Euphotic Zone Metabolism in the North Pacific Subtropical Gyre Based on Oxygen Dynamics

We report in situ rates of gross oxygen production (GOP), community respiration (R), and net community production (NCP) in the North Pacific Subtropical Gyre derived from mixed layer O₂/Ar measurements. The measurements were conducted between November 2013 and January 2019 at the site of the Hawaii Ocean Time-series program. Biological O₂ concentration anomalies in the mixed layer showed a consistent diel variation, with values increasing during daytime due to net community production and decreasing during nighttime due to respiration. In situ mixed layer GOP and R, determined from these variations, covaried but showed no clear seasonal pattern, averaging 0.9 and 0.8 mmol O₂ m⁻³ d⁻¹, respectively. In situ rates of NCP determined from mixed layer O₂/Ar ranged between −1.2 and 16.6 mmol O₂ m⁻² d⁻¹. Our analyses indicate that at certain times of the year the diapycnal flux of O₂ across the base of the mixed layer may be non-negligible and, therefore, a fraction of O₂/Ar-derived NCP may form below the mixed layer. The seasonal climatology of NCP below the mixed layer (down to 150 m) was also estimated using near-monthly changes in dissolved O₂ concentrations. These calculations allowed us to estimate NCP for the entire euphotic zone (0–150 m), which shows pronounced seasonality, with a maximum in April-May and a minimum in December, when the ecosystem becomes temporarily net heterotrophic. Annual NCP was estimated to be 2.1 ± 0.6 mol O₂ m⁻² yr⁻¹, approximately 1.7 times the export of C through sinking particles captured in sediment traps at 150 m.