Strong impact of wintering waterbirds on zebra mussel (Dreissena polymorpha) populations at Lake Constance, Germany

Abstract

1. Since zebra mussel invaded Lake Constance in the 1960s the number of wintering waterbirds increased fourfold. We studied the impact of predation by waterbirds (tufted duck Aythya fuligula, pochard Aythya ferina and coot Fulica atra) on the population of Dreissena polymorpha in winter 2001/2002. These three species, with monthly peak numbers of approximately 230 000 individuals, currently comprise up to 80% of the waterbird population wintering at Lake Constance. 2. Four different study sites and four depths, that represent typical and characteristic habitats of mussels in Upper Lake Constance, were chosen. 3. Zebra mussels were sampled before, during and after predation by waterbirds. Their biomass in shallow areas decreased by >90%; the biomass reduction in deeper areas was highly variable and dependent on the substratum. With one exception, no changes could be detected at the greatest depth (11 m). 4. Concurrent exclosure studies revealed that the decrease in zebra mussels was caused by waterbird predation. A GIS-based approximation revealed that in an area of 1 km 2 a total of approximately 750 t mussel fresh mass was removed by birds, which is equivalent to 1390 g mussels per bird per day. 5. Wintering waterbirds have a strong structural impact on the littoral community of Lake Constance and could be the key predator of zebra mussels.

Environmental factors shaping the distribution of common wintering waterbirds in a lake ecosystem with developed shoreline

“…For 400 instance, there were strong positive correlations between mussel biomass and the abundance of 401 diving ducks and coots, as found in other studies (Werner et al 2005). However, the positive 402 relationship between diving duck abundance and mussel biomass was significant only during the 403 normal water level period when the entire shoreline was under water.…”

Active pockmarks in a large lake (Lake Constance, Germany): Effects on methane distribution and turnover in the sediment

“…As in other sandy 'nonpockmark' substrates, the mussels were simply laying on the surface (Werner et al 2005) both inside and outside the pockmarks. Their abundance was similar to other sites within this area (M. Mö rtl pers.…”

Ecosystem services provided by waterbirds

“…In the USA alone, the damages and associated control costs were estimated at $1 billion/year (Pimentel, Zuniga & Morrison, 2005). Exclosure experiments have shown that diving ducks often greatly reduce the abundance of these bivalves in shallow areas (Hamilton, Ankney & Bailey, 1994;Werner et al, 2005). Eurasian coots Fulica atra also feed actively on the mussels and seem to prevent greater colonization of boats, associated infrastructure and floating vegetation in shallow invaded systems, all of which are likely to reduce the economic costs of the invasion (G. van der Velde, personal communication).…”