Using triple oxygen isotopes and oxygen-argon ratio to quantify ecosystem production in the mixed layer of northern South China Sea slope region

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.