Abstract: The year 2022 marked a period of extreme hydrological drought in the Changjiang Basin. Research on the distribution characteristics and sources of particulate biogenic elements at the Changjiang Estuary under drought plays an important role in understanding the marine biogeochemical cycles. Based on the observational data of particulate organic carbon, nitrogen, and phosphorus in the Changjiang Estuary during October to November (autumn), we analyzed the spatial distributions of particulate organic carbon, nitrogen, and phosphorus in the estuary waters under drought season, as well as the main controlling factors. In conjunction with the particulate carbon stable isotope data, a two-end-member mixing model was used to quantitatively estimate the contributions of riverine input organic matter and marine authigenic organic matter to the sources of particulate organic matter. The results showed that the concentrations of particulate organic carbon, nitrogen, and phosphorus decreased along the salinity gradient and were positively correlated with suspended particulate matter (SPM) and negatively correlated with temperature and salinity. Those results indicated that the changes of SPM in the Changjiang Estuary waters and the mixing process of salty and fresh water were important factors regulating the spatial distribution of particulate carbon, nitrogen, and phosphorus. Areas with a significant positive deviation from the theoretical dilution line in the concentrations of particulate organic carbon, nitrogen, and phosphorus were found at the estuary and in the bottom layer of nearshore waters, corresponding to the highvalue areas of SPM and chlorophyll a. This suggested that sediment resuspension and biological processes played an additive role. Compared to particulate organic carbon and particulate nitrogen, sediment resuspension had a stronger additive effect on particulate phosphorus, causing the ratios of carbon to nitrogen to phosphorus deviate from the Redfield ratio. The twoendmember mixing model showed that from the estuary to the nearshore and then to the offshore, the proportion of riverine input organic matter gradually decreased, while the proportion of marine authigenic organic matter gradually increased, with the proportion of marine authigenic organic matter nearshore being greater than that of riverine input. Our results provided a new perspective for understanding the response of the biogeochemical cycles in the Changjiang Estuary to extreme climate change.

Key words: Changjiang Estuary, particulate organic carbon, nitrogen and phosphorus, carbon stable isotope, spatial distribution, source