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 ( p CO 2 ) and other physical and biogeochemical parameters. Early in the deployment, surface p CO 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 p CO 2 decrease observed during 10–23 September. Subsequently, in association with strong winds, the mixed layer rapidly deepened and surface p CO 2 increased sharply (by about 108 μatm in late September and again in early October). Vertical mixing accounted for more than half of this p CO 2 increase, which offset more than the earlier biologically driven CO 2 drawdown. In the presence of such strong temporal variations of p CO 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 −2 · day −1 or ±63% error.