Optimization of exergy in a structural dynamic model

The hypothesis that ecosystems are optimizing exergy and the effect of applying exergy as optimizing function in modelling are tested in a structural dynamical model. The model used was developed to describe the development of the phytoplankton community in a Danish shallow lake as a result of biomanipulation. Exergy can be seen as the free biogeochemical energy of a system compared to its surroundings. The function has previously been proposed as a goal function for ecosystem development. In other words, exergy is thought to act as a quality indicator and to direct and govern the development of parameters in the ecosystem modelled. Traditional mathematical unconstrained optimization was found not to be adequate in the case described, since unrealistic values were reached during the optimization runs. Constraints on parameters were introduced by transforming them into a hyperbolic cosine function. The effects of changing optimization intervals, i.e. the frequency of optimization during the simulation, and the possible change per interval, i.e. the rate of change, were analysed. The first was to mimic a response time of the system illustrating either memory or resilience of the system. The second was thought to reflect the capability of variation of the genetic pools in the system. Both optimization interval and the allowed change per interval affected the rate with which the model evolved with time. Decreasing intervals and increase of rate speeds up the system. The sequence in which the parameters were manipulated was in accordance with observed dominance in natural ecosystems. The parameter most often changed by the optimizer was the maximum growth rate of the algae.