Damming alters the particulate organic carbon sources, burial, export and estuarine biogeochemistry of rivers

The long-term changes in composition and export of particulate organic carbon (POC) by rivers due to dam construction are poorly known. Based on observations, incubation experiments and modelling, this study analyzed the sources, spatial and temporal distribution and annual export of POC in the Changjiang River during recent decades to explore the POC changes due to dam construction. Changes in POC composition resulted from both increasing POC sequestration and carbon fixation in the river basin. The allochthonous POC (from terrestrial input) export by the Changjiang River to its estuary decreased by ∼90% between 1956 and 2002 and 2013–2019. We estimated that 2.3 ± 0.5 Mt C/yr allochthonous POC has been sequestered in the Changjiang watershed since 2013. The autochthonous (entirely from in-stream riverine primary production) contribution increased from 1% to 55% of total POC export during the same period due to improved light transparency caused by decreased sediment discharge. Global POC trapping in nine large rivers strongly impacted by dams is 12% of the global riverine POC export. The reduced POC export and increase in labile autochthonous POC strongly impact the estuarine carbon cycle. Therefore, with continued dam construction in the future, important changes in the riverine and estuarine carbon cycle can be expected.