Blue-green algae are not algae but bacteria. Most species are blue-green in color. Blue-green algae nuisance usually occurs during warm weather in nutrient-rich water. This is because blue-green algae grow optimally at temperatures between 20 and 30 ºC; at even higher temperatures they grow faster. Blue-green algae occur mainly in fresh stagnant water such as ponds but also in swimming pools.

They float on the surface of the water, forming a layer that resembles oil. As the layer thickens and the algae have less space, they die. This creates a greenish, smelly mush. The green goo causes less light to enter the water. The breakdown and lack of light reduces oxygen in the water and can also kill fish and other aquatic animals. Blue-green algae produce toxins. These are harmful to humans and animals. Common symptoms include skin rashes, headaches, fever and gastrointestinal problems. Small children may suffer earlier because they regularly ingest some water while swimming.

Water quality monitoring

The official swimming season for natural waters runs from May 1 to October 1. During this period, water quality is measured at least every 2 weeks at designated swimming locations in South Holland, including the Zoetermeerse Plas (Noord Aa). Here, samples are taken at the beach and the play pond. The monitoring is done by the Hoogheemraadschap van Rijnland on behalf of the Province. The water is examined for pathogenic bacteria, clarity and the presence of blue-green algae.

Results of research with hydrogen peroxide

In 2018, screens were placed between the beach and parts of the lake. Highly diluted hydrogen peroxide was added to the water. This was to such a dilute degree that it did not harm humans, animals and plants in the water.

The research was a project from the Academy of Sciences led by the University of Amsterdam and Arcadis. The research worked partially well. The hydrogen peroxide caused the blue-green algae, which are extra sensitive to oxygen, to be removed from the water. The downside was that the screens surrounding the children's stand proved to be very fragile. As a result, blue-green algae was still regularly released from the pond itself into the water of the children's beach. The study further showed that the addition of an extremely small amount of hydrogen peroxide was not harmful to other fauna in the water, such as water fleas.

The research was paid for by the Royal Dutch Academy of Sciences; the Zoetermeerseplas was a testing ground for this. The technique was not yet developed to the point where we could treat the entire puddle with peroxide. This was the original intention.

Longer-term solutions

Together with the Rhineland, the municipality is working to redesign the water system so that the supply of phosphate (fertilizer for the blue-green algae) to the Zoetermeerseplas will stop or at least reduce significantly.

For the long term and most effective solution, the development of natural water purification in the pond and in the areas around the pond is underway. This process only takes a long time. Think years. The water purification is done using a marsh zone that consists of nine marsh pearls. These are locations around the Zoetermeerseplas that purify the pond's water and allow for more biodiversity in nature. Marsh pearls are slightly wetter locations where reeds and other plants can grow. They filter the water and absorb nutrients, reducing fertilizers in the pond. By isolating the pond and reducing fertilizer inputs, LG-Sonic buoys are likely to better prevent blue-green algae problems.

The marsh pearls mainly contribute to biodiversity in the pearls and Zoetermeerseplas. They occur in the bank or just on solid ground next to the pond. They provide better living conditions for plants and animals, including rare and legally protected species. This helps to improve the ecosystem in the Zoetermeerseplas .