Waterfowl herbivory on submerged macrophytes in eutrophic lakes

Abstract: Submerged macrophytes are thought to stabilize clear-water states of shallow lakes. The aim of this thesis was to elucidate how waterfowl herbivory can affect abundance, distribution and species composition of submerged macrophytes in eutrophic lakes. I found that in a lake with sparse vegetation, which was in a colonization phase, small plants were uprooted and large plants were heavily grazed upon. In three macrophyte-dominated lakes, grazing did not reduce biomass severely. This suggests that the risk of waterfowl grazing causing lakes to enter a turbid state is low at moderate waterfowl densities, unless the amount of vegetation is sparse or in a phase of colonization. However, waterfowl grazing can have differentiated effects on different species of macrophytes, with consequences for species composition. In autumns, migrating waterfowl can reduce belowground tubers of Potamogeton pectinatus substantially, and particularly at intermediate water depth. Mainly large waterfowl (swans) dig in the sediment and their foraging lower the competitive ability of P. perfoliatus, and P. pectinatus can dominate the heavily grazed areas. It seems like P. pectinatus, at least partly, can compensate for the reductions of tubers in autumn by producing more tubers per plant the next summer. Direct effects of waterfowl herbivory can thus be a mechanism, behind plant species depth distribution.