Three Gorges Reservoir construction induced dissolved organic matter chemistry variation between the reservoir and non-reservoir areas along the Xiangxi tributary

Highlights: • Both natural and anthropogenic sources of DOM were revealed in Xiangxi tributary. • Higher terrestrial input was observed in reservoir area than that in non-reservoir area. • DOM diversity among reservoir and non-reservoir area was caused by water intrusion from mainstream. As one of the most dynamic and reactive compound pools, DOM plays a crucial part in various biochemical processes such as element cycling and nutrient export. Although the reservoir DOM has been investigated extensively, the variation of DOM between reservoir area and non-reservoir area induced by reservoir construction is not comprehensively assessed. By the combination of a series of complementary techniques including stable carbon isotope, optical spectroscopy, and ultrahigh-resolution mass spectrometry, here we show that hydrological alterations induced by Three Gorges Reservoir (TGR) construction were responsible for the variation in DOM molecular composition between reservoir area and non-reservoir area in Xiangxi tributary. With water intrusion from mainstream induced by reservoir construction and operation, reservoir area had relatively higher terrestrial input and more recalcitrant DOM than those in the non-reservoir area with limited influence of reservoir operation, whereas, relatively more autochthonous source and higher molecular lability of DOM were observed in the non-reservoir area. The water intrusion from mainstream to tributaries induced by reservoir construction is likely the main factor controlling DOM variation between reservoir area and non-reservoir area, and might devote to the organic carbon burial in the reservoir. This research provides new insight into spatial variations of riverine DOM composition induced by reservoir construction, underlining the important role of the reservoir in DOM cycling.