Biomanipulation and ecological modelling

Abstract

A structural dynamic model based on the thermodynamic concept of exergy is used to help understand the application of biomanipulation. Exergy is used as a goal function assuming that the highest possible values indicate what is happening in nature. The model with six state variables including zooplanktivorous and a predatory fish aims at explanation of the observed switch between two possible states in shallow lakes: a bream state and a pike state. The two states appear in certain range of nutrients and this fact may explain why biomanipulation not always has been used successfully For search of the phosphorus levels 54 simulations were carried out with different combinations of two parameters (phosphorus uptake rate by algae and grazing rate by zooplankton). Simulations lasted 1,100 time steps to ensure that either steady state, limit cycles or chaotic behaviour is attained. The parameter values giving highest exergy index suggest that in the ecosystem smaller species of zooplankton are selected (due to interactions with fish) above the levels of P concentration of 100-150 mu g . l(-1). This result is consistent with published information.