Anthropogenic input of sulfate (SO) in reservoirs may enhance bacterial sulfate reduction (BSR) under seasonally hypoxic conditions in the water column. However, factors that control BSR and its coupling to organic carbon (OC) mineralization in seasonally hypoxic reservoirs remain unclear. The present study elucidates the coupling processes by analyzing the concentrations and isotopic composition of dissolved inorganic carbon (DIC) and sulfur (SO, sulfide) species, and the microbial community in water of the Aha reservoir, SW China, which has high SO concentration due to the inputs from acid mine drainage about twenty years ago. The water column at two sites in July and October revealed significant thermal stratification. In the hypoxic bottom water, the δC-DIC decreased while the δS-SO increased, implying organic carbon mineralization due to BSR. The magnitude of S isotope fractionation (ΔS, obtained from δS-δS) during the process of BSR fell in the range of 3.4‰ to 27.0‰ in July and 21.6‰ to 31.8‰ in October, suggesting a change in the community of sulfate-reducing bacteria (SRB). The relatively low water column stability in October compared to that in July weakened the difference of water chemistry and ultimately affected the SRB diversity. The production of DIC (ΔDIC) scaled a strong positive relationship with the ΔS in July (p < 0.01), indicating that high OC availability favored the survival of incomplete oxidizers of SRB. However, in October, ΔC-DIC was correlated with the ΔS in the bottom hypoxic water (p < 0.01), implying that newly degraded OC depleted in C could favor the dominance of complete oxidizers of SRB which caused greater S isotope fractionation. Moreover, the sulfide supplied by BSR might stimulate the reductive dissolution of Fe and Mn oxides (Fe(O)OH and MnO). The present study helps to understand the coupling of C and S in seasonally hypoxic reservoirs characterized by high SO concentration.
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