Enhanced biological pump and carbonate pump synergy: The primary pathway for phosphorus clearance in the century-long dynamics of a karst lake

Abstract

Lakes exhibit a biological pump (BP) effect, akin to marine systems, acting as a carbon sequestration mechanism. In karst lakes, the BP works in synergy with a more efficient carbonate pump (CP), enhancing phosphorus (P) removal. This synergy provides a plausible explanation for the observed P enrichment in karst lake sediments. It is hypothesized that an enhanced BP effect drives the CP, accelerating the co-precipitation of carbonates with P, resulting in the accumulation of calcium-bound P compounds (CaP). However, these hypotheses remain largely unquantified and unverified. Research into centennial-scale multi-proxy sedimentary records from Fuxian Lake, a representative karst lake in China, showed significant correlations among organic and inorganic carbon, including contents, fluxes, and isotopic profiles, demonstrating the interplay between the BP and the CP. Carbonate δ18O data further support the conclusion that the BP is the primary driver of the CP. Moreover, a strong correlation (r > 0.6, P < 0.0001) between organic-inorganic carbon proxies and sedimentary P has been identified. Compared to phytoplankton, submerged plants significantly boost P co-precipitation (forming CaP) through the BP mechanism by facilitating a more efficient CP. CaP, the main component within stable sedimentary P reservoirs, saw a post-1950 burial flux increase to 3.5 times that of the preceding period, reaching 0.24 g P m−2 yr−1. This underscores the CP's effectiveness in P sequestration under increased anthropogenic pressure. Given the uniquely high weathering rates, Ca2+, pH, and dissolved inorganic carbon content of karst lakes, we suggest that their strong BP effect, combined with the CP, significantly enhances P removal, offering an autogenic solution to eutrophication.