blue green algae

blue green algae

Improving water quality

Verspreiden Phoslock

Drijflaag blauwalgen

Bouwen zanddam

Zwanen in recreatieplas



Blue green algal problems occur regularly in the Gouden Ham recreational area and problems for recreational users occur frequently. The Recreation Corporation was started in 2005 to control this problem based on a range of research activities. These research activities are organized in collaboration with:

  • The Province Gelderland

  • Bussiness Association De Gouden Ham

  • Radboud University Nijmegen

  • Rijkswaterstaat Limburg

  • Water Service Rijkswaterstaat

  • Local authority West Maas en Waal

Short term measures

A variety of short term measures aimed at preventing or postponing the formation of scums have been implemented at various locations. The results of these short term measures are used in the research.

Control measures investigated are:

  • Keeping the water in motion by use of pumps

In 2006 and 2007 the Recreation Corporation used pumps to keep the water at the swimming location the Maaslanden in motion to prevent the formation of scum layers. At first this seemed to help, but when optimal conditions for blue green algal growth developed this method appeared to be insufficient and the concentration of blue green algae rose excessively.

  • Use of oil screens

In 2008 the use of pumps will probably be continued and oil screens will be placed near the inlet of the Maaslanden. The purpose of these is to keep the scums outside the areas used for swimming. A disadvantage of the use of these screens is maintenance as they have to be placed at the beginning of the summer season and then dismantled again at the end of the season.

  • Use of ultrasound waves in combination with oil screens

In May 2007, an experiment was started using ultrasound waves at the Het Groene Eiland swimming area. Oil screens were placed at the inlets to the lake and three sonar devices installed at different locations across the the lake. This method was also ineffective in keeping scums out of the swimming areas.

  • Closure of the Het Groene Eiland swimming area in combination with the use of Phoslock and additional measures

In early 2008, this Groene Eiland swimming area was permanently sealed off from the other water of the Gouden Ham through the construction of three dams. This means that the area is now a lake and water quality within the lake can be controlled independently of the outside water. The following additional measures have also been implemented to limit the growth of blue green algae:

  • Removal of inclining plants at the shores.

  • Lowering the phosphate content by Phoslock.
    Phoslock is a natural product based on bentonite clay which binds phosphate in the water in a form which cannot be used as a food source for algae.

  • Use of pumps and streamers
    When the water is in motion, it will take up more oxygen. In the short term this has been achieved through the use of an electrical pump, however in the longer term the possibility of using a windmill is under consideration.

  • Planting reeds
    To support the recovery of the water quality, a reed bed of about 2000 items has been planted. Every two years the reeds will be harvested. The reed beds form a part of the shores of the bay.

The effect of this measure package will be scientifically researched and guided by the University of Wageningen Research (WUR) and engineering consultants DHV with a monitoring program of a year.


The results of the measures undertaken are very encouraging. During the 2008 season, no blue green algae blooms have been detected in this bathing location, nor has there been a swimming ban.

Below you can find interim monitoring reports.

A note has to be made that this approach cannot be the final solution for the recreation area. The bathing bay is about 20 ha and shallow. The water in the Gouden Ham covers 200 ha and is at several places rather deep.

Monitoring results

Below you can find links to reports of measurements which have been performed in the framework of the short term results.

Totaal fosfaat 2006

Analyse juli 2007

Analyse augustus 2007

Totaal fosfaat september 2007

Analyse apr-mei 2008

Tabel P-gebonden sediment apr-mei 2008

Analyse RWS mei-juni 2008

Analyseresultaten april-september 2008



Aanleggen van de dam voor de zwembaai

Construction of dams for the swimming area






Strooien van Phoslock in de zwembaai

Spreading Phoslock in the swimming area







Blauwalgen buiten de zwembaai

The greenish water colour in this picture is caused by a fairly high concentration of blue green algae outside the swimming area.








Helder,veilig water in de zwembaai

Clear , save water inside the swimming area. (Both pictures are made at nearly the same time. The difference is significant.)