Effects of Biological Production and Vertical Mixing on Sea Surface pCO₂ Variations in the Changjiang River Plume During Early Autumn: A Buoy‐Based Time Series Study

Abstract

A monthlong, high-resolution buoy time series from the surface ocean of the Changjiang River plume in early autumn 2013 (30-min sampling frequency) shows great variability in the partial pressure of carbon dioxide (pCO 2 ) and other physical and biogeochemical parameters. Early in the deployment, surface pCO 2 decreased by~117 μatm in a single day (11-12 September, from an initial value of~527 μatm); a similar decline of 62 μatm occurred five days later (to~378 μatm). Both drawdown events were associated with strong vertical stratification, high chlorophyll a concentrations, and oxygen supersaturation. A one-dimensional mass balance model suggests that biological production was responsible for more than half the pCO 2 decrease observed during 10-23 September. Subsequently, in association with strong winds, the mixed layer rapidly deepened and surface pCO 2 increased sharply (by about 108 μatm in late September and again in early October). Vertical mixing accounted for more than half of this pCO 2 increase, which offset more than the earlier biologically driven CO 2 drawdown. In the presence of such strong temporal variations of pCO 2 , sampling frequency exerts a substantial influence on air-sea CO 2 flux calculations for the Changjiang River plume and similar coastal areas. Compared to daily sampling, even weekly sampling would result in a bias of up to ±4.7 mmol C · m À2 · day À1 or ±63% error.Plain Language Summary Determining the mechanisms that control sea surface pCO 2 and its variability in coastal waters is an important step toward estimating global air-sea CO 2 fluxes and projecting future atmospheric CO 2 levels. In this study, a 31-day high-resolution buoy time series from the surface ocean of the Changjiang River plume in early autumn 2013 show great variability in the partial pressure of carbon dioxide (pCO 2 ) and other parameters. Early in the deployment, surface pCO 2 decreased sharply in short period (one to five days). The pCO 2 drawdown events were associated with strong vertical stratification, high chlorophyll a concentrations, and oxygen supersaturation. A mass balance model suggests that biological production was responsible for more than half the pCO 2 decrease observed. Subsequently, in association with strong northeast winds, the mixed layer rapidly deepened and surface pCO 2 increased sharply. Vertical mixing accounted for more than half of this pCO 2 increase. In the presence of such strong temporal variations of pCO 2 , sampling frequency exerts a substantial influence on air-sea CO 2 flux calculations for the Changjiang River plume and similar coastal areas.With respect to the partial pressure of carbon dioxide (pCO 2 ) at the sea surface, the biological processes triggered and maintained by subsurface and riverine nutrient inputs are particularly important in lowering surface pCO 2 values (Cao et al.

Hypoxic Bottom Waters as a Carbon Source to Atmosphere During a Typhoon Passage Over the East China Sea

“…Furthermore, we observed significant relationship between p CO 2 and Chl a with a slope value of −34.49 (supporting information Figure S2), which is close to that obtained using underway data in the East China Sea shelf (−49.8; Tseng et al, ). Very significant relationship between p CO 2 and DO was also observed (Figure c), which was similar to previous studies (Li et al, ; Zhai & Dai, ). Such significant relationships suggest the controlling effect of biological production on surface p CO 2 during Period III.…”

Temporal variability and driving factors of the carbonate system in the Aransas Ship Channel, TX: A time-series study

“…Estuaries are challenging to fully understand because of the heterogeneity between and within estuaries that is driven by diverse processes operating on different time scales such as river discharge, nutrient and organic matter loading, stratification, and coastal upwelling (Jiang et al, 2013;Mathis et al, 2012). The traditional sampling method for carbonate system characterization involving discrete water sample collection and laboratory analysis is known to lead to biases in average pCO2 and CO2 flux calculations due to daytime sampling that neglects to capture diel variability (Li et al, 2018). Mean diel ranges in pH can exceed 0.1 unit and single day ranges can exceed 1 pH unit, with especially high diel variability in biologically productive areas or areas with higher mean pCO2 (Challener et al, 2015;Cyronak et al, 2018;Schulz and Riebesell, 2013;Semesi et al, 2009;Yates et al, 2007).…”

Tidal mixing of estuarine and coastal waters in the Western English Channel controls spatial and temporal variability in seawater CO<sub>2</sub>

“…The study demonstrated that, despite estuaries representing <10 % of the domain area, interactions with shelf seas must be considered as the estuarine waters gave off more CO 2 than the shelves drew down. Eulerian studies in the estuarine zone have identified large tidal signals in pCO 2 with data collected from research vessels (Borges and Frankignoulle, 1999;Jeffrey et al, 2018a) and from moorings or fixed sites (Dai et al, 2009;Jeffrey et al, 2018a;Li et al, 2018;Bakker et al, 1996;Call et al, 2015;Ferrón et al, 2007;Santos et al, 2012). pCO 2 levels at the mouth of different rivers have been observed to co-vary with changes in river flow rate and with the tidal cycle (Ribas-Ribas et al, 2013;Canning et https://doi.org/10.5194/bg-2021-166 Preprint.…”