Submerged vegetation and the variation in the autumn waterfowl community at Lake Tåkern, southern Sweden

Abstract

We analysed data from 25 years of standardised mid-September counts of waterfowl at eutrophic Lake Tåkern, southern Sweden. For eight species, counts were compared with a national index constructed from similar counts at numerous lakes and coastal sites. For Crested Grebes and Mute Swans there was a correlation between counts at Lake Tåkern and the national index, suggesting that they simultaneously vary in number over a large geographic area . Indirect gradient analysis (PCA) revealed a strong temporal trend in the data, which was covaried out in a partial PCA to expose residual patterns . This ordination separated Pochards and Tufted Ducks at one end of the first axis from Cormorants and Goosanders at the other. A direct gradient analysis, with biomass of submerged macrophytes (recorded for 14 of the 25 years) as the sole environmental variable, showed that bird species composition varied significantly with plant biomass. The piscivores, Cormorants and Goosanders, were abundant in years with small amounts of plant biomass, while several species were most abundant in years with large amounts. Our analyses suggest that the abundance of submerged macrophytes is an important determinant of the bird community composition in eutrophic lakes