Influences of catastrophic floods on the biogeochemistry of organic matter and nutrients in the Changjiang River estuary

Climate change is causing an increase in the intensity and frequency of extreme weather events globally. In the summers of 2016 and 2020, two catastrophic floods occurred in the Changjiang (Yangtze) River basin, resulting in the first and second largest monthly discharges since 2000. Understanding of how the coastal environments response to these extreme floods is important. Therefore, during each of these two years, two cruises (four in total) were conducted in the Changjiang River estuary and the adjacent shelf, just before and after those floods. The floods brought more low-salinity and high-SPM (suspended particulate matter) waters into the shelf areas, along with higher concentrations of particulate and dissolved organic carbon (POC and DOC). Lower POC (%) and lower POC to particulate nitrogen (PN) ratios (POC/PN) also implied that SPM and POC substances in the surface of estuarine and shelf seawaters became more terrestrial-material dominated after the floods. The extra nutrient inputs did not stimulate uptake rates and enhance non-conservative degrees of nutrients in the shelf, suggesting that phytoplankton growth and biological biomass were not elevated by these floods. We hypothesized that the higher turbidities induced by the floods limited phytoplankton production by reducing the amount of sunlight penetration. The results of the current study further revealed that significant evolutions of nutrient regimes over long time scales of at least 10 years might have happened, i.e., lower uptake rates of nutrients in the estuarine area than before. The more frequently occurring catastrophic floods might have contributed, at least to some degree, to those evolutions.