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 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.
Highlights
The Changjiang estuary plume is a typical marginal sea with a coastal continental shelf that has large spatial and temporal variations in carbon sinks/sources
The results demonstrate the importance of biological processes in the regulation of estuarine carbon sources and sinks, and they show the gradients of trophic statuses that are influenced by Changjiang-diluted water in the Changjiang River estuary plume
Changjiang-diluted water has a strong influence on the Changjiang River estuary plume by virtue of a water discharge of about 944 × 109 m−3 year−1 [17] that carries a large amount of nutrients and sediments [18]
Summary
The Changjiang estuary plume is a typical marginal sea with a coastal continental shelf that has large spatial and temporal variations in carbon sinks/sources. The influence of physical processes, such as strong winds, and the large amount of dissolved inorganic carbon produced by respiration under the mixed layer turns the region into an atmospheric carbon source [4]. The water mass compositions in the mixed layer of the Changjiang River estuary plume are determined primarily by the Changjiang Diluted Water and the Kuroshio Surface Water. On shorter time scales (e.g., 24-h), the complicated physical (upwelling, wind, tidal mixing, etc.) and biogeochemical (including the release of CO2 from organic matter degradation by microbes and CO2 uptake by phytoplankton) effects on the coastal and shelf ecosystems lead to complex transitions between carbon sinks and sources [7]. Observations at high temporal resolutions are urgently needed to study the effects of biological processes on carbon sinks and sources
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