Abstract: We evaluated the effects of dam interception on sulfur cycle in Longtan Reservoir, a deepwater reservoir in the upper reaches of the Pearl River. Water samples were quarterly collected from tributaries, stratified water and underflow water in April, July, October 2019 and January 2020. Hydrochemical parameters, SO₄^2- concentrations and isotopic compositions (δ³⁴S_SO4 and δ¹⁸O_SO4) in water samples were analyzed. Based on redundancy analysis and Bayesian stable isotope mixing model, we examined the impacts of altered hydrodynamic conditions under the influence of damming on sulfur cycle in the water column of Longtan Reservoir. The results showed that the main source of SO₄^2- was sulfide oxidation (with O₂ or Fe³⁺ as electron acceptors), and also included atmospheric precipitation and anthropogenic sources. After damming and impoundment, the fluctuation range of SO₄^2- concentration and δ³⁴S_SO4 and δ¹⁸O_SO4 values in the water column significantly increased. Water storage after dam impoundment led to an increase in relative water column stability (RWCS), which significantly enhanced sulfide oxidation and sulfate reduction in the reservoir. Thus, RWCS is an important hydrodynamic factor driving the sulfur cycle in the reservoir. This study is helpful to further understand hydrodynamic mechanisms driving sulphate transport and transformation processes in reservoirs, providing a theoretical basis for the scientific utilization and protection of water resources in deep-water reservoirs.

Key words: sulfur cycle, relative water column stability, Bayesian stable isotope mixing model, Longtan Reservoir.