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

In oligotrophic oceans, the stoichiometric ratios of apparent oxygen utilization (AOU) to nutrients often deviate from the classical Redfield ratio, yet the mechanisms remain poorly constrained. Contrary to the commonly held view that these ratios are typically elevated, we found that the mean ratios of AOU to dissolved inorganic nitrogen (DIN) and AOU to dissolved inorganic phosphorus (DIP) over the upper 2000 m of the oligotrophic Northwest Pacific are only 6.28 and 86.79, respectively, both below the classical values of 8.6 and 138. Strong stratification indicates that physical mixing alone cannot explain these integrated patterns, while depth-resolved regressions further reveal distinct controls among water layers. In upper waters, nutrient limitation promotes phytoplankton to produce TEPs with high C:N ratios, decoupling carbon fixation from nutrient assimilation, a process in which Pelagibacter may play a key role in recycling small organic molecules. In intermediate waters, preferential degradation of low C:N compounds within sinking organic matter enhances nutrient regeneration relative to oxygen consumption. In deep water, AOU / nutrient relationships are not statistically significant, indicating that oxygen consumption and nutrient accumulation become increasingly decoupled; this decoupling is likely influenced by Alteromonas activity in polymer degradation and phosphorus mobilization, alongside archaeal nitrification. 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.