A potential explanation for the anomalously low nitrate to phosphate ratio in the well-oxygenated East/Japan Sea

Abstract. The East/Japan Sea (EJS), a well‑oxygenated marginal sea, exhibits an anomalously low nitrate (NO₃⁻) to phosphate (PO₄³⁻) ratio (~ 2.6:1), diverging from the canonical Redfield ratio (16:1). To resolve this long‑standing biogeochemical enigma, we examined nitrogen (N) cycling genes and bacterial communities across depths and seasons. External phosphorus inputs – riverine, atmospheric, and crustal – were insufficient to explain the imbalance. Instead, high abundances of N‑reducing genes and affiliated taxa suggest a plausible role for bacterially mediated N loss throughout the water column. We propose that N removal may occur within particle-associated microenvironments (i.e., oxygen-depleted microzones inside sinking organic aggregates), despite oxygen-rich conditions. A dual-scale feedback – short-term anthropogenic N deposition enriching surface waters (i.e., increasing NO₃⁻:PO₄³⁻ ratio in the upper waters) and longer-term deoxygenation, driven by the weakening of deep-water formation, potentially favoring subsurface N loss (i.e., decreasing NO₃⁻:PO₄³⁻ ratio in the deep waters) – may promote a vertically stratified NO₃⁻:PO₄³⁻ regime in the future EJS. Our findings highlight the EJS as a sentinel system for how combined anthropogenic and climatic forces could reshape marine nutrient balances.