Blue-green algae
In our management area, we regularly see intense blooms of blue-green algae in surface water in many places. This is a sign that water quality and the nature that depends on it are not doing well. Blue-green algae sometimes produce toxins that can be poisonous and cause a lot of nuisance: swimming bans, foul odors, and mass fish deaths. This affects recreational users, residents, and businesses.
What are blue-green algae?
Blue-green algae are not algae, but bacteria with a blue-green color. They occur in all seasons and in all waters. Usually, we don't notice them much; their densities are so low that they cannot be seen with the naked eye. But if there is a lot of blue-green algae in the water, a 'bloom' occurs. When there is a lot of bloom, floating layers form: a blue-green, paint-like, sludgy substance that can smell terrible.
There are different types of blue-green algae, but what they have in common is that they grow and flourish when the water gets warmer. They feed on phosphates and nitrogen, which they can extract from the water. They also need light and air. Because there are more than enough phosphates and nitrogen in our waters, they can often grow here without restriction. This process begins in early spring and peaks during the summer months.
Watch out!
Blue-green algae can be toxic to humans and animals. Toxins can certainly be found in floating layers. However, it is difficult to say whether toxic substances are always present in floating layers. The same type of blue-green algae can sometimes produce toxins and sometimes not. And some species produce stronger toxins than others. For safety reasons, we always check the swimming water for the presence of toxic blue-green algae. We err on the side of caution: if there is blue-green algae in the water, we always put up warning signs.
Swimming water monitoring
We measure the quality of the official swimming locations in our area. We measure two types of bacteria and we measure the density and toxicity of blue-green algae. You can view the results of the measurements and whether they are good or bad atwww.zwemwater.nl. For more information, see the Swimming Water page.
Approach to blue-green algae
Because most bodies of water contain phosphates and nitrogen, prevention is impossible. Only if we can reduce the source, the phosphates and nitrogen, to very low levels will the blue-green algae disappear. But we live in an area where there is a lot of phosphate in the surface water, partly naturally and partly because humans have introduced it.
It is only possible to a limited extent to flush locations where blue-green algae occur with blue-green algae-free water. Sometimes it is not possible to flush. In other cases, the undesirable effect may occur that the blue-green algae move to another location.
Rain and strong winds cool the water and reduce blue-green algae. So a bad summer is also bad for blue-green algae. And in the fall, they decline anyway.
More information
More information about blue-green algae can be found on the websites ofSTOWA(Foundation for Applied Water Management Research), the Information Point for the Environment, and Rijkswaterstaat.
