Using triple oxygen isotopes and oxygen-argon ratio to quantify ecosystem production in the mixed layer of northern South China Sea slope region
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Abstract: Quantifying the gross and net production is an essential component of carbon cycling and marine ecosystem studies. Triple oxygen isotope measurements and the O2/Ar ratio are powerful indices in quantifying the gross primary production and net community production of the mixed layer zone, respectively. Although there is a substantial advantage in refining the gas exchange term and water column vertical mixing calibration, application of mixed layer depth history to the gas exchange term and its contribution to reducing indices error are unclear. Therefore, two cruises were conducted in the slope regions of the northern South China Sea in October 2014 (autumn) and June 2015 (spring). Discrete water samples at Station L07 in the upper 150 m depth were collected for the determination of δ17O, δ18O, and the O2/Ar ratio of dissolved gases. Gross oxygen production (GOP) was estimated using the triple oxygen isotopes of the dissolved O2, and net oxygen production (NOP) was calculated using O2/Ar ratio and O2 concentration. The vertical mixing effect in NOP was calibrated via a N2O based approach. GOP for autumn and spring was (169±23) mmol/(m2·d) (by O2) and (189±26) mmol/(m2·d) (by O2), respectively. While NOP was 1.5 mmol/(m2·d) (by O2) in autumn and 8.2 mmol/(m2·d) (by O2) in spring. Application of mixed layer depth history in the gas flux parametrization reduced up to 9.5% error in the GOP and NOP estimations. A comparison with an independent O2 budget calculation in the diel observation indicated a 26% overestimation in the current GOP, likely due to the vertical mixing effect. Both GOP and NOP in June were higher than those in October. Potential explanations for this include the occurrence of an eddy process in June, which may have exerted a submesoscale upwelling at the sampling station, and also the markedly higher terrestrial impact in June.
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Figure 1. Study area (a) and sampling stations (b). In image a, the black box indicates the location of plot b. GOP represents gross oxygen production, NOP net oxygen production, and TS time series station.
Figure 2. Surface salinity distribution in October 2014 (a) and June 2015 (b) as revealed by grid stations (diamonds). The red star indicates the location of Station L07 where O2 samples were collected. Note that salinity ranges and gradients in both seasons are the same.
Figure 3. Wind speed before the sampling date (0 d is the sampling date). The blue dashed line (7.4 m/s) indicates the mean wind speed of the 60 d for observation in 2014 and the red dashed-dot line (5.6 m/s) indicates that for 2015. Data derived from
www.remss.com .
Figure 4. Modeled mixed layer depth history until the sampling date (a. October 2014; b. June 2015). The observed mixed layer depth at the sampling date is shown as the red asterisk on the right y axis. The black dashed line indicates 5 d (a) and 8.5 d (b) before the sampling date, which gets close to the O2 residence time within the mixed layer. The bottom layer of the mixed layer criteria is a 0.125 kg/m3 difference from the surface. Note that the x-axis is reverse in direction relative to Fig. 3.
Figure 5. Pooled piston velocity k values in October 2014 (a) and June 2015 (b). mws, mean wind speed method (k″ in the text); tw (fixed mixed layer), time-weighted method with mixed layer unchanged (k′ in the text); tw (varied mixed layer), time-weighted method with mixed layer changed. Note that 7.49 and 2.52 were accepted as the k value in calculating the gross oxygen production for October 2014 and June 2015, respectively, and the x axis is disproportional.
Figure 6. O2 properties (δ18O, ([O2]/[O2]eq)bio, and Δ17O in per meg),
${{\rm {NO}}_3^-} $ concentration (μmol/L), and fluorescence signal at Station L07 in October 2014 (a, b) and June 2015 (c, d). The illumination condition is shown by dashed lines, PAR represents photosynthetic active radiation.
Figure 7. δO2/Ar values plotted against time at 5 m depth of the time series station. Note that the x axis is disproportional.
A1. Sea level anomaly (m) on October 13, 2014 (a) and June 15, 2015 (b) in this study. Note that both plots share the same contour range and color scale. Star indicates Station L07. Sea level anomaly data downloaded from https://las.aviso.altimetry.fr/las/.
A2. 14C-P rate versus depth within the euphotic zone in autumn (Station L07, mixed layer depth of 39.5 m) and spring (Station L05, mixed layer depth of 27 m). The dashed line indicates the bottom of mixed layer.
A3. δ18O plotted against [O2] (as ln(fraction of remained O2)) in this study for both spring and autumn samples beneath the mixed layer.
A4. Nutrients plotted against O2 properties in the depths where oxygen is depleted (i.e., ([O2]/[O2]eq)bio < 1). AOV, apparent oxygen utilization.
Table 1. Main physical parameters in spring (June) and autumn (October) in the present study
Parameter Spring Autumn Mixed layer depth/m 21.5 39.5 k (time-dependent)*/(m·d–1) 2.524 7.485 k′ (time-independent)/(m·d–1) 2.285 7.509 SST/°C 30.208 26.957 Salinity 33.808 34.061 [O2]eq/(μmol·L–1) 195 205 ([O2]/[O2]eq)bio 1.0111 1.0009 GOP/(mmol·m–2·d–1) (by O2) 189±26 169±23 NOP/(mmol·m–2·d–1) (by O2) 8.2±1.1 1.5±0.7 GOP/14C-P – 5.4 Note: GOP, gross oxygen production; NOP, net oxygen production; k, pooled piston velocity; [O2] and [O2]eq represent the measured and equilibrated O2 concentration in the water, respectively; 14C-P represents 14C-based primary production; – represents no data; * this value was considered in the final GOP calculation. 
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Table 2. Pooled production rates for the SCS shelf/slope region and other subtropical regions
Date INP/(mmol·m–2·d–1) (by C) IPP/(mmol·m–2·d–1) (by C) Region Data Source Jul./Aug. 2006 – >75 – Song et al. (2012) Jul. 2003, Jun./Jul. 2004 5–14 68.0–72.5 SCS shelf and slope regions Chen and Chen (2006) Oct. 2002 2.5–18 12.5–51.0 SCS shelf region Chen (2005) May 2005 – 27 SCS basin region Zhang et al. (2018) Jun./Jul.1998 – 19 SCS basin region Ning et al. (2004) Oct. 2014 – 34 – present study Jun. 2015 – 31 – present study Oct. 2014 1.5±0.7 169±23 SCS slope region present study Jun. 2015 8.2±1.1 189±26 SCS slope region present study Aug. 2012 0–50 200 SCS slope region Wang et al. (2014) July – 270–376 SCS shelf edge Hung et al. (2020) Annual 10.7 – SCS basin Huang et al. (2018) Annual 14±4 50–240 subtropical Pacific Quay et al. (2010) Aug. to Sep. 2006 5.9±0.9 121±34 western equatorial Pacific Stanley et al. (2010) Annual 6–8.3 29–103 BAT Luz and Barkan (2009) Annual – 0–280 BAT Nicholson et al. (2012) Annual – 25–210 HOT Nicholson et al. (2012) Note: BAT, Bermuda Atlantic time series study; HOT, Hawaii Ocean time series study; INP, integrated net production; IPP, integrated primary production; SCS, South China Sea; –, no data.
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