Simulating critical nutrient loadings of regime shift in the shallow plateau Lake Dianchi

The external inputs of nitrogen and phosphorus are crucial driving forces for regime shifts in lakes. However, the altered flow caused by anthropogenic activities and climate change complicates the transition process. Identifying critical nutrient loadings for state transitions and their relationship with flow change has become an increasingly crucial scientific issue for managing lake ecosystems. Lake Dianchi, a shallow plateau lake that suffers from long-term eutrophication and extensive water regulation, was used as a case study to explore the critical nutrient loading under changing flow regimes. To this end, we established the PCLake+ model for Lake Dianchi using the parameters from native submerged macrophytes and long-term observational data. The results showed the following. (1) From 2001 to 2018, the lake's trophic status was moderately eutrophic with slight inter-annual fluctuations, and the most severe period occurred in the summer and autumn. (2) The critical total nitrogen (TN) and total phosphorus (TP) loadings of the lake from the clear-water state dominated by submerged macrophytes to the turbid-water state dominated by algae were 45.89 mg N m-2d-1 and 3.53 mg P m-2d-1, respectively. Those from the turbid to the clear state were 26.78 mg N m-2d-1 and 2.06 mg P m-2d-1, respectively, which were close to the actual external inputs of TN and TP loadings, indicating that the lake entered a critical period for restoration. (3) The water level and inflow significantly changed the critical TN and TP loadings and impacted the restoration threshold more than degradation. Decreasing water level or increasing water inflow can effectively rise the critical TN and TP loadings of both degradation and restoration, which is conducive to accelerating the evolution of aquatic ecosystems towards a clear-water state dominated by submerged macrophytes.