Abstract:

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.

Key words: eutrophication, regime shift, recovery time, phosphorus recycling model

摘要：

为了揭示富营养湖泊恢复时间和负荷削减强度之间的非线性关系及其影响因素, 以典型湖泊为理论分析对象, 基于经典磷循环模型, 通过数值模拟, 计算得到不同负荷削减强度下的湖泊恢复时间, 即不同负荷下, 湖泊从初始富营养状态, P浓度逐渐降低回到清水稳态所需要的时间。通过计算不同参数取值下湖泊恢复时间的变化, 探究湖泊不同方面的特征对湖泊恢复时间的影响及可能的管理契机, 得到如下结果。1) 湖泊由浊水稳态向清水稳态的恢复时间和负荷削减强度之间存在非线性关系。若将外源负荷强度控制在略低于“浊水–清水”转换阈值, 则恢复时间在40年以上, 若加大削减强度, 则可缩短恢复时间; 但随着削减强度的持续增加, 其边际效应将逐渐减弱。2) 湖泊形态和状态会对恢复时间产生明显的影响。在同样的负荷削减强度下, 寒冷地区较深的湖泊恢复时间更短; 沉积物释放较快的湖泊恢复时间更长; 水力停留时间短的湖泊恢复时间也更短。因此, 从理论上讲, 通过生态修复工程措施来降低沉积物P的释放或改善水动力条件, 能够缩短富营养湖泊的恢复时间; 湖泊状态的改善还可降低“浊水–清水”稳态转换阈值, 进而降低治理难度。

关键词: 富营养化, 稳态转换, 恢复时间, 磷循环模型