Swans in the marginal lakes of the IJsselmeerpolders

Swans are responding positively to the increase in aquatic plants triggered by the improved water quality in the marginal lakes of the IJsselmeerpolders.

Trends

Both the mute swan and Bewick's swan live on aquatic plants, particularly duckweed and pondweed, as well as on grass and stubble on farmland. In recent years, more swans have been staying longer on the Veluwemeer, because the aquatic plants have returned to this lake in response to improvements in the water quality.

Since 1980, when the sewerage works in Harderwijk started to remove phosphates from water before discharging it, the Veluwemeer has been flushed through with phosphate-poor water, and the water quality in the marginal lakes of the IJsselmeerpolders has improved. The water quality has also benefited from the fall in bream numbers brought about by fishing. As a result, there has been a sharp drop in the high levels of nutrients, which has benefited the aquatic plants. Vegetations of aquatic plants began to recover in 1990, particularly in the central marginal lakes, like the Veluwemeer. In the Gooimeer, however, the aquatic plants have not returned; although the water quality has improved, the lake is too deep for them.

Since the swans prefer to feed on aquatic plants, they are grazing less on the farmland near the Veluwemeer. Near the Gooimeer, however, they still prefer to graze on the farmland.