Blue-green algae: how to spot it, risks and warnings

What is blue-green algae?

Blue-green algae is not actually an algae but a group of cyanobacteria that occur naturally in fresh water. In warm, still, nutrient-rich water — typically in the summer months — they can multiply explosively and form a floating layer. Some species produce toxins while doing so.

A bloom often shows as a green to blue-green, oily-looking layer or streaks in the water, sometimes with a musty smell. But beware: blue-green algae is not always visible to the naked eye, and not every green layer is blue-green algae. Never rely on what you can see alone.

What can it do to you?

Contact with blooming water can cause skin irritation and itching. Swallowing water can lead to stomach and intestinal complaints, headache or flu-like symptoms. Complaints are usually mild and pass on their own, but children swallow water more easily and are more at risk.

For dogs, blue-green algae is considerably more dangerous: they drink the water and lick their fur clean. Do not let your dog swim in water with an algae warning. If you have persistent complaints after swimming, contact your doctor and mention that you swam in open water.

Warning, negative advice or ban

The provinces monitor official swimming locations during the bathing season (1 May – 1 October) and issue measures you will find on the signs at the water and on zwemwater.nl. A warning means algae has been found and swimming may cause complaints. Negative swimming advice is more urgent: swimming is advised against. A swimming ban means swimming is not allowed.

On this site, algae measures show as a violet label on the spot, a violet ring around the map pin, and you can hide affected spots with the 'Hide algae warnings' filter. The data comes from the official zwemwater.nl publications and is refreshed daily during the bathing season.

Reducing the risk yourself

Check the current status of the spot before you go and always read the on-site signs — the situation can change from day to day. Avoid water with floating layers or streaks, shower after swimming, and when in doubt pick another spot: with hundreds of official locations there is almost always a clean alternative nearby.

Frequently asked questions

How do I recognise blue-green algae?

Often as a green to blue-green, oily-looking floating layer or streaks, sometimes with a musty smell. It is not always visible though — rely on the official warnings on the signs at the swimming spot and on zwemwater.nl, not just on your own eyes.

Is swimming with blue-green algae dangerous?

Contact can cause skin irritation; swallowing water can lead to stomach complaints, headache or flu-like symptoms. Complaints are usually mild and temporary. For dogs it is more dangerous — keep them out of the water during a warning.

When does blue-green algae occur in the Netherlands?

Mainly in the summer months (June to September), during sustained warm weather in still or slow-moving fresh water such as lakes and recreational ponds. After a heat wave the number of warnings usually rises quickly.

Where do I find current algae warnings?

On the signs at the official swimming location and on zwemwater.nl. This site shows the same official measures per spot, refreshed daily during the bathing season, with a violet algae label and a map filter.