Copyright © 2005 by the author(s). Published here under license by the Resilience Alliance. Amemiya, T., T. Enomoto, A. G. Rossberg, N. Takamura and K. Itoh. 2005. Lake restoration in terms of ecological resilience: a numerical study of biomanipulations under bistable conditions. Ecology and Society 10(2): 3. [online] URL: http://www.ecologyandsociety.org/vol10/iss2/art3/ Research Lake Restoration in Terms of Ecological Resilience: a Numerical Study of Biomanipulations under Bistable Conditions Takashi Amemiya 1 , Takatoshi Enomoto 1 , A. G. Rossberg 1 , Noriko Takamura 2 , and Kiminori Itoh 1 ABSTRACT. An abstract version of the comprehensive aquatic simulation model (CASM) is found to exhibit bistability under intermediate loading of nutrient input, supporting the alternative-stable-states theory and field observations for shallow lakes. Our simulations of biomanipulations under the bistable conditions reveal that a reduction in the abundance of zooplanktivorous fish cannot switch the system from a turbid to a clear state. Rather, a direct reduction of phytoplankton and detritus was found to be most effective to make this switch in the present model. These results imply that multiple manipulations may be effective for practical restorations of lakes. We discuss the present results of biomanipulations in terms of ecological resilience in multivariable systems or natural systems. Key Words: alternative stable state; biomanipulation; bistable; comprehensive aquatic simulation model (CASM); resilience INTRODUCTION Several mathematical models of ecosystems have been proposed and used for understanding complex aspects of ecosystems (Walker et al. 1981, DeAngelis et al. 1989, Hulot et al. 2000, Scheffer 2001). Among these models, the comprehensive aquatic simulation model (CASM) (DeAngelis et al. 1989) has been found to be useful in applications for several aquatic ecosystems, and has been used for ecological risk assessment (Bartel et al. 1999, Naito et al. 2002). A simplified, more abstract version of CASM introduced by DeAngelis et al. (1989), abbreviated as abs.-CASM hereafter, is more suitable for a theoretical analysis; it was previously employed for studying stability of ecosystems (DeAngelis et al. 1989). Abs.-CASM retains important characteristics of aquatic ecosystems: material circulation, species interactions among three trophic levels, and the effects of human activities on aquatic ecosystems. DeAngelis et al. (1989) have analyzed abs.-CASM in terms of resilience, sensu Pimm (1984), i.e., the return time following perturbations (“engineering resilience,” Holling 1996). For their analyses, monostable states were examined as a function of nutrient loading. Many ecosystems, including shallow lakes, are known to exhibit bistable states or alternative stable states (Scheffer et al. 2001). Scheffer (2001) and Beisner et al. (2003) have shown bistable behavior in two- or three-variable aquatic models. We show that abs.-CASM also exhibits bistable behavior under a realistic range of parameter values, and that it is useful to study the properties of alternative stable states. Ecological resilience, below just referred to as resilience, is introduced to describe the properties of alternative stable states in ecosystems (Holling 1973). Resilience concerns large and drastic changes in ecosystems. It characterizes “the amount of change the system can undergo and still retain the same controls on function and structure” (Holling 1973, 1996), and is used as a novel concept for ecosystem management (Scheffer et al. 2001, Bellwood et al. 2004, Carpenter and Brock 2004). Resilience is usually illustrated by using a sigmoidal bifurcation curve for a simple one-variable system, as shown in Fig. 1. The horizontal axis is an environmental factor, i.e., control parameter, such 1 Yokohama National University, 2 National Institute for Environmental Studies