascade reservoir sediments has been identified as a critical gap in understanding the effective carbon sink of inland waters. In this study, nine sediment cores were collected from cascade reservoirs in the Wujiang and Pearl Rivers. We analyzed lignin phenol (?8), total organic carbon (TOC) content, and stable carbon isotopic composition (delta C-13) of the sediments, focusing on the changes of terrigenous OC, and the variation of OC source in cascade reservoir sediments after damming. Our results showed that the sigma 8 and TOC contents decreased from upstream to downstream reservoirs, indicating the significant interception of terrigenous OC by cascade damming. Additionally, the ?8 content in the Pearl River reservoir sediments was much higher than that in the Wujiang River. From the three-end-member mixing model, we estimated that OC in reservoir sediments mainly comes from soil and plankton. After damming, the proportion of plankton OC in TOC slightly increased in seasonal and annual regulation reservoirs due to the limnetic evolution of the reservoir. These findings suggest that the cascade damming increases the interception capacity of the river to terrigenous OC and nutrients, and that slowing of water velocity caused by damming affected primary productivity and fluvial carbon cycling.