Damming effects on river sulfur cycle in karst area: A case study of the Wujiang cascade reservoirs

•Biogeochemical processes of sulfur were enhanced by damming, especially in summer.•Damming decreased seasonal differences but increased spatial differences of SO42– and δ34S-SO42−.•SO42– decreased whilst δ34S-SO42– increased from upstream to downstream.•Cumulative effects on δ34S-SO42− could derive from HRT and mean depth of reservoir.•SO42− and δ34S-SO42− could be useful indicators in assessing the damming effects.