Stoichiometric deviation and regulatory mechanisms of AOU-nutrient ratio in the oligotrophic Northwest Pacific Ocean

Abstract. In oligotrophic oceans, the stoichiometric ratios of apparent oxygen utilization (AOU) to nutrients often deviate from the classical Redfield ratio, yet the mechanisms driving these deviations remain poorly constrained. Contrary to the commonly held view that ratios of AOU to nutrients are typically elevated, our study found that the mean ratios of AOU to dissolved inorganic nitrogen (DIN) and AOU to dissolved inorganic phosphorus (DIP) in the upper 2000 m of the oligotrophic Northwest Pacific are substantially lower than the classical Redfield ratios (8.6 and 138, respectively), measuring only 6.28 and 86.79, respectively. Physical mixing alone cannot explain these low ratios, as the region is strongly stratified. This persistent vertical isolation drives chronic nutrient limitation in surface waters, promoting phytoplankton to produce carbon‑rich transparent exopolymer particles (TEPs) with high C:N ratios. Meanwhile, the microbial community, dominated by Pelagibacter and Alteromonas, exhibits functional partitioning. Pelagibacter efficiently recycles small organic molecules, while Alteromonas degrades complex polymers and actively releases phosphate. This selective processing enhances nutrient regeneration relative to carbon oxidation, depressing the AOU/nutrient ratios. These findings suggest that biogeochemical models should account for such biological feedbacks to improve predictions of ocean carbon export and nutrient cycling under future climate scenarios.