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

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.