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 O/Ar measurements. The measurements were conducted between November 2013 and January 2019 at the site of the Hawaii Ocean Time-series program. Biological O concentration anomalies in the mixed layer showed a consistent diel variation, with values increasing during daytime due to net primary production and decreasing during nighttime due to respiration. In situ mixed layer GOP and R, determined from these variations, co-varied but showed no clear seasonal pattern, averaging 0.9 and 0.8 mmol O m d, respectively. In situ rates of NCP determined from mixed layer O/Ar ranged between -0.7 and 17.6 mmol O m d. Our analyses indicate that at certain times of the year the diapycnal flux of O across the base of the mixed layer may be non-negligible and therefore a fraction of O/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 O concentrations. These calculations allowed us to estimate NCP for the entire euphotic zone (0-150 m), which shows pronounced seasonality, with a maximum in May and a minimum in December, when the ecosystem becomes temporarily net heterotrophic. Annual NCP was estimated to be 2.4 ± 0.5 mol O m yr, approximately twice the export of C through sinking particles captured in sediment traps at 150 m.