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

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 similar to 117atm in a single day (11-12 September, from an initial value of similar to 527atm); a similar decline of 62atm occurred five days later (to similar to 378atm). 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 108atm 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 CO2 drawdown. In the presence of such strong temporal variations of pCO(2), sampling frequency exerts a substantial influence on air-sea CO2 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 4.7mmolCm(-2)day(-1) 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 CO2 fluxes and projecting future atmospheric CO2 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 CO2 flux calculations for the Changjiang River plume and similar coastal areas.