Carbon sequestration and decreased CO₂ emission caused by biological carbon pump effect: Insights from diel hydrochemical variations in subtropical karst reservoirs

While river damming has been extensively investigated, there remains unclear in the diel biogeochemical processes and carbon control mechanisms in karst reservoirs, under the influence of the biological carbon pump (BCP) effect. We investigated the diel variation of Hongjiadu (HJD) and Wujiangdu (WJD) reservoirs, which exhibit typical periodic thermal stratification, in the Wujiang River located in Southwest China. We found that diel variation of dissolved inorganic carbon (DIC) (HJD: 1668.6 +/- 10.4 mu mol/L; WJD: 1777.9 +/- 31.5 mu mol/L), delta 13CDIC (HJD: -3.1 +/- 0.1 %o; WJD: -4.6 +/- 0.4 %o), and other parameters in the lentic zones were relatively stable. Moreover, the Revelle factor in the lentic zones (HJD: 44.3-49.0; WJD: 44.3-53.9) were substantially higher than those of the fluvial zones (HJD: 24.4-33.4; WJD: 14.2-35.1). The stable diel variations indicate that the HJD and WJD reservoirs may have distinct stability regulatory mechanisms. In the lentic zones, delta 13CDIC, nitrogen-oxygen isotope (delta 15N and delta 18O), and the correlation of C-N indicated that the conversion of DIC and NO3- is primarily influenced by primary production, and there was coupled absorption of carbon and nitrogen nutrients during photosynthesis. Meanwhile, the C/N weight ratio (6.0-10.1) and the delta 13CPOC (-30.0 %o to -24.7 %o) also revealed a gradual transition from soil organic matter (OM) in the fluvial zone to phytoplankton source in the intermediate-lentic zone. Thus, two reservoirs had a significant BCP effect and converted a significant amount of DIC to autochthonous organic carbon (AOC) burial in the lentic zones. The BCP effect in subtropical deep karst reservoirs buried lots of carbon, increased carbon sequestration and decreased CO2 emissions in the lentic zone. Simultaneously, the stability of reservoirs promotes BCP effect and intensifies carbon sequestration, which is of great significance for the study of the global carbon sources-sinks pattern.