Effectively mitigated eutrophication risk by strong biological carbon pump (BCP) effect in karst reservoirs

Karst reservoirs can significantly enhance the effect of biological carbon pump (BCP), a crucial process for carbon sequestration, water purification, and eutrophication mitigation. However, the effects of BCP on the fate of carbon (C), nitrogen (N), and phosphorus (P) and its role in regulating eutrophication within river-reservoir systems, remains insufficiently understood, particularly across different geological settings. We investigated the Hongfeng Reservoir (HFR), a typical karst reservoir, analyzing water chemistry, nutrient concentrations, and stable isotopes of dissolved inorganic carbon (δ¹³C_DIC) and nitrate (δ¹⁵N-NO₃^-) to uncover the underlying mechanisms governing the migration of biogenic elements and the process of eutrophication. Our findings reveal a strong BCP effect in the reservoirs that leads to substantial CO₂ and HCO₃^- uptake via phytoplankton photosynthesis during the warm-wet season, resulting in decreased dissolved inorganic carbon (DIC) concentrations and increased pH in the epilimnion. The δ¹³C_DIC (-4.0 ± 0.5 ‰) values in the epilimnion relatively increased in response to phytoplankton photosynthesis that preferentially absorbs the lighter isotope of ¹²C. Compared with the inflow, the δ¹⁵N-NO₃^- (7.4 ± 0.2 ‰) in the epilimnion of the reservoir is significantly depleted, with the water predominantly aerobic or oxygen-supersaturated. This suggests that nitrification is the dominant process during the warm-wet season. The high NO₃^- concentrations (44.3 ± 10.1 μmol/L) indicate a sufficient N supply for biological uptake. The strong BCP effects in the epilimnion convert substantial amounts of DCO₂ and nutrients into autochthonous organic matter. The resulting increase in pH further reduces the availability of DCO₂. Furthermore, BCP-induced calcium carbonate precipitation enhances P removal through co-precipitation, thereby accelerating nutrient depletion and carbon sequestration, which collectively contribute to the mitigation of eutrophication risks. To assess the broader applicability of these findings, we analyzed data from 129 lakes and reservoirs globally. Our results show that karst reservoirs, with their strong BCP effect, exhibit an average Carlson trophic status index (CTSI) 9.8 % lower than non-karst reservoirs, indicating a reduced risk of eutrophication. These insights offer valuable implications for the management of water resources in karstic reservoirs globally.