Floods mobilize dissolved organic carbon (DOC) from watersheds to estuaries, yet how flood magnitude regulates DOC bioavailability and estuarine oxygen dynamics remains poorly understood. Here we conducted in-situ monitoring, laboratory incubations, and hydrodynamic modeling in a dam-regulated subtropical monsoon estuary. Our results reveal two contrasting flood-oxygen response pathways. Low-flow floods (peak discharge 310~1030 m³/s) mobilize phytoplankton-derived DOC from upstream reservoirs. This DOC is highly biodegradable (bioavailable fraction up to 39%) and has low aromaticity, as indicated by low SUVA₂₅₄ and high spectral slope ratio. These moderate events also trigger in-situ phytoplankton blooms (2.3- to 10.8-fold increase in Chl-a) through modest nutrient pulses, improved light availability, and extended water residence time. The senescing bloom provides additional labile carbon. Under nitrogen-replete, carbon-limited conditions (BDOC-to-DIN ratio ≤ 0.79), this combined bioavailable pool fuels rapid microbial respiration, driving a net DO decline of 3~12% (up to 17% in a typical event). In contrast, high-flow floods deliver predominantly terrestrial, aromatic, and refractory DOC, but no concurrent Chl-a peaks were observed following these events. Heavy turbidity suppresses bloom development, while shortened residence time and enhanced vertical mixing prevent oxygen depletion, yielding net DO replenishment. These findings reveal low-flow floods as overlooked drivers of hypoxia in regulated monsoon estuaries.

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