Charisma: a spatial explicit simulation model of submerged macrophytes

An important problem in lake management is the replacement of submerged plant species by competing species with undesirable properties. Although dynamics of rooted aquatic macrophytes have been extensively modeled, competition between different species has hardly been addressed so far. We present an individual-based spatially explicit model (‘Charisma’) designed to simulate the growth of one or more competing species of submerged macrophytes. The model behavior easily becomes rather complex and puzzling if we study more than one species. This probably reflects the complexity as it really occurs in nature, but poses a challenge for calibrating and understanding the model. To control this complexity, we used a flexible design facilitating the elimination of feedback loops and processes to trace the causes of the complex behavior of the model. The resulting ‘realistic, yet transparent’ model appeared useful in many aspects for bridging the existing gap between theory and reality. As an example, we present an analysis of competition between the charophyte Chara aspera Deth. ex Willd. and pondweed Potamogeton pectinatus L. The model suggests that alternative equilibria may arise as a result of two positive feedbacks: the enhancement of transparency by macrophytes and a feedback caused by bicarbonate competition.