A simple model for evaluating the costs and benefits of aquatic macrophytes

Dense beds of aquatic macrophytes often cause nuisance to boaters and swimmers, and may obstruct water flow. Management of aquatic vegetation is, therefore, often aimed at reducing the biomass of the plants. If the nuisance is caused by exotic invasive species, there usually is no controversy with nature conservation aims. In shallow lakes, however, the interests of recreational users may conflict with nature conservation because the promotion of indigenous submerged vegetation is considered an important tool for lake restoration. Aiming at intermediate vegetation biomass seems a good solution for this controversy at first sight. However, we argue that such a compromise is often not the best policy from a welfare economic point of view. We present preliminary results of a graphical model, showing that the overall benefit for all ecosystem users may be minimal at the intermediate vegetation biomass. Furthermore, even if there is an optimal benefit at an intermediate macrophyte biomass, we argue that it is may not always be feasible to force the vegetation biomass to the desired level. Due to ecological feedback mechanisms, the system can have two alternative stable states: one with high vegetation biomass and one with little or no vegetation. It is concluded that it will often be better to realise a management strategy aimed at keeping some lakes (or parts of lakes) free of aquatic plants, whereas allowing others to be densely vegetated.