Distribution characteristics of nutrients in the Changjiang River Estuary under the watershed extreme drought in July 2023

The freshwater discharges of the Changjiang (Yangtze) River into the sea largely control the extension range of the Changjiang Diluted Water in the estuary and the inherent biogeochemical behaviors of biogenic elements. In July 2023, a summer drought event occurred in the Changjiang River basin, with extremely lower river discharge. In order to study the distribution characteristics of nutrients in the Changjiang River Estuary in response to this drought, samples were collected from 32 stations in the Changjiang River Estuary that month, and the results were further compared with those obtained before and after the arrivals of the Changjiang River watershed floods occurring in the summers of 2016 and 2020. Under the drought condition in July 2023, the estuarine mixing area of freshwater and seawater greatly shrank, and the time and space for the occurrences of non-conservative processes of NO⁻₃-N and SiO²⁻₃-Si were largely restricted, resulting in that these two nutrients being more conservative compared 3 to those in the flood seasons of 2016 and 2020. At the same time, since the surface seawaters with low-to-medium salinity values (<25) under the drought condition were confined to areas with shallower water depths,-PO³⁻₄ released through sediment resuspension from the bottom was more easily transported to the surface, and the PO³⁻₄ P concentrations at surface increased and displayed a “source” mode. As the salinity continued to increase, the impact of the “sink” mode owing to surface phytoplankton assimilation gradually became dominant. In addition, with the increasing water depths, the PO³⁻₄-P released via bottom sediment resuspension could not penetrate into the surface layer anymore. The combined effect of the above two processes led to a rapid decrease in PO³⁻₄-P concentrations at surface when salinity was higher than 25. Thus in July 2023, the surface PO³⁻₄ -P concentrations showed a special “concave” trend. This study enriches and deepens our understanding on the response and feedback mechanisms of biogeochemical processes in the Changjiang River Estuary, to the large inter-annual variability in the Changjiang River discharges. This study also helps to further answer those key scientific questions regarding the triggering mechanism of algal blooms in the Changjiang River Estuary, a typical PO³⁻₄ -P -limited large-river estuary in the world.