*2.1. Study Area*

Changjiang-diluted water has a strong influence on the Changjiang River estuary plume by virtue of a water discharge of about 944 × 10<sup>9</sup> m<sup>−</sup><sup>3</sup> year<sup>−</sup><sup>1</sup> [17] that carries a large amount of nutrients and sediments [18]. The eutrophic Changjiang-diluted water enters the upper estuary area, resulting in phytoplankton blooms that can absorb a substantial quantity of atmospheric CO2 [19]. At the same time, fluvial carbon input [20], as well as the decomposition and regeneration of organic matter in primary production, causes the estuary to release CO2 into the atmosphere [21]. In addition, the Changjiang River estuary plume has a regular semidiurnal tide [22], which results in periodic changes in sea surface temperature and salinity. The largest monthly water discharge at Datong Station, which is 624 km from the river mouth, occurred in July, with the second largest occurring in August [23].
