Abstract
Abstract. The dissolved oxygen-to-argon ratio (O2∕Ar) in the oceanic mixed layer has been widely used to estimate net community production (NCP), which is the difference between gross primary production and community respiration; it is a measure of the strength of the biological pump. In order to obtain the high-resolution distribution of NCP and improve our understanding of its regulating factors in the slope region of the northern South China Sea (SCS), we conducted continuous measurements of dissolved O2, Ar, and CO2 with membrane inlet mass spectrometry (MIMS) during two cruises in October 2014 and June 2015. An overall autotrophic condition was observed in the study region in both cruises with an average Δ(O2∕Ar) of 1.1 % ± 0.9 % in October 2014 and 2.7 % ± 2.8 % in June 2015. NCP was on average 11.5 ± 8.7 mmol C m−2 d−1 in October 2014 and 11.6 ± 12.7 mmol C m−2 d−1 in June 2015. Correlations between dissolved inorganic nitrogen (DIN), Δ(O2∕Ar), and NCP were observed in both cruises, indicating that NCP is subject to the nitrogen limitation in the study region. In June 2015, we observed a rapid response of the ecosystem to the episodic nutrient supply induced by eddies. Eddy-entrained shelf water intrusion, which supplied large amounts of terrigenous nitrogen to the study region, promoted NCP in the study region by potentially more than threefold. In addition, upwelling brought large uncertainties to the estimation of NCP in the core region of the cold eddy (cyclone) in June 2015. The deep euphotic depth in the SCS and the absence of correlation between NCP and the average photosynthetically available radiation (PAR) in the mixed layer in the autumn indicate that light availability may not be a significant limitation on NCP in the SCS. This study helps us to understand the carbon cycle in the highly dynamic shelf system.
Highlights
Oceanic carbon sequestration is partially regulated by the production and export process of biological organic carbon in the surface ocean
chlorophyll a (Chl a) concentration ranged from 0.01 to 0.71 μg L−1 and was on average 0.18 ± 0.13 μg L−1, which is comparable to the 11-year mean value (∼ 0.2 mg m−3) in the same region in October reported by Liu et al (2014). (O2/Ar) values were in the range of −2.9 %–4.9 % and slightly oversaturated in most areas (Fig. 2d)
Most of the (O2/Ar) values were positive in the study region, whereas the negative values were concentrated along Transect 4 (Fig. 3f)
Summary
Oceanic carbon sequestration is partially regulated by the production and export process of biological organic carbon in the surface ocean. Net community production (NCP) corresponds to gross primary production (GPP) minus community respiration (CR) in the water (Lockwood et al, 2012) and is an important indicator of carbon export. NCP is equivalent to the rate of organic carbon export and is a measure of the strength of the biological pump (Lockwood et al, 2012). NCP effectively couples the carbon cycle and oxygen (O2) production through photosynthesis and respiration in the euphotic layer; many previous studies have measured the mass balance of O2 to quantify NCP (e.g., Emerson et al, 1991; Hendricks et al, 2004; Huang et al, 2012; Reuer et al, 2007). Dissolved O2/Ar has been developed as a proxy for NCP in a water mass
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