Hydrologic heterogeneity induced variability of dissolved organic matter chemistry among tributaries of the Three Gorges Reservoir

Dissolved organic matter (DOM) chemistry in rivers regulates aquatic food web dynamics, water quality, and carbon storage. The operation of reservoirs represents one of the major human modifications on the natural flow of rivers, which can affect DOM chemistry. Although hydrologic heterogeneity has been observed in different segments of the reservoir, whether it will structure DOM chemistry is poorly assessed, which is critical to better constrain the carbon cycle in reservoirs. By the combination of a series of techniques including stable carbon isotopes, optical spectroscopy, and ultrahigh-resolution mass spectrometry, here we showed that hydrologic heterogeneity induced changes in DOM molecular composition between two large tributaries, named Shennongxi (SR) and Xiangxi (XR) rivers, of the Three Gorges Reservoir (TGR). With water intrusion from mainstream, SR had relatively higher terrestrial and more recalcitrant DOM than XR, where no water intrusion from the mainstream was observed. In contrast, the averaged relative abundance of autochthonous input and biological lability of DOM along upstream to downstream transect were higher in XR than that in SR. The presence or absence of water intrusion from mainstream to tributaries induced by hydrologic management is likely the main factor controlling DOM chemistry in different tributaries of the TGR. By linking DOM chemistry in the water column with that in surface sediments, we suggest that hydrologic management of reservoir likely affects the preferential preservation of recalcitrant DOM in surface sediments, which further affects the organic carbon burial and the river carbon cycle. With reservoir construction increasing worldwide, further studies are encouraged to investigate the DOM chemistry under different hydrologic management of reservoirs to better constrain and predict the carbon cycling in fluvial ecosystems.