The tufted duck and mussels in the Markermeer

The Markermeer is of international importance for the tufted duck, but in the last 10 years the numbers of birds overwintering here have fallen sharply because of the decline in their main source of food: the zebra mussel.

Trends

In the winters of the 1980s about 6-10% of the European population of tufted duck overwintered in the Markermeer, but since 1990 the mean number of overwintering birds has gradually halved. The cause of this decline is the decrease in numbers of zebra mussels, which are the staple food for tufted duck in the Markermeer during the winter. In 2000 the mean biomass of the mussels was 47% of what it was in 1993.

The zebra mussel is a filter feeder, which feeds on phytoplankton. As an average mussel filters about a litre of water per day in its quest for food, if the density of mussels is high enough the water becomes clearer. Since 1990 the transparency (a measure of how clear the water is) of the Markermeer has declined (see graph), partly because of the decline in mussels. A possible reason for the mussels' decline is the increase in the silt content of the bottom sediment, which makes it more difficult for the mussels to anchor themselves to the lake bed.