Bioturbation and the effects on benthic flux of nitrogen in a large eutrophic lake: Insights from 224Ra/228Th disequilibrium and inverse geochemical modelling

Abstract

Benthic fluxes are critical pathways for constituent exchanges and biogeochemical interactions across sediment–water interface. To comprehensively evaluate the modulation of bioturbation on the benthic fluxes of nutrients, we present a novel inverse geochemical modelling framework in near-surface sediments of Lake Taihu, a highly eutrophic freshwater lake in eastern China. This approach incorporates the disequilibrium of ²²⁴Ra/²²⁸Th and concentration profiles of additional constituents (SO4^2-, NH4⁺, NO3^– and NO2^–) through simulation and Bayesian theorem, enabling the estimation of bio-irrigation coefficient (α) and corresponding reaction rates. Based on the sediment columns collected from different segments of the lake, the model demonstrates vertical variability of α, with an average range of 0.60 × 10^-4 to 5.01 × 10^-4 s⁻¹. The calculated reaction rates and microbial taxa indicate the dominance of chemoheterotrophy, where NO3^– serves as an electron acceptor during the degradation of organic matter in the anoxic sediment environment to generate NH4⁺. The estimation of nitrogen benthic flux reveals that bioturbation predominates the export from sediments to the lake, and the directions of the internal nutrient fluxes across sediment–water interface are primarily controlled by nutrient loadings in the overlying lake water. This study introduces a systematic quantification to advocate that bioturbation is crucial in regulating nutrient variability by influencing both the reaction and flux rates, and the workflow expands the application of ²²⁴Ra/²²⁸Th disequilibrium to near-surface sediments in freshwater lacustrine systems, advancing the technique in tracing proxies for benthic fluxes in lakes.