**Contribution of Biological E** ff**ects to the Carbon Sources**/**Sinks and the Trophic Status of the Ecosystem in the Changjiang (Yangtze) River Estuary Plume in Summer as Indicated by Net Ecosystem Production Variations**

### **Yifan Zhang, Dewang Li, Kui Wang \* and Bin Xue**

Key Laboratory of Marine Ecosystem and Biogeochemistry, State Oceanic Administration & Second Institute of Oceanography, Ministry of Natural Resources, P.R. China, Hangzhou 310012, China; angela06z@163.com (Y.Z.); dwli@sio.org.cn (D.L.); xuebin119@sio.org.cn (B.X.)

**\*** Correspondence: wangkui@sio.org.cn

Received: 6 May 2019; Accepted: 13 June 2019; Published: 17 June 2019

**Abstract:** We conducted 24-h real-time monitoring of temperature, salinity, dissolved oxygen, and nutrients in the near-shore (M4-1), front (M4-8), and o ffshore (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 (pCO2bio) 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 CO2 from organic matter degradation by microbes and CO2 uptake by phytoplankton) to the CO2 flux in the Changjiang River estuary plume. Results show that seawater in the near-shore region is a source of atmospheric CO2, and the front and o ffshore regions generally serve as atmospheric CO2 sinks. In the mixed layer of the three regions, pCO2bio has an overall positive feedback e ffect on the air–sea CO2 exchange flux. The contribution of biological processes to the air–sea CO2 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 o ffshore 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 o ffshore regions are autotrophic systems in the mixed layer, and all three regions are heterotrophic under the mixed layer.

**Keywords:** biological processes; air–sea CO2 exchange flux; net ecosystem production; potential CO2 emissions; trophic status; Changjiang River estuary plume
