Biological carbon pump drives co-removal of phosphorus and heavy metals in karst lakes

Karst lakes are dynamic aquatic systems where carbonate weathering, biological production, and mineral precipitation jointly regulate carbon cycling, nutrient retention, and contaminant fate. However, field evidence linking the biological carbon pump (BCP) to phosphorus (P) removal and heavy metals (HMs) immobilization in natural karst lakes remains limited. This study investigated Tianchi Lake (TCH) in Southwest China using high-frequency hydro-chemical monitoring, seasonal sampling, sediment P fractionation, HMs analysis, and SEM-EDS characterization. Results showed pronounced seasonal covariation among NEP and hydrochemical variables commonly associated with BCP-related processes. During summer, positive NEP coincided with higher pH, dissolved oxygen (DO), and calcite saturation and with lower Ca²⁺, HCO₃^-, pCO₂, and water-column total phosphorus (TP). These coordinated seasonal patterns were consistent with enhanced net autotrophic metabolism and photosynthetic DIC uptake. The cumulative NEP over the approximately one-year monitoring period was 9.15 g C m^-2, indicating net autotrophy at the ecosystem-metabolism level. Daily mean NEP was positive only in summer, when the cumulative NEP reached 53.86 g C m^-2. Sedimentary P was also highest in summer, with total P reaching 330.98 mg kg^-1. The BCP-related P fractions HCl-P, BD-P, and NaOH-SRP reached 32.64, 85.51, and 116.39 mg kg^-1, respectively, and together accounted for an average of 65.19% of sedimentary P. HMs (Cr, Zn, Pb, Fe, Mn) showed coupled water-sediment redistribution, with lower dissolved concentrations but stronger sediment enrichment in spring and summer respectively. Total sedimentary metals peaked at 188.99 mg kg^-1 in summer. During the monitoring period, the coordinated seasonal patterns of NEP, carbonate-system variables, sedimentary P fractions, and metal distributions were consistent with a conceptual mechanism in which carbonate precipitation and Fe-oxide formation under periods of stronger net autotrophic metabolism may contribute to coupled carbon sequestration, P retention, and partial HMs immobilization.