Waterfowl in temperate eutrophic lakes - Interactions with macroinvertebrates, submerged vegetation and fish

The aim of this thesis was to explore some of the important links between waterfowl and the biota in shallow eutrophic lakes in temperate areas. Waterfowl depend on macroinvertebrates as sources of protein and calcium, which are essential for egg production and growth. Macroinvertebrates in turn depend on submerged vegetation for food and shelter. Increased density of vegetation thus leads to increased density of epiphytic macroinvertebrates. However, at the same time as the amount of food to waterfowl increase with submerged vegetation, the availability to the food may decrease, because the invertebrates may become less available in dense vegetation. In a field study, I found that feeding ducks avoided areas with dense stands of submerged vegetation in spite of their high density of invertebrates. In another study, I also found that the invertebrates avoided the surface layer of the vegetation, and that some taxa migrated vertically up to the surface of the vegetation during night and down towards the sediment during daytime. This behaviour should decrease the availability of invertebrates to waterfowl, and possibly contribute to the low waterfowl utilization of areas with dense vegetation. Experiments showed that the vertical distribution and migration of the epiphytic invertebrates are related to the avoidance of fish predation, but also other factors such as avoidance of ultraviolet radiation may be important. Besides the behavioural effects by which fish may affect the availability of invertebrates to waterfowl, my studies suggest that even rather low densities of fish can reduce the amount of invertebrates by predation. Waterfowl themselves seldom significantly affect the amount of invertebrates by predation. In some cases, however, the grazing by large herbivorous waterfowl may have indirect negative effects on the invertebrates. Although long lasting reduction of submerged vegetation due to waterfowl grazing is not common, high numbers of autumn migrating swans and geese can at least temporary reduce the density of macroinvertebrates.