From External Pressure to Internal Drivers: Anthropogenic Disturbance and Positive Feedback Trigger Deep Lake Regime Shifts

Regime shifts in aquatic ecosystems have attracted widespread attention. While research has extensively focused on shallow lakes, such shifts in deep lakes remain less studied. This study investigates the mechanisms influencing regime shifts in Lake Fuxian, China's largest deep freshwater lake, which has experienced cyanobacterial blooms and rapid algal growth despite strict nutrient input regulations. We examined trends in diatoms, cyanobacteria, and geochemical elements over the past century using rate‐of‐change analysis, F ‐statistics and random forest regression models to identify the time of ecological shifts and the factors associated with them. We found two significant ecological shifts during the 1990s and 2010s. Both were accompanied by increases in TOC, hypolimnetic anoxia and cyanobacterial abundance. The 1990s shift was primarily associated with invasive fish species, whereas the 2010s shift was linked to phosphorus enrichment. Notably, phosphorus enrichment was not mainly associated with external nutrient inputs but by an internal positive feedback mechanism. Specifically, hypolimnetic anoxia facilitates phosphorus release from sediments, promoting algal growth during mixing periods, which increases oxygen demand. This feedback further alters nutrient dynamics, favouring the proliferation of algal biomass, particularly cyanobacteria, and may ultimately trigger an ecosystem regime shift. These results suggest that even oligotrophic deep lakes are susceptible to regime shifts, emphasising the importance of disrupting positive feedback mechanisms for effective management.