Declining particulate organic carbon flux to estuary yet rising oceanic flux over the past 20 years: a case study of the Pearl River Estuary

Abstract

A substantial amount of particulate organic carbon (POC) is transported by the Pearl River to the Pearl River Estuary (PRE) and the northern South China Sea (NSCS) via its dense river network and eight outlets. The basin's high-intensity human activities and the estuary's complex hydrodynamic environment result in spatiotemporal variability in the POC flux entering both the estuary and the sea. Utilizing 30-meter spatial resolution Landsat-7/8 satellite data with the the Finite Volume Community Ocean Model (FVCOM), this study estimated the monthly POC fluxes entering the PRE and the NSCS from 2001 to 2020. The results indicate that the annual mean POC flux entering the PRE is 0.13 Tg C/yr. Over the past 20 years, increased dam construction in the basin has resulted in a decline in the POC flux entering the PRE and a reduction in its annual variability. Additionally, the decrease in sediment concentration has increased the proportion of autochthonous POC, leading to a significant rise in POC (%TSM) in the Pearl River. Approximately 25% of the POC from the river deposites in the PRE each year, while the remaining 0.098 Tg C of POC flows into the NSCS. Over the past 20 years, with a reduced POC flux entering the estuary, increased runoff, and a higher proportion of northerly winds, the POC deposition in the PRE has decreased by 47%, and the POC flux entering the NSCS has increased by 8.7%. Thus, despite the decrease in POC flux entering the estuary over the past 20 years, the increase in POC flux entering the sea influences seasonal hypoxia, carbon source-sink patterns and the nutrient structure in the PRE. The combined use of satellite data and numerical model provides a comprehensive and effective method for estimating POC transport in estuaries, thereby obtaining effective sea flux estimates.