To investigate the nonlinearity between recovery time of eutrophic lakes and the intensity of external load reduction, as well as the factors that could modify this time span, a quantitative analysis was conducted by model simulation. The authors employed a widely-applied phosphorous recycling model, and calculated the recovery time of a eutrophic lake to revert to clear state under different reduction rate. The parameters were set to different values to uncover how different attributes of the lake ecosystem could influence the recovery time. The model results showed that, there was a significant nonlinear relationship between load reduction and recovery time. When the external load reduced to slightly below the threshold, the recovery time would be longer than 40 years. Increasing reduction rate would result in significant decrease in recovery time, while its marginal effect became less significant. Lake type and morphology has significant influence on recovery time. Under the same reduction rate, recovery time of deeper lakes in colder regions is shorter; high sediment release rate requires longer recovery time; and longer hydraulic retention time leads to longer recovery time. Therefore, ecological remediation to reduce sediment release, or improve the hydro-dynamic conditions, may be effective. Moreover, this would both lower the threshold for clear phase, which lead to lower load reduction, and also shorten the recovery time, which made the remediation much easier.

Special attention has been paid to three types of processes, that is, source process, removal process and internal cycling. Through the summary of literatures, the mechanisms of typical cycling processes, such as sediment release and denitrification, were discussed. The comparison of contributions of different cycling processes was listed as well. Moreover, the main research approaches of experiment and modeling in this field were well summarized and compared, raising a general framework for the study of nutrients cycling in lakes. With the purpose of looking into the cycling of nutrients, different approaches should be combined together. For example, mechanism models and observation for the mass balance of nutrients, then the microcosmic experiments for the impact factors of cycling processes. It is an efficient way to explore the cycling of nutrients in lakes.