June 10, 2025, 8:31 am | Read time: 5 minutes
The name might suggest otherwise, but blue-green algae are actually bacteria. If a lake is heavily contaminated with them, it can be harmful to swimmers’ health—and even deadly for dogs. Learn from TRAVELBOOK how increased levels of blue-green algae can occur in bodies of water and how to identify them in swimming lakes.
It is typically the warm season that draws swimmers to the lake. Another, less desirable side effect of warmth and sunlight is the possibility of what’s known as a water bloom. This refers to a significant increase in blue-green algae. It’s important to recognize a blue-green algae infestation to avoid affected waters, as these microorganisms can release toxins, making contact with them potentially very harmful.
Overview
Characteristics of Blue-Green Algae–and What Makes Them Dangerous
Despite their name, blue-green algae are not true algae but bacteria, specifically cyanobacteria. What they share with algae and plants is the ability to perform photosynthesis, though they do it somewhat differently. As explained by the Brandenburg Ministry of the Environment (MLEUV), cyanobacteria use not only the same light as green plants but also other wavelengths, thanks to special pigments called phycobilins. This allows them to efficiently capture energy under various light conditions. Depending on which pigments dominate, they can appear bluish, reddish, green, or black.
Another difference from “true” algae is that certain types of blue-green algae produce cyanotoxins, which are released into the water. Swimmers in lakes can ingest these toxins, leading to symptoms such as nausea, vomiting, and diarrhea in humans. Contact with blue-green algae can also cause skin and mucous membrane irritation. If the toxins reach the lungs, they can cause respiratory problems.
Unpleasant for Humans, Potentially Deadly for Dogs
For humans, blue-green algae generally do not pose serious dangers. However, the MLEUV advises caution with small children, as they are more likely to swallow water while playing near the shore. The symptoms described above can be unpleasant. A blue-green algae poisoning can be life-threatening for dogs, as explained in detail by the colleagues at PETBOOK.
What Promotes the Mass Proliferation of Blue-Green Algae
“In Brandenburg, mass developments occur primarily with high to very high nutrient availability (…) and higher water temperatures as well as good light conditions,” TRAVELBOOK learns from the Brandenburg State Office for the Environment (LfU). Simply put, warmth and sunlight provide ideal conditions for microorganisms to multiply. “Nutrient availability” means that, depending on various factors, nitrogen and phosphorus can be present in lakes. These nutrients promote the growth of blue-green algae, acting like fertilizer. They can enter bodies of water when, for example, agricultural runoff is washed into rivers and lakes by rain or through household wastewater.
Or: because swimmers urinate in the water. You should avoid this, as TRAVELBOOK explains in this article.
How to Recognize Blue-Green Algae
Blue-green algae, or cyanobacteria, are difficult to recognize, warns the LfU. Even experts often need a microscope to identify them definitively. However, there are some visible signs that may indicate an infestation. These include:
- Milky cloudiness of the water, which is typical for a certain type of cyanobacteria.
- Bright turquoise-blue slime on the water surface or shore, caused by dead cyanobacteria.
- Reddish-brown to chocolate-brown masses in certain species.
- Streaks, clumps, or mats on the water surface.
- Large algae accumulations on windward shores.
In summary, one should be cautious of water that appears cloudy or unusually discolored. Dogs should be kept away from the shore if algae masses accumulate there, as the concentration of possible toxins is highest in these areas.
Those who enjoy swimming in lakes may have used special test strips that claim to assess water quality. However, one should not rely too heavily on them, as they do not react to blue-green algae. “There are no test strips that can detect cyanobacteria or other health hazards at the lakeshore,” assures the LfU. Reliable detection requires standardized laboratory methods, as prescribed by health departments for their testing mandates.
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Blue-Green Algae Do Not Automatically Lead to Swimming Bans
TRAVELBOOK also spoke with Markus Kaschubski from the Bremen Health Department. He advised checking with the responsible authorities before visiting a swimming area in the summer. This is generally very easy. In his state, the necessary information can be found on all websites of the Food Monitoring, Animal Protection, and Veterinary Service of the State of Bremen (LMTVet).
In Germany, EU guidelines for bathing water safety apply. However, warnings about blue-green algae are not centrally published by the federal government. The states and municipalities are responsible for providing current and local information on blue-green algae developments. Whether swimming warnings are issued or even bans are imposed depends on the specific situation. Generally, a light to moderate concentration of blue-green algae leads to a warning, recognizable by signs like “Swim at your own risk” or “No contact with water.” These warnings should be taken seriously, especially for children and dogs. In cases of higher blue-green algae concentrations due to a mass proliferation of bacteria, authorities typically impose bans.