Aquatic macrophytes : restore, eradicate or is there a compromise?

Promoting indigenous submerged macrophytes is considered an important measure to restore shallow lakes. On the other hand, dense stands of aquatic vegetation often cause nuisance for boating, swimming and by obstruction of water flow. Consequently, the interests of recreational users may conflict with nature conservation. At first sight, aiming lake management at intermediate vegetation biomass seems a good solution for this controversy. However, as can be shown with a simple economical model, such a compromise may not always be an optimal solution in terms of total welfare across all users of a lake. Here, we show that a management strategy aimed at moderate plant biomass can also be unfeasible for ecological reasons. This is shown by applying two very different models: a minimal logistic model of plant growth which is easy to understand, and the relatively complex and realistic simulation model Charisma which, among other things, includes the seasonal cycle and a detailed description of the dynamics of light availability for macrophytes. The qualitative results of both models were remarkably similar. If the ecosystem has no alternative stable states, it is possible to set vegetation to any desired sustainable level using an appropriate harvesting strategy. However, if an intermediate vegetation biomass is to be realized the costs of harvesting are high, because the yield is predicted to be maximal in that case. If the ecosystem has alternative stable states, harvesting becomes risky because the vegetation may collapse entirely below a certain, in practice unknown, biomass. Also, even moderate harvesting may reduce the resilience of the vegetated state, making the ecosystem more vulnerable to adverse conditions such as unfavorable weather conditions or bird grazing.