Blue-green algae and algal blooms: avoid the water

Cyanobacteria are ancient, tiny organisms that, like plants, get energy from sunlight. They occur naturally in many waters. When it is warm, sunny, the water is still and rich in nutrients – for example from fertiliser or sewage – they can multiply explosively. This is called a bloom. Wind can push the masses to the shore, where they collect as scum or a mat.

Some species produce toxins called cyanotoxins. They can harm the liver, the nervous system or the skin. Whether a particular bloom is toxic cannot be seen with the naked eye – blooms that look the same can be harmless or dangerous, and that can change from day to day.

The toxins are dissolved in the water or held inside the cells. Boiling does not destroy them; boiling only evaporates water, and the toxins can even become more concentrated. Common outdoor filters, tablets and UV devices are made for germs, not for such substances, and do not reliably remove them. Waterworks use special treatment for this that is not available in the field.

Typical signs are green, blue-green, brownish or reddish colouring of the water, streaks that look like spilled paint, scum or deposits along the shore and a musty smell.

Also look out for warnings: bathing bans, signs at swimming spots, official notices in apps or local media. Blooms occur mainly in late summer in warm, nutrient-rich lakes, ponds and slow rivers.

Important: the absence of visible signs does not mean safety. A bloom can shift within a short time, and toxins can still be in the water after the visible streaks have gone. If in doubt or if there are warnings, avoid the water.

Dogs are especially at risk. They drink from the water, play at the shore where scum collects, and lick their fur afterwards. Vomiting, staggering, seizures or breathing difficulties after contact are an emergency: get veterinary help immediately. Keep dogs on a lead at such places.

In people, skin contact can cause irritation; after contact, wash skin, fur and equipment with clean water. Anyone who has swallowed water and develops symptoms seeks medical help; in an emergency call the local emergency number (112 in the EU).

Water with an algal bloom cannot be made drinkable in the field – by any method. The only right response is to find another source: flowing water, a spring or rainwater. And that water is treated as usual too.

The name ‘blue-green algae’ is actually misleading: cyanobacteria are bacteria, not algae, and they are not always blue. The term has nevertheless become established in everyday language.

Cyanobacteria and their toxins are covered in the WHO drinking-water guidelines. Because warm summers and nutrient-rich waters favour blooms, the topic is discussed more often in many regions. For you on the move the rule stays simple: recognise, avoid, switch.