Hydrological regime mediates stoichiometry regime and source-sink alternation in shallow lakes.

Shallow lakes are increasingly subjected to pronounced alterations in hydrological regimes and exacerbated nutrient stoichiometric imbalances due to climate change and anthropogenic factors. Understanding the interactions between watershed eco-hydrological processes and lake systems, particularly their impact on nutrient balance dynamics deserves further investigation. Employing seasonal-trend decomposition (STL), Copula modeling, and the Lindeman-Merenda-Gold (LMG) algorithm, this study systematically analyzed eco-hydrological processes in Poyang Lake basin and identified hydrological regime as the key factor governing lake nutrient balance. Notably, over the past three years (2022-2024), water level fluctuations intensified with the annual coefficient of variation (CV) rising by 15 % compared to the 15-year average (2010-2024). Moreover, the average N/P ratio was 15.40 (ranging from 6.29 to 32.73) over the past 45 years, with a transition trend from N&P co-limitation to P limitation. Low water levels amplified both lake nutrient imbalance and eutrophication risk, with an approximately twofold increase in overall risk probability compared to normal level. River inputs dominated N/P variations during low water level period (55 %), whereas lake sediment release became predominant during high water level phases (42 %). By extending the analysis to subtropical lakes globally, this study provides a foundation for hydrological management strategies that address eutrophication and support ecosystem resilience in floodplain lakes.