Hydrological management shifts dissolved organic matter chemistry in the world’s largest reservoir

The transport of dissolved organic matter (DOM) from rivers to coastal oceans is a critical component of the global carbon cycle. With more than 60% of rivers are affected by dam construction, dam reservoir construction and hydrological management are one of the most far-reaching human modifications on natural river flow and therefore carbon cycling of inland waters. However, how hydrological management affect DOM dynamics in river reservoirs remains poorly known. As such, a tributary named Xiangxi Bay (XB) and mainstream of Three Gorges Reservoir (TGR), the largest reservoir in the world, were chosen as a typical case assessing how hydrological management affects DOM cycling in inland waters. The spatiotemporal dynamics of the DOM was investigated by a series of bulk, optical and molecular (Fourier transform ion cyclotron resonance mass spectrometry, FT-ICR MS) approaches. The linkage between optical properties and molecular characteristics was also established. Higher algal inputs but lower terrestrial and anthropogenic inputs in XB than mainstream were observed during the drainage period (160m intermediate water level) instead of the storage period (175m highest water level), which was mainly attributed to the specific hydrodynamics (i.e. water intrusion from mainstream to XB) during the drainage period. Lower anthropogenic input and higher algal input were observed during the drainage period than the storage period. The higher terrestrial characteristics (higher AI_mod, polyphenols, humidification index, and SUVA₂₅₄) of DOM during the storage period is likely caused by the additional leaching of DOM from soils of the water-level fluctuation zone, which is also supported by a leaching experiment. Bio-incubation experiments further revealed that the overall DOM bio-lability is higher during the drainage than the storage period. Photo-incubation suggested that compounds with higher AI_mod were preferentially degraded, whereas some aliphatic compounds with high O/C were photo-produced. This study illustrates that hydrological management significantly affect both composition and reactivity of DOM at TGR. Further studies are needed to assess how hydrological management of reservoirs affect inland carbon cycle at regional and even global scales.