Diagnosis guided restoration implementing a package of measures.
Site description
The Bleiswijkse Zoom was created in 1970. It is an elongated park north of Rotterdam, close to the river Rotte. The park has a recreational function and includes a lake, which is approximately 2.5 km in length and 140 to 5 m wide. The northern part is called Zeeltje (11.3 ha) and the southern part Galgje (3.1 ha). The north part includes an official bathing site with a depth of approximately 1 meter, and the southern part has a depth of 1.5 - 2 meters. The lake suffered annually from massive cyanobacteria blooms.
The Regional Water Authority and the Recreation Board Rottemeren wanted to minimize cyanobacterial blooms and to meet the Water Framework Directive targets of good ecological condition. The aim was to transfer the lake from its turbid, cyanobacteria-dominated state into a clear water lake with submerged vegetation (at least 50% coverage) enhancing the water quality and amenity and promoting recreational use of the lake.
Restoration measures applied
A system analysis guided package of restoration measures was applied to reduce the external and internal nutrient load and to increase the lake’s resilience. External load was reduced using a bypass to end the inflow of nutrient-rich water from the Rotte, by constructing an interception layer to bind nutrients from inflowing groundwater and pruning trees. The internal nutrient load was lowered by dredging followed by a P-binder (Phoslock) to immobilize phosphate remaining after dredging. Before dredging all fish were removed, and fish without carp and bream were re-stocked. The shores were reconstructed, a flexible water level implemented and a run-off interception ditch made. A maintenance plan is implemented allowing the use of P-binder to reduce phosphate levels when needed as a gradual increase is expected from ongoing diffuse pollution.
Current goals of restoration at the site
Increase ecological status, allow recreational use, and swimming.
Sectors involved in lake basin management
Regional Water Authority, Municipalities (via Recreation Board), anglers, citizens
Further reading