Marti, J., S. Gammeter & L. Schifferli
(* = Kurzbeitrag)
Die Entwicklung von Wandermuschel- und Wasservogelbeständen am Walensee 1967 bis 2003.
(von 1994 bis 2006 vergeben)
Dreissena polymorpha, Aythya ferina, Aythya fuligula, Aythya marila, Bucephala clangula
Bestandesentwicklung, Nahrungsangebot, Wasserqualität
Aythya ferina, Aythya fuligula, Aythya marila, Bucephala clangula, Fulica atra
Tafelente, Reiherente, Bergente, Schellente, Blässhuhn
Walensee, Glarus, St. Gallen
Effects of the colonization by Dreissena polymorpha on wintering waterbirds in a lake on the northern edge of the Swiss alps, 1967 to 2003. The Walensee is a medium-sized (24 km2), oligotrophic lake (10 mg PO4-P/m3) with steep banks (3.7 % shallow water <10 m) on the northern edge of the Swiss alps (419 m a.s.l.). It was invaded by the Zebra Mussel Dreissena polymorpha in 1982. Larval abundance has been quantified as an indicator of the adult mussels present the previous winter. Changes in the phytoplankton and the PO4-P concentration, have also been recorded since 1972. These three variables are compared with population fluctuations in the Tufted Duck Aythya fuligula (255, mean mid-January waterbird counts, 1967–2003, Tab. 1), the Common Pochard A. ferina (150), the Common Goldeneye Bucephala clangula (50) and the Common Coot Fulica atra (919). Phytoplankton and phosphate concentration both declined in parallel (Fig. 2) and were therefore correlated (Fig. 3). A 10-fold decline in phosphates, from the maximum of 27 mg PO4-P/m3 to a mean of 2 mg in the 1990s led to a decline by half in the phytoplankton. However, there were no correlations of these two variables with the larval numbers of the Zebra Mussel nor the four waterbird species. Winter populations of the Tufted Duck, the Common Goldeneye and the Common Coot (Fig. 5, 6, 7), but not the Common Pochard, were related to the number of Dreissena larvae. A 9-fold increase in larval numbers, from 1800/m 2 in the first three years following colonization in 1982 to a maximum of 15 700 in 1990, was followed by a doubling in Common Goldeneye and a 3-fold increase in Common Coot and Tufted Duck. After the peak, larvae numbers fluctuated widely at a lower level (Fig. 3). Tufted Ducks declined during this period to almost pre-Dreissena levels (Tab. 1). Common Goldeneye and Common Coot maintained higher winter populations than before the arrival of the mussel. Nevertheless, the new food resource had much less spectacular effects on the waterbird populations than on other Swiss lakes with a 5- to 10-fold in-crease. This might be due to the structure of the Walensee (only a small area of this deep lake is accessible to diving waterbirds) and perhaps a relatively low density of Dreissena in the nutrient-poor water.
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