Gross oxygen production and microbial community respiration in the oligotrophic ocean

Uncertainties in the temporal and spatial patterns of marine primary production and respiration limit our understanding of the ocean carbon (C) cycle and our ability to predict its response to environmental changes. Here we present a comprehensive time-series analysis of plankton metabolism at the Hawaii Ocean Time-series program site, Station ALOHA, in the North Pacific Subtropical Gyre. Vertical profiles of gross oxygen production (GOP) and community respiration (CR) were quantified using the ¹⁸O-labeled water method together with net changes in O₂ to Ar ratios during dawn to dusk in situ incubations. Rates of ¹⁴C-bicarbonate assimilation (¹⁴C-based primary production [¹⁴C-PP]) were also determined concurrently. During the observational period (April 2015 to July 2020), euphotic zone depth-integrated (0–125 m) GOP and ¹⁴C-PP ranged from 35 to 134 mmol O₂ m⁻² d⁻¹ and 18 to 75 mmol C m⁻² d⁻¹, respectively, while CR ranged from 37 to 187 mmol O₂ m⁻² d⁻¹. All biological rates varied with depth and season, with seasonality most pronounced in the lower portion of the euphotic zone (75–125 m). The mean annual ratio of GOP to ¹⁴C-PP was 1.7 ± 0.1 mol O₂ (mol C)⁻¹. While previous studies have reported convergence of GOP and ¹⁴C-PP with depth, we find a less pronounced vertical decline in the GOP to ¹⁴C-PP ratios, with GOP exceeding ¹⁴C-PP by 50% or more in the lower euphotic zone. Variability in CR was higher than for GOP, driving most of the variability in the balance between the two.