The Changjiang River plume shifts from carbon source to sink when net community production exceeds a threshold in early autumn

Di Wu; Jianfang Chen; Kui Wang; Xiaobo Ni; Dewang Li; Dingyong Zeng; Wei Fan; Dawei Xu

doi:10.1016/j.scitotenv.2023.164126

The Changjiang River plume shifts from carbon source to sink when net community production exceeds a threshold in early autumn

Sci Total Environ. 2023 Aug 25:888:164126. doi: 10.1016/j.scitotenv.2023.164126. Epub 2023 May 13.

Authors

Di Wu¹, Jianfang Chen², Kui Wang³, Xiaobo Ni², Dewang Li², Dingyong Zeng², Wei Fan⁴, Dawei Xu⁵

Affiliations

¹ Ocean College, Zhejiang University, Zhoushan, Zhejiang 316021, China; Donghai Laboratory, Zhoushan, Zhejiang 316021, China.
² Second Institute of Oceanography, Ministry of Natural Resources, Hangzhou, Zhejiang 310012, China.
³ Ocean College, Zhejiang University, Zhoushan, Zhejiang 316021, China; Donghai Laboratory, Zhoushan, Zhejiang 316021, China; Second Institute of Oceanography, Ministry of Natural Resources, Hangzhou, Zhejiang 310012, China. Electronic address: kwi@zju.edu.cn.
⁴ Ocean College, Zhejiang University, Zhoushan, Zhejiang 316021, China.
⁵ State Key Laboratory of Marine Geology, Tongji University, Shanghai, Shanghai 200092, China.

PMID: 37187391
DOI: 10.1016/j.scitotenv.2023.164126

Abstract

Estuaries are crucial components of the global ocean carbon cycle due to their high productivity. However, our understanding of the carbon source-sink dynamics at the air-sea interface of estuaries is incomplete, largely due to the rapidly changing environmental conditions. To address this, we conducted a study in early autumn 2016 using high-resolution biogeochemical data collected through buoy observations in the Changjiang River plume (CRP). Using a mass balance approach, we examined the factors driving changes in the sea surface partial pressure of carbon dioxide (pCO₂) and quantified the net community production (NCP) in the mixed layer. We also explored the relationship between NCP and the carbon source-sink dynamics at the air-sea interface. Our results revealed that biological activities (64.0 %) and seawater mixing (19.7 %, including lateral transport and vertical mixing) were the dominant factors controlling changes in sea surface pCO₂ during the study period. Moreover, NCP in the mixed layer was affected by factors such as light availability and the presence of respired organic carbon associated with vertical mixing of seawater. Notably, we observed a strong correlation between NCP and the difference in pCO₂ between air and sea (δpCO₂), with a threshold NCP value of 308.4 mmol m^-2 d^-1 identified as an indicator of the transition from a CO₂ source to a sink in the CRP. Hence, we suggest that the NCP in a specific ocean box has a threshold, beyond which the air-sea interface in estuaries will change from a carbon source to a carbon sink, and vice versa.

Keywords: Air-sea interface; Carbon source-sink dynamics; Changjiang River plume; Net community production; Seawater mixing.