Duckweed and the water quality of the Veluwemeer
Duckweed species have expanded greatly in the Veluwemeer (Lake Veluwe) since the amounts of nutrients in the water have been reduced.
Trends
In the early 1960s, the Veluwemeer and the Drontermeer were overgrown with duckweed, but by the end of the decade the water in the Veluwemeer had become more turbid because inflowing nutrients had encouraged bluegreen algae to multiply. An increase in bream also contributed to the turbidity of the water. The result was that by 1969 the duckweed had almost disappeared. In 1979 measures were taken to abate the inflow of nutrients; these resulted in duckweed reappearing in the north-east of the Veluwemeer in the mid-1980s. Since then the duckweed has steadily expanded. It is accelerating the recovery of the water quality, as it stabilises the bottom sediment. Currently, about 60% of the bed of the Veluwemeer is covered with duckweed.
Other factors that have encouraged the water to regain its clearness are the fishing of bream and the return of the zebra mussel. In recent years the increased transparency and the large amounts of duckweed have led to an increase in the numbers of fish species and of birds that eat fish and duckweed.
Technical note
The maps are based on field surveys of the Regionale Directie IJsselmeergebied of the Directorate-General of Public Works and Water Management. The grid cells are 100 x 100 m.
References
- Berg, M.S. van der, R.W. Doef en J. Postema (2001). Waterplanten in het IJsselmeergebied. De Levende Natuur, 102: 179-183.
- Nat, E., J. Simons, M.A.A. de la Haye en H. Coops (1996). Verspreiding van kranswieren in Nederland. RIZA werkdocument 94.148. Lelystad.
