Short‐term net community production enhanced by internal wave–driven nutrient supply in the lower euphotic zone of the western subtropical North Pacific

Abstract

The metabolic status of lower euphotic zones (EZs) in subtropical oceans—whether they are net autotrophic or heterotrophic—has been a subject of debate. In this study, we demonstrated net community production (NCP) enhanced by internal wave–driven nutrient supply in the lower EZ of the western subtropical North Pacific. Near 6‐h interval observations conducted from July 20, 2016 to July 28, 2016 at a fixed station revealed oxygen production and nutrient consumption in the deep layer around a potential density of 25 σθ (129–161 m depth), where relative irradiance to the surface was less than 1%. On July 25–26, internal waves generated strong turbulence, driving an upward nutrient supply. Following this event, the 25 σθ isopycnal was maintained at a shallower depth for one and a half days, providing favorable conditions for nutrient utilization by photoautotrophs. Over the study period, increased oxygen in the upper 200 m was estimated at 0.95 ± 0.40 mol m−2, corresponding to decreases in nitrogen and phosphorus of 0.049 ± 0.042 and 0.0030 ± 0.0027 mol m−2, respectively. This indicated that their O2 : N : P stoichiometry (313 ± 305 : 16 ± 20 : 1) overlapped with the canonical Redfield stoichiometry. The observed NCP statistically corresponded to approximately half of the annual NCP in subtropical oceans, although this short‐term NCP may have been offset by respiration over time. This study demonstrated that the lower EZ could temporarily exhibit net autotrophy, with NCP enhanced by internal wave–driven nutrient supply.