Blue algae

Wageningen UR’s Flock & Lock method helps solve the Netherlands’ blue algae problem. Blue algal bloom in ditches, waterways and lakes is increasingly resulting in oxygen-depleted water, stench, fish mortality and swimming prohibitions.

The pollution of fresh surface water with fertiliser has become the world’s major water-quality problem. When the conditions are favourable algae can grow at an explosive rate in the presence of these indispensable nutrients for algae. This results in algal bloom. Blue algal bloom creates a turbid algae soup with paint-like floating layers (website in Dutch) on the water surface. This results in fish mortality. In addition, swimming is no longer safe. Members of the Aquatic Ecology and Water Quality Management Group are carrying out extensive blue algae research (website in Dutch) to map the current risks of blue algae in the Netherlands. They are also reviewing the potential consequences of climate change and suitable methods for tackling the nuisance caused by blue algae.

Toxic compounds

Blue algae are the oldest organisms on Earth. Although they look attractive and innocent under the microscope, many species produce annoying toxic compounds (website in Dutch). Contact with or the ingestion of blue algae can cause health complaints. Advanced techniques such as liquid chromatography and mass spectrometry – used, respectively, to separate liquids and identify molecules – enable the researchers to identify some 25 toxic compounds. Extensive measurement programmes provide an insight into which toxic compounds are present in the water in which concentrations, where, and when. For example, 88 official Dutch swimming locations were examined for the presence of blue algae toxic compounds and blue algae species. The researchers wish to establish which blue algae species produces which toxic compound, the risks, and the effects of climate change (website in Dutch) on the types and numbers of species in the water.

Chemical warfare

Climate change is beneficial to species such as Cylindrospermopsis raciborskii that is now advancing all over the world. This blue alga is also increasingly frequently encountered in the Netherlands. The great majority of the research is focused on a number of strains of this species in an endeavour to determine the potential consequences for the ecosystem and public health, work which Wageningen UR’s blue algae team is carrying out in close cooperation with Brazilian universities. One of the main elements of this study is the review of the chemical warfare between species of blue algae and blue algae and other algae: they release toxic compounds to retard each other’s growth.

Flock & Lock

Since 2006, water managers have been overwhelmed with offers of products that can purportedly tackle the blue algae problem. Examples of these products include the use of ultrasound (website in Dutch) and mud balls containing microorganisms that compete with the blue algae for nutrients. Their effect was examined in a study commissioned by the Directorate-General of Public Works and Water Management. The effects were disappointing. In the intervening period the Wageningen research team has developed the ‘Flock & Lock’ method. This method is based on the use of a flocculent that causes the blue algae to clump together and sink to the water bed, followed by the use of a phosphate fixative in a covering layer. This fixative prevents the release of free phosphate from the water bed and ensures that this essential nutrient is no longer available to blue algae. The blue algae are then unable to develop at an enormous rate.


The Flock & Lock method was subjected to exhaustive tests and then used in the De Rauwbraken swimming lake in Berkel-Enschot. What had previously been a turbid pool with few aquatic plans and profuse blue algae has now become a lake with clear water, an abundance of aquatic plants and virtually no algae – or, in other words, the technique was a success. The De Kuil swimming lake in Prinsenbeek has since been treated with Flock & Lock and now also has no problems with blue algae. This treatment is part of the Water Framework Directive Kansrijke innovatieve maatregelen bestrijding blauwalgenoverlast (‘Promising innovative measures to control blue algae hindrance’) innovation project.  This sub-project is focused on a further investigation of the effects of approaches such as dredging, Flock & Lock and fish population management on the improvement of water quality and the reduction of the hindrance caused by blue algae.