Contribution of Biological Effects 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

• Main Authors: Yifan Zhang, Dewang Li, Kui Wang, Bin Xue
• Format: Article
• Language: English
• Published: MDPI AG 2019-06-01
• Series: Water
• Subjects: biological processes; air–sea CO₂ exchange flux; net ecosystem production; potential CO₂ emissions; trophic status; Changjiang River estuary plume
• Online Access: https://www.mdpi.com/2073-4441/11/6/1264

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 (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 offshore regions generally serve as atmospheric CO₂ sinks. In the mixed layer of the three regions, pCO₂bio has an overall positive feedback effect 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 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.