How extreme floods and droughts regulate total phosphorus exchange flux in China’s largest freshwater lake

Abstract

Understanding the evolving patterns of total phosphorus (TP) flux balance in freshwater ecosystems is essential for effective water environment management and planning. Extreme hydrological events driven by climate change have notably altered lake hydrodynamics, yet the TP flux response in river-connected lakes remains unclear. This study investigates the temporal evolution of TP flux in Poyang Lake from 2003 to 2022 based on hydrodynamic analysis, and quantifies the impact of extreme floods and droughts on the TP balance. The contribution of extreme floods to the annual average TP inflow and outflow was 18.42 % and 4.53 %, respectively, while that of extreme droughts dropped to 6.08 % and 9.06 %. We defined Flood Onset Abruptness Rate (FOAR) and Comprehensive Drought Index (CDI) to describe how tributaries respond differently to extreme events. Notably, the Rao River, characterized by short-duration and high-intensity floods, exhibited a flood-induced increase in annual average TP inflow of up to 35.45 %, while showing limited sensitivity to drought conditions. Net TP fluxes in Poyang Lake shifted from net inflow to net outflow, and eventually stabilized at a persistent net inflow condition between 2007 and 2020, with a mean annual net flux of 1.47 × 10⁶ kg, 92.7 % of which was attributed to extreme flood events. These findings highlight extreme floods as a primary driver of TP imbalance in large river-connected lakes and provide new insights into phosphorus cycling under intensifying extreme flood and drought regimes.