FAQ Frequently asked questions about blue-green algae

Blue-green algae carpet

Blue-green algae are bacteria, not algae. They have a bluish-green colour and are therefore also called cyanobacteria. There are many different types of blue-green algae. Like plants, they use sunlight and carbon dioxide to grow. In warm and sunny weather, blue-green algae can multiply rapidly. If the wind blows the blue-green algae onto the beach, you should be careful.

Blue-green algae can often be seen with the naked eye. The water turns greenish or bluish, and sometimes small flakes float on the surface. The water may also smell musty. Depending on the wind direction, blue-green algae can occur more frequently in certain areas of the water, but when the wind changes direction, they are usually driven to other areas of the lake very quickly, sometimes within a few hours. This means that a carpet of blue-green algae can quickly appear on the bathing beach and disappear just as quickly.

Yes, some species of blue-green algae produce toxins. These can cause skin irritation, mucous membrane irritation, conjunctivitis, allergic reactions or earache if you come into contact with contaminated water. Respiratory diseases are also possible. If you accidentally swallow the water, it can cause diarrhoea. When small children play near the shore, they may accidentally ingest large amounts of sand and water due to their play behaviour and frequent hand-to-mouth contact. Older children may also swallow large amounts of water when playing in shallow water.

• Pay attention to warning signs at bathing lakes. Blue-green algae are not always easy to spot.
• Do not go into the water where it is green or bluish in colour or smells musty.
• Avoid areas where the water is cloudy. You can tell if this is the case if you stand knee-deep in the water and cannot see your feet.
• Talk to parents if their children are playing in the water despite warning signs.
• Report blue-green algae if you discover any and no warning signs have been put up. Inform the DLRG, the environmental authority or the health department.

Blue-green algae grow particularly quickly in warm temperatures, but other environmental factors such as water temperature, sunlight, heat, rain or wind can also influence blue-green algae growth in a very short time, making it virtually impossible to predict when blue-green algae will occur. Blue-green algae can occur in all of Bremen's lakes. However, a bathing ban has not yet had to be imposed.

The Senator for the Environment, Climate and Science and the Bremen Health Authority monitor the bathing lakes regularly from May to September. The German Life Saving Association also monitors the situation on site. If blue-green algae are suspected, water samples are taken and tested on the same day. If the suspicion is confirmed, warning signs are put up.

On the internet:

• On the Senator for Environment, Climate and Science's page on bathing in Bremen, you will find information on water quality, water temperatures and possible blue-green algae blooms.

By telephone:

• The answering service of the Senator for Environment, Climate and Science can provide you with information on +49 421 361 5500.
• The health department will answer your questions on +49 421 361 155 13
  (Monday, Wednesday and Friday from 9 a.m. to 1 p.m.).