Sources, variations, and flux of settling particulate organic matter in a subtropical karst reservoir in Southwest China

The biological carbon pump (BCP) effect is a key mechanism in karst surface freshwater ecosystems, and can transform inorganic carbon from carbonate weathering into organic carbon via photosynthesis and sedimentation. In the process, an understanding of particulate organic carbon sources, their flux, and their variability is important for understanding BCP effects, but these relative processes are still poorly understood. Therefore, we used a sediment trap experiment to collect settling particulate organic carbon (SPOC) and conducted a systematic study to understand the sources and factors that influence SPOC, as well as determine the amount of SPOC generated by the BCP effect by combing meteorological data monitoring and modern hydrochemical monitoring in the Dalongdong (DLD) reservoir, a karst groundwater-fed reservoir. Based on δ¹³C, δ¹⁵N, and molar C/N data, as well as relative models, the SPOC sources came from algae, resuspened matter, soil, and domestic sewage. Among these, resuspension (45.27%) and algae (31.57%) were the main sources for SPOC. Thus, the seasonal variations in different sources were mainly affected by water movement caused by climate changing with respect to air temperature and rainfall. In addition, domestic sewage discharge in the DLD reservoir also affected the sources for SPOC. The results suggested that the SPOC from algal sources in the DLD reservoir was 17.07 gC/m²/a at the surface and 9.26 gC/m²/a at the bottom via the BCP process. The SPOC deposition efficiency (OCDE) of algae was 54.25%. Organic carbon from algal source via the BCP effect at the surface of water column was finally deposited and buried in sediment. According to estimation of historical data, the stored organic carbon through assimilating DIC from carbonate rock weathering by BCP effect was as much as 10.84 tC/a. The quantity of organic carbon stored and fixed by BCP effect cannot be ignored in the reservoirs.