Contrasting impacts of flooding and tropical cyclones on estuarine plume environments

Plume environments in large river estuaries are particularly sensitive to extreme hydrological and atmospheric events, especially when these occur concurrently. In summer 2020, the Changjiang River experienced a severe fluvial flood lasting over 1.5 months, with a high peak discharge, while two tropical cyclones passed nearby. Observations demonstrated an unusual eastward extension of the low-salinity plume, accompanied by elevated surface chlorophyll concentrations. A coupled physical-biogeochemical model was used to identify key environmental changes during this period, and numerical experiments were conducted to quantify individual and compound effects. Floodwater persistently increased stratification and created sustained favorable light conditions for phytoplankton growth. In contrast, the passage of cyclones increased vertical mixing, weakened stratification, and reduced regional productivity. During the neap tide after the first cyclone passage, re-stratification and enhanced nutrient supply created favorable conditions for phytoplankton growth, consistent with the observed high chlorophyll concentration. Although tropical cyclone impacts largely counteracted those of the flood, the compound extreme event still exhibited overall positive effects on net primary production within the plume system. Heterogeneous responses and variable preconditions underscore the need for high-resolution, high-complexity, and flexible models to contextualize observations and enable accurate forecasting of future extreme events.