Seasonal oxygen deficiency reveals carbon-nitrogen decoupling and dissolved-nitrogen accumulation in a coastal sea.

Dissolved organic nitrogen (DON) is a major component of the fixed-nitrogen pool in coastal seas and is known to accumulate in oxygen-deficient waters. Yet whether DON decouples from dissolved organic carbon (DOC) under deoxygenation, and how the wider dissolved-nitrogen inventory responds, remains unresolved in a coastal time series. Here we use six summer cruises across a Yellow Sea coastal oxygen-deficient zone to characterize this response. As bottom-water oxygen fell from a June mean of ∼7.7 to a minimum of 2.3 mg L-1, DON increased (ρ = -0.53) while DOC remained constant (ρ = 0.18), decoupling the two elements and lowering the DOC:DON ratio below the Redfield reference (29% of samples; minimum 5.44), an unusually nitrogen-rich stoichiometry for marine dissolved organic matter (DOM). Ammonium and nitrite rose in parallel (NH4+ ρ = -0.82; NO2- ρ = -0.67), with the inorganic pool shifting toward reduced forms. Contemporaneous surface-bottom comparisons provide the primary evidence that this is a bottom-layer effect: the nitrogen enrichment of the bottom layer intensified with declining oxygen (ΔNH4+ ρ = -0.83; ΔDON ρ = -0.66, and -0.51 after control for covarying seasonal factors) while DOC showed the opposite vertical structure. A diffusive mass balance gives a central benthic contribution of ∼8% of the observed DON increase (median 13% across plausible parameterizations), and because the porewater-bottom-water gradients deliver carbon and nitrogen in a ratio of six to one, a benthic source large enough to explain the DON increase would raise bottom-water DOC by twice its observed variability, which is not seen; the carbon budget therefore excludes benthic supply as the source of the increase, implicating water-column processes; whereas for the reduced inorganic-nitrogen pool, regional benthic fluxes are large enough to constitute a plausible contribution. DOM optical properties shifted toward more humified, aromatic material, and the microbial community shifted toward taxa with established low-oxygen ecophysiology (SUP05 ρ = -0.46; SAR324 ρ = -0.43), which we read as an indicator of the bottom-layer redox state rather than as agents of DON production. As coastal oxygen deficiency expands, this carbon-nitrogen-decoupled retention of fixed nitrogen in the bottom layer may shift coastal seas toward internal nitrogen recycling rather than removal, weakening the nitrogen sink and reinforcing eutrophication.