Reservoir operation regulates the dynamics of dissolved organic matter in sediments

Dissolved organic matter (DOM) in sediments is pivotal in biogeochemical processes of aquatic ecosystems. Given the influence of reservoir operation on riverine ecosystems, the dynamics of DOM in reservoir sediments remain unclear. In this study, focusing on the Three Gorges Reservoir (TGR), one of the world’s largest reservoirs, we investigated the mechanisms underlying variations in sedimentary DOM using radiocarbon(Δ¹⁴C), optical, and molecular techniques. Furthermore, a DOM molecule–based numerical model was developed to assess the monthly and annual variations in sedimentary DOM from 2011 to 2080. Laboratory analysis demonstrated that there was more autochthonous DOM in sediments with a declining pattern from upstream to downstream in the wet season, and more allochthonous DOM in sediments with no spatial trend in the dry season. The findings suggested that variations of primary productivity and hydrological conditions influenced by reservoir operation likely modulated the dynamics of DOM in sediments of TGR. Moreover, based on the numerical simulation, from 2011 to 2080, July, April, and September hold major (>50%) of the year’s accumulation of allochthonous and autochthonous DOM in sediments. By 2080, the quantities of allochthonous and autochthonous DOM in sediments in TGR would reach 1166×10⁴t and 129×10⁴t, respectively. This study provides detailed insights into the dynamics of organic matter pools in reservoirs and enhances our understanding of the ecological impacts of reservoir construction on aquatic ecosystems.