The restoration of a shallow lake by introducing Potamogeton spp.: The impact of waterfowl grazing

Submerged macrophytes were introduced by transplantation and enclosure experiments were performed in shallow, eutrophic Lake Engelsholm, Denmark. We investigated whether or not transplantations were possible, whether protection from grazing had a significant positive influence on the recolonization potential of plants and whether the impact of grazing on macrophytes differed over the season. Enclosure experiments showed that transplanted Potamogeton crispus disappeared in early summer if not protected against waterfowl grazing. The results were supported by subsequent experiments that showed partial disappearance of Potamogeton pectinatus in early summer and a significant biomass reduction later in the season. We suggest that it is primarily waterfowl (coot, Fulica atra , and mallard, Anas platyrhynchos ) that up‐root young plants and graze on larger plants. Therefore, in order to promote recolonization of submerged, vascular macrophytes after fish manipulation, P. pectinatus , Potamogeton perfoliatus and Potamogeton lucens were transplanted into protected areas. During the first two years following transplantation, plant density development varied widely, from a decrease in initial density to an overall six‐fold increase. We conclude that macrophytes can be introduced by means of transplantation, although grazing might have a significantly negative influence even when waterfowl densities are relatively low (< 5.6 individuals ha −1 ). Therefore, we suggest that transplants should be protected from waterfowl until established stands are formed to facilitate submerged macrophyte growth and dispersal, and to optimize their positive effects on the lake ecosystem as a whole. Furthermore, we suggest that protection from grazing is more important in early summer than in late summer and autumn.