The numbers of overwintering waterbirds at Lake Constance, a large lake on the borders of Germany, Austria and Switzerland, have increased fourfold since the early 1960s, with a daily average of 162 500 waterbirds from September to April as shown by a study published in the august issue of Freshwater Biology. This massive increase coincides with the major expansion of zebra mussels in the lake, originating in the Caspian-Aral basin, and a major pest in Europe and North America.
The main avian population changes concerned the mussel-eating tufted duck, pochard and coot; which currently make up some 80% of all wintering waterbirds at Lake Constance. Stefan Werner, Martin Mrtl and Karl-Otto Rothhaupt from the Limnological Institute at the University of Konstanz; and Hans-Gnther Bauer from the Max-Planck Institute for Ornithology in Radolfzell found that the diving ducks and coots deplete mussel populations by more than 90% in water as deep as 11 m each winter. Each bird consumes about 1.4 kg of mussels (fresh mass) per day. At one study site, a shallow lake at Konstanz, the birds removed a total of about 750 tons of mussels per km2 in the course of winter. It is apparent that wintering waterbirds exert a strong top-down effect on the littoral community, but the mussel stocks recover every summer during the absence of their main predators.
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