Promoting low-canopy macrophytes to compromise conservation and recreational navigation in a shallow lake

Abstract

The shift from a turbid-water state to a clear macrophyte-dominated state in the shallow lake Veluwemeer (The Netherlands) has led to nuisance for recreational navigation. The nuisance concerns the dense beds of Potamogeton perfoliatus in particular, whereas the low-canopy forming charophytes cause much less harm. On the other hand, the importance of macrophyte cover for the stability of the clear-water state has been recognised. To assess the potential of mechanical removal of dense macrophyte beds, several cutting regimes were simulated in a mixed vegetation of P. perfoliatus and Chara aspera, using the individual-based model Charisma. These species occupy a wide range of water depths between 0.5 and 2.5 m, with C. aspera dominating the shallower zone and P. perfoliatus dominating the deeper zone; intermediary is a zone where either species may dominate as alternative equilibria. Both the cutting height and timing affected the amount of biomass present in summer. The effect of cutting was more profound for treatment later in the season. With a cutting level above the Chara-canopy, the simulations showed an increased biomass of C. aspera and reduction of P. perfoliatus. In the zone of alternative equilibria, it was possible to provoke a sustainable shift from P. perfoliatus dominance to C. aspera dominance. To achieve this, annual repetition of cutting for a number of years was necessary. A harvesting regime aimed at shifting the vegetation dominance from P. perfoliatus towards C. aspera could be an option for management of the lake, since it holds the perspective of decreased management effort after a number of years. Moreover, removing only high-canopy vegetation implies high conservation values and recreational use can be combined.