PNNL

Abstract

Damming impact on carbon emission and dissolved organic matter (DOM) composition in river reservoirs remains elusive. In this study, we investigated CO2 and CH4 effluxes, chemical and molecular compositions of DOM, DOM degradation, and microbial community in eleven samples from the mainstream and four tributaries of the Three Gorges Reservoir (TGR). Terrigenous DOM is one major component in the dissolved organic carbon pool in the TGR. The CH4 and CO2 efflux at the studied sites were 0.03-1.17?104 µg C m2 day-1 and -12.45-22.53?104 µg C m2 day-1, respectively. The CO2 efflux was the lowest near the Three Gorges Dam among all the studied upstream sites. This decreased CO2 efflux could be ascribed to low microbial degradability of DOM, which was evidenced by high average aromaticity of DOM molecular formulae. Furthermore, fast water discharge from the dam aerated the river water to enhance microbial degradation and thus mineralization of DOM, resulting in abrupt rise of CH4 and CO2 effluxes and dramatic shift of DOM composition in the downstream sites. In addition, microcosm experiments demonstrated that DOM in the TGR can be degraded and even mineralized by aquatic microbial populations, among which members of the Betaproteobacteria may play crucial roles. Taken together, this study expands our understanding of damming impact on carbon emission and DOM composition in reservoir ecosystems, and has great implications for optimizing dam management in major rivers.