Dynamics of inorganic carbon and pH in a large subtropical continental shelf system: Interaction between eutrophication, hypoxia, and ocean acidification

Abstract

We examined the dynamics of dissolved inorganic carbon (DIC) and pH in the Pearl River Estuary (PRE) and the adjacent northern South China Sea (NSCS) shelf in summer, aiming for a better understanding of the interaction between eutrophication, hypoxia, and ocean acidification. Using a semi‐analytical diagnostic approach based on validated multiple end‐member water mass mixing models, we showed a −191 ± 54 μmol kg−1 deficit in DIC concentrations in an extensive surface plume bulge, corresponding to a significant pH increase of ∼ 0.57 ± 0.19 units relative to conservative mixing. In contrast, DIC additions in the bottom hypoxic zone reached ∼ 139 ± 21 μmol kg−1, accompanied by a decrease in pH of −0.30 ± 0.04 units. In combination with stable carbon isotopic compositions, we found biological production and CO2 outgassing to be responsible for DIC deficits in surface waters, while degradation of organic matter (OM) accounted for DIC additions in bottom waters. The PRE‐NSCS plume system as a whole served as a net source of atmospheric CO2 from the perspective of Lagrangian observations, because strong CO2 outgassing in the inner estuary overwhelmed the CO2 uptake in the plume despite strong phytoplankton blooms. Using a two‐layer box model, we further estimated that at least ∼ 45 ± 13% of eutrophication‐driven OM production in the surface plume accounted for 67 ± 18% of the DIC addition and oxygen consumption in bottom waters. Eutrophication also buffered ocean acidification in surface waters while hypoxia enhanced it in bottom waters, but their effects on acid‐base buffering capacity were secondary to the amplification of coastal ocean acidification caused by freshwater inputs.

Destruction and reinstatement of coastal hypoxia in the South China Sea off the Pearl River Estuary

“…The endmember values of brackish plume water were thus determined from the surface water samples near the mouth of the PRE with a salinity of ~ 16.9, mainly consisting of a mixture of riverine freshwater and offshore surface water. The DIC endmember of brackish plume water here was consistent with the predicted value using the endmember values of riverine/plume water reported by Su et al (2017), whereas it was higher than that calculated by using the endmember values of riverine freshwater from Zhao et al (2020) since the riverine DIC concentrations might be diluted by abnormally high river discharge in 2017 (Guo et al 2008). For simplification, DO concentrations in offshore surface water were assumed to be saturated, in equilibrium with the atmosphere, while the upwelled subsurface water was assumed oxygen-deficient by ~ https://doi.org/10.5194/bg-2020-346 Preprint.…”

“…Additionally, a smaller-scale hypoxic zone appeared under the surface bloom near the Huangmaohai sub-estuary, a region which was not fully covered by survey measurements. The general pattern of hypoxic zones was similar to that found in summer 2017 (Zhao et al 2020), yet with a slight offshore shift. The minimum oxygen level, 3.5 μmol kg -1 (~ 0.1 mg L -1 ), was observed in the bottom layer at Station F303, lower than the previously reported minimum (~ 7 μmol kg -1 at Station F304 (Su et al 2017)).…”

“…S2a. We adopted the endmember values of offshore surface water and upwelled subsurface water from Zhao et al (2020), as our sampling was limited almost exclusively within the 30-m isobaths (Fig. 1b) and these values were consistent with those found in previous studies (Cao et al 2011, Guo and Wong 2015, Su et al 2017.…”

“…1e). The extensive hypoxia observed in summer 2017 also occurred during the transformation from a spring tide to a neap tide (Zhao et al 2020). Hypoxia off the PRE thus could survive from the spring tides under the conditions of a widespread plume and weak winds.…”

Spatial Variation in Aragonite Saturation State and the Influencing Factors in Jiaozhou Bay, China

“…First, Ω arag and TA were calculated based on pH and DIC. The uncertainty of pH measurements made with electrodes calibrated on the NBS scale may cause a certain level of inaccuracy [62,63]. Thankfully, the spatiotemporal changes of pH in JZB were large enough, and the conclusions were valid based on these data.…”