| Summary: | We conducted 24-h real-time monitoring of temperature, salinity, dissolved oxygen, and nutrients in the near-shore (M4-1), front (M4-8), and offshore (M4-13) regions of the 31° N section of the Changjiang (Yangtze) River estuary plume in summer. Carbon dioxide partial pressure changes caused by biological processes (pCO<sub>2</sub>bio) and net ecosystem production (NEP) were calculated using a mass balance model and used to determine the relative contribution of biological processes (including the release of CO<sub>2</sub> from organic matter degradation by microbes and CO<sub>2</sub> uptake by phytoplankton) to the CO<sub>2</sub> flux in the Changjiang River estuary plume. Results show that seawater in the near-shore region is a source of atmospheric CO<sub>2</sub>, and the front and offshore regions generally serve as atmospheric CO<sub>2</sub> sinks. In the mixed layer of the three regions, pCO<sub>2</sub>bio has an overall positive feedback effect on the air–sea CO<sub>2</sub> exchange flux. The contribution of biological processes to the air–sea CO<sub>2</sub> exchange flux (Cont) in the three regions changes to varying extents. From west to east, the daily means (±standard deviation) of the Cont are 32% (±40%), 34% (±216%), and 9% (±13%), respectively. In the front region, the Cont reaches values as high as 360%. Under the mixed layer, the daily means of potential Conts in the near-shore, front, and offshore regions are 34% (±43%), 8% (±13%), and 19% (±24%), respectively. The daily 24-hour means of NEP show that the near-shore region is a heterotrophic system, the front and offshore regions are autotrophic systems in the mixed layer, and all three regions are heterotrophic under the mixed layer.
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