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

The East/Japan Sea (EJS), a well-oxygenated marginal sea, exhibits an anomalously low nitrate (NO3-) to phosphate (PO43-) ratio (∼ 12.6 : 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 widespread potential for bacterially mediated N reduction 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. Our results further reveal a vertically structured stoichiometric regime in which anthropogenic N deposition is associated with elevated surface NO3- : PO43- ratios, whereas long-term changes in ventilation and deep-water properties coincide with lower ratios at depth. Together, these findings highlight the importance of considering nutrient stoichiometry across distinct temporal and vertical scales and identify the EJS as a sentinel system for detecting biogeochemical responses to ongoing environmental change. More broadly, our study provides a mechanistic framework for interpreting persistent nutrient stoichiometric anomalies in oxygenated marginal seas.