Time‐series observations of photosynthetic oxygen production in the subtropical western North Pacific by an underwater profiling buoy system

Abstract

We conducted time-series observations in the northwestern part of the North Pacific subtropical gyre (30 N, 145 E) from July 2012 to March 2013 using a profiling buoy system equipped with a fast repetition rate fluorometer and dissolved oxygen (DO) sensor in order to understand vertical and temporal variations in gross oxygen production (GOP) by phytoplankton and its contribution to the oxygen cycle in the upper ocean. Under stratified conditions (July-November), daily GOP slightly exceeded 2 mmol O 2 m −3 d −1 and only in near-surface waters. However, during periods of vertical mixing (December-March) when phytoplankton blooms occurred, daily GOP increased in the upper 30 m and often exceeded 5 mmol O 2 m −3 d −1 . The depth-integrated daily GOP within the euphotic layer ( Ð GOP EL ) was relatively low (84.0 AE 23.6 mmol O 2 m −2 d −1 ) during stratification, but increased gradually with vertical mixing and reached 460 mmol O 2 m −2 d −1 on 17 March 2013. Additionally, during stratified conditions, a subsurface oxygen maximum (SOM) formed below the mixed-layer depth (MLD) because the steep density gradient constrained upward diffusion of oxygen produced below the MLD. During the period of vertical mixing, the SOM disappeared, and DO concentrations within the upper 100 m increased through both increasing Ð GOP EL and atmospheric oxygen uptake. Using our buoy observations and air-sea gas flux model, we estimated that 24% AE 7% of Ð GOP EL was emitted to the atmosphere during the stratification period, and Ð GOP EL during the mixing period was about 2.4 times the rate of oxygen uptake by the ocean from the atmosphere. 18 O spike (Bender et al. 1987) incubation methods. These methods rely on the change in oxygen concentrations or the evolution of 18 O 2 from 18 O-H 2 O during water splitting in incubation bottles containing the phytoplankton. However, despite its recognized importance, information on

Global distribution and variability of subsurface chlorophyll a concentration

“…In region C, the surface Chl-a concentrations increased after the shallowing of the mixed layer and with sufficient light in summer (Figure 8h and 8i). These features are consistent with regional studies in the literature (e.g., Baldry et al, 2020;Fujiki et al, 2020;Mignot et al 2014;Sverdrup, 1953). The latitudinal dependence of the occurrence of the subsurface Chl-a maxima also pointed out in Cornec et al (2021).…”

Global distribution and variability of subsurface chlorophyll <i>a</i> concentrations

“…10h, i). These features are consistent with regional studies reported in the literature (e.g., Baldry et al, 2020;Fujiki et al, 2020;Mignot et al, 2014;Sverdrup, 1953). The latitudinal dependence of the subsurface Chl a maxima occurrence was also noted by Cornec et al (2021).…”

“…This is consistent with an f ratio of 15 % reported at station ALOHA (23 • N 158 • W; Karl et al, 1996), considering a small but nonzero seasonal dissolved inorganic carbon drawdown reported in the subtropical surface layer (Yasunaka et al, 2013(Yasunaka et al, , 2021. It should be noted that oxygen production is not always associated with an increase in biomass at shallow depths in the subtropics (Fujiki et al, 2020).…”

Comment on os-2021-88

“…This is consistent with the f-ratio of 15% reported at station ALOHA (23°N 158°W; Karl et al, 1996), considering small but nonzero seasonal dissolved inorganic carbon drawdown reported in the subtropical surface layer (Yasunaka et al, 2013(Yasunaka et al, , 2021. However, it should be noted that oxygen production is sometimes not associated with biomass increase in shallow layers in the subtropics (Fujiki et al, 2020).…”