Cascade reservoirs affected chemical compositions of dissolved organic matter and greenhouse gas dynamics in the Lancang River.

Abstract

Dissolved organic matter (DOM) is an important component in aquatic systems. There has been much debate about the potential effects of cascade reservoirs on the transport and transformation of DOM. Here, through a survey of source to leave-boundary section of Lancang River (LCR) in June and November of 2017-2018, our results revealed that weak spatiotemporal variations were observed for DOC content, whereas DOM parameters were significantly different between natural and reservoir reaches. And DOM showed higher humification degree from allochthonous sources with increasing autochthonous matter in reservoir reach, may due to high particulate organic matter and releasing autochthonous DOM from phytoplankton blooms in the LCR, which can be evidenced by depleted DIC, enriched δ¹³C_DIC and higher BIX. A unique fluorescent fraction (C5) appeared in the reservoir reach and increased along water flow, which was strongly associated with dissolved CO₂ and N₂O. Meanwhile, BIX value decreased with increasing dam height, hydraulic residence time (HRT), and reservoir capacity, which may promote CH₄ production, highlighting variation of DOM compositions in understanding the effect of greenhouse gas (GHG) dynamics in the LCR. The findings were essential for comprehending the influences of cascade reservoirs on carbon cycle, and informed policy development for the sustainable management of transboundary water resources like the LCR.