**1. Introduction**

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. In summer, the East China Sea generally acts as a carbon sink for atmospheric CO2 (−4.6 ± 1.3 mmol m<sup>−</sup><sup>2</sup> day−1) [1–3]. 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. However, the originally deep (50 m) subsurface water of the Kuroshio [5] will rise and form an upwelling around 123◦ E, where there is a trough [6]. 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) e ffects on the coastal and shelf ecosystems lead to complex transitions between carbon sinks and sources [7]. Thus, observations at high temporal resolutions are urgently needed to study the e ffects of biological processes on carbon sinks and sources.

The di fference between gross primary production (GPP) and respiration (R) in an ecosystem is defined as the net ecosystem production (NEP) [8]. Negative NEP indicates that the ecosystem is heterotrophic, and positive NEP indicates that it is autotrophic; therefore, NEP can be used as an indicator of the trophic status, which is an important factor in the assessment of a specific ecosystem [9,10]. For example, Li [11] estimated the nutrient flux, primary production, and NEP in the Changjiang River estuary in the four seasons using the budget box model. Xu [12] used in situ sampling data and the "muddy" LOICZ (land–ocean interaction in the coastal zone) model to evaluate the tropical status of the Changjiang River estuary plume in summer and winter. NEP is also used to distinguish biogeochemical controls from other controls of carbon sinks and sources in marginal environments [13,14]. For instance, Borges established the relationship between mixed-layer NEP and air–sea CO2 flux in order to detail the function of biogeochemical processes in European coastal seas [15]. Studies of NEP in the Changjiang River estuary plume have mostly applied the biogeochemical budget model on a seasonal scale. However, the contributions of biological processes to the impact of air–sea CO2 flux using continuous monitoring data have rarely been reported. In addition, the quantification of potential CO2 flux under the mixed layer using NEP remains to be studied in depth.

In this study, data from 24 hours of continuous monitoring in the Changjiang River estuary plume in summer were used to further explore these processes. The diel variations in parameters such as carbon dioxide partial pressure (pCO2) and NEP in the near-shore, front, and o ffshore regions were calculated by a mass balance model to separate the controlling processes of pCO2. We di fferentiated the air–sea CO2 exchange flux associated with physical and biological processes and then quantified the contribution of biological processes to the total air–sea CO2 exchange flux in the mixed layer. We also attempted to calculate the quantitative potential CO2 emission under the mixed layer. Moreover, the NEP vales of the three regions were compared to assess the trophic statuses of the di fferent ecosystems. The results demonstrate the importance of biological processes in the regulation of estuarine carbon sources and sinks, and they also show the gradients of trophic statuses that are influenced by Changjiang-diluted water in the Changjiang River estuary plume.

Our research on the carbon sinks and sources and assessment of the trophic statuses is based on a 24-h dataset. Although this maybe a shorter period than the timescale at which pCO2 variation occurs in a carbonate system because of the bu ffer capacity of seawater, this study is meaningful from the perspective of the steady state over several months in summer in the Changjiang River estuary plume [16].

### **2. Materials and Methods**
