Managing Regime Shift in Lake Systems by Modelling and Simulation

Abstract

Large, abrupt and persistent change in ecosystem structures and dynamics, known as regime shift, is not always observed in advance. This lack of early warning poses problem for prevention and mitigation measures. Regime shift can be driven by natural processes or anthropogenic activities that push the ecosystem across a threshold. Lake or reservoir eutrophication is an example of regime shift that developed gradually over a long period due to persistent high levels of nutrients in the stagnant system. Driven by excessive nutrient loading, eutrophication involves the abrupt change from a clear-water low algal state (oligotrophic) to a turbid algal-dominated condition (eutrophic). Eutrophication degrades water quality, and impairs the supply of safe drinking water, leading to public health risks and economic losses. Hence, identifying the regime shift threshold for an oligotrophic or eutrophic lake is vital for determining the effective intervention measures or restoration measures, respectively. For this purpose, mathematical models linking algal concentration (\(\mu \)g/L chlorophyll a) to phosphorus concentration (\(\mu \)g/L) are formulated for two case studies. The first case study involves a temperate, large, deep and oligotrophic lake known as Lake Fuxian in China while the second examines a tropical, small, shallow and highly eutrophic lake known as Tasik Harapan (TH) in Universiti Sains Malaysia, Penang. Model simulations coupled with bifurcation analysis of the mathematical model identify the regime shift threshold and determine the type of lake response to nutrient loading. Model simulations revealed that the reversible and deep Lake Fuxian would only become eutrophic (algal concentration \(\ge \) 10 \(\mu \)g/L) by year 2380 if the current external phosphorus loading rate continues to increase linearly by a modest rate of 0.00016 \(\mu \)g/L/d per year. However, Lake Fuxian could abruptly shift from oligotrophic to eutrophic in just three years if there is a large increase in external phosphorus loading beyond the critical threshold of 0.0765 \(\mu \)g/L/d. For the highly eutrophic tropical and shallow TH, model analysis suggests that TH shifted to eutrophic state when the external phosphorus loading exceeded 0.01595 \(\mu \)g/L/d. In its current condition, a large reduction in the phosphorus input through dredging and flushing is needed to restore the irreversible state of TH.