Unraveling the dual impacts of damming and eutrophication on dissolved organic carbon dynamics in a large River

Abstract

Dissolved organic carbon (DOC) plays an essential role in riverine carbon cycling, yet the combined effects of damming and nutrient enrichment on its in-stream dynamics remain poorly constrained. This knowledge gap limits our ability to predict how large river systems respond to intensifying human activities. Here, we investigate how damming reshapes DOC composition and its seasonal variability, and how nutrient enrichment modifies DOC stoichiometry. Our results indicate that damming alters the temporal dynamics of DOC by mobilizing refractory DOC stored in reservoir sediments, thereby changing downstream carbon fluxes. The relationship between DOC degradation and discharge is positive during high-flow flushing but reverses during sediment retention, reflecting hydrological control on carbon processing. Nutrient enrichment promotes a higher proportion of autochthonous DOC, which is enriched in nitrogen but depleted in phosphorus relative to carbon, signaling ecological responses to nutrient imbalance. An increased dominance of autochthonous DOC under damming further shifts the elemental composition of riverine exports to the ocean. Our findings indicate that damming and eutrophication jointly reconfigure DOC sources, reactivity, and stoichiometry, with important implications for river-ocean carbon and nutrient linkages under global change.