Marked interannual variability in the relative dominance of phytoplankton over submerged macrophytes rather than regime shifts in a shallow eutrophic lake: Evidence from long-term observations

•Research has shown that the aquatic environment of Lake Yilong has experienced abrupt changes during past four decades.•Statistical analysis of the relationship between environmental factors and state variables indicates the absence of regime shifts in Yilong Lake.•Nutrients and water depth are the primary environmental factors driving the marked inter-annual variability.•An adaptive strategy including external nutrient loading reduction, sustainable water level management and biomanipulation is necessary for lake restoration. The theory of regime shift is rooted in the concept of fold catastrophe and posits the existence of alternative stable states, such as a lake dominated by phytoplankton or macrophytes under similar environmental conditions. Although regime shifts have been reported in many shallow lakes, recent evidence has revealed that a linear rather than dual relationship between chlorophyll-a (Chla) and nutrients is more common, indicating that regime shifts are rare in nature. Here, long-term field observations and the coverage of the submerged macrophyte area (SMA) retrieved from remote sensing (RS) images of Yilong Lake, in which regime shifts have been reported, were used to re-examine the presence or absence of regime shifts and to explore the drivers of abrupt changes. During the past three decades, Yilong Lake has experienced three abrupt changes in water quality, phytoplankton biomass (Chla) and the SMA. However, the relationship between Chla and nutrients was significantly linear, and the state variables (Chla, Secchi depth and SMA) changed along the gradient of environmental stress, which demonstrates the absence of regime shifts and instead highlights the presence of marked interannual changes. Moreover, we found that nutrients and water depth are the main environmental factors driving the establishment/collapse of the SMA and shaping the relative dominance of phytoplankton over the SMA, further inducing rapid changes in nutrient cycling and water quality in Yilong Lake. The significant interannual variability in the SMA and Chla in Yilong Lake reflects a pronounced weakening of resilience. To enhance resilience, an adaptive strategy involving external nutrient loading reduction, water level management and biodiversity restoration is proposed.