Successional Patterns of Plants, Invertebrates and Ducks in a Man-Made Lake

(1) A shallow lake of 35 ha was created in northern Sweden by flooding a sedge meadow. Changes in water chemistry, the vegetation and the populations of invertebrates and ducks were followed between the third and eighth years after flooding. (2) Water pH, conductivity, K, Na and S04 contents in the lake outflow were higher than in the inflow. The chemical oxygen demand of the central lake water was negatively correlated with lake age. (3) The number of macrophytic species increased and the dominant submerged hydrophyte increased its coverage from a few percent to about 30%. (4) The Chironomidae were early colonizers, but between the third and eighth years the autumnal larval biomass decreased from 55 g per m2 to less than 10 g. Mean individual larval weight also decreased. The Asellus population increased steadily during the first five years of the lake's existence, thereafter decreasing. Coleoptera and Heteroptera were relatively early colonizers, while larger flying insects, e.g. Trichoptera, Ephemeroptera and Odonata became abundant later on. Snails were first noticed when the lake was 6 yr-old, thereafter they increased rapidly. (5) During the years studied no linear change in the species diversity of ducks was recorded. Peak numbers of ducks occurred in the fourth and fifth years after flooding. Thereafter they decreased. Duck production values paralleled the changes in value of their main food items. (6) The last finding stresses the importance of the early secondary successional stages, which provide an abundance of duck food at the right time of the season.