Eutrophication bifurcation analysis for Tasik Harapan restoration.
Regime shift is characterized by large, abrupt and persistent changes in ecosystem structure and dynamics. Bifurcation analysis is commonly used to identify regime shift equilibrium states and to distinguish their stability characteristics. Eutrophication in lake, a regime shift from clear-water oligotrophic state, is a stable equilibrium state that can persist for long duration. Characterized by undesirable turbid water condition, eutrophication has been known to impair valuable ecosystem services provided by lakes worldwide. The high incidence of eutrophication in Malaysian lakes (62%) mandates urgent need for lake restoration. The three-fold objectives of this paper are (1) to develop a mathematical model for analysing bifurcation criteria in regime shift, (2) to identify regime shift thresholds and (3) to propose effective ecosystem management strategy for shallow tropical lakes such as Tasik Harapan. A mathematical model consisting of four compartments: algae, phosphorus, dissolved oxygen and biochemical oxygen demand is formulated to analyse the eutrophication dynamics in the highly eutrophic Tasik Harapan (TH), a small shallow lake in Universiti Sains Malaysia (USM). Bifurcation analysis is performed by means of XPPAUT to identify the regime shift thresholds and to determine the type of lake response. Identified as irreversible, the eutrophication state of TH mandates an urgent lake restoration program to remove nutrients in the lake. Two restoration methods for reducing nutrients are assessed: (1) flushing of lake water and (2) hypothetical application of the invasive water hyacinth. Bifurcation analysis indicates that a flushing rate exceeding 0.042 day-1 is needed to restore TH to oligotrophic state. A complicated strategy of water hyacinth application would reduce the algae concentration from 300 μg/l to 120 μg/l after 9.6 years. A severe shortfall is the complicated and uncertain process of frequent removal of water hyacinth to prevent the lake from being overwhelmed by the invasive plants. The feasibility and sustainability of these two lake restoration methods are discussed. The insights gained would be useful to the relevant authorities in determining and implementing the best remediation measures for TH.