MEGAPLANT - A SIMULATION-MODEL OF THE DYNAMICS OF SUBMERGED PLANTS

A generic simulation model for the dynamics of submerged macrophytes i s presented, called MEGAPLANT ('Model Evaluating General Aquatic Pland Laws And New Theories'). Although the model is kept relatively simple , it includes many aspects that have an effect on vegetation dynamics during the seasonal cycle, such as grazing, overwintering strategies, wave action and seed dispersal. The characteristics of both the plant and the environment can be adjusted by the user. The model is meant as an aid to enhance insight into the interplay of processes that influe nce vegetation dynamics. As an example, it is used to analyze mechanis ms that determine the survival of vegetation in turbid eutrophic lakes . The following results were found. (1) Hot spots of propagule-produci ng vegetation can permanently maintain vegetation stands at sites that would otherwise be devoid of macrophytes. (2) Moderate fluctuations i n environmental factors that normally have little impact on the vegeta tion can cause large year-to-year fluctuations in biomass of vegetatio n growing under suboptimal conditions. (3) Grazing by herbivores such as coots can cause the existence of two alternative stable states: und er-exploited and overgrazed. This implies that herbivores may prevent the recovery of submerged macrophytes in lakes that are recovering fro m eutrophication, but also that temporary exclusion of herbivores can trigger permanent recovery of the vegetation. It is shown that the qua ntitative simulation results are sensitive to the setting of several p arameters but that the qualitative results presented are quite robust against changes in parameter setting. It is argued that although the p rospects of accurate predictive simulation of aquatic vegetation are p oor, relatively simple models like the one presented can be a useful t ool to improve the theoretical framework for understanding the dominan t mechanisms regulating submerged plant dynamics.