The Hirschstetten swimming pond in Vienna’s Donaustadt district was temporarily closed earlier this month after rapid tests confirmed suspected cyanobacteria, ORF Vienna reported. According to MA 45, Vienna Waterways, greenish-blue streaks had been seen in shallow areas, including the children’s bathing area known as Baby Bay.
The whole pond was closed as a precaution after samples from the north bank, east bank and dog-walking area on the south bank were “not of bathing water quality” – but the area is again open for bathing. The City of Vienna’s water-quality page for natural bathing sites says the ban at Hirschstetten swimming pond was lifted on June 11th, after tests found no evidence of harmful cyanotoxins.
The case is still a reminder for anyone swimming in Austria’s lakes, ponds and natural bathing areas this summer: cyanobacteria can appear quickly, and some types can cause health problems for people and serious danger for dogs.
What are cyanobacteria?
Cyanobacteria are often called blue-green algae, or Blaualgen in German, but they are not actually algae. They are bacteria that occur naturally in surface waters such as ponds and lakes.
They become a problem when conditions allow them to multiply rapidly, creating what is often called an algal bloom. According to Austria’s Agency for Health and Food Safety, AGES, blooms are more likely when water temperatures and nutrient levels are high. Excessive sun and heat can also help them spread.
Many types are harmless. The concern is that some cyanobacteria can produce toxins, known as cyanotoxins.
READ ALSO: How to stay safe when swimming in Austria's lakes and rivers this summer
How can you recognise a possible bloom?
The warning signs are fairly visual.
AGES describes possible signs as bluish-green turbidity, streaks in the water, algae-like mats on the surface and reduced visibility, especially if visibility is less than one metre.
It tells you not to swim if you can't see your feet because of green-blue cloudiness in the water. It's important to keep this in mind because even if Austria has bodies monitoring the quality of water, cyanobacteria can show up quickly (in between checks) and some small lakes aren't monitored at all.
What symptoms can cyanobacteria cause?
If harmful cyanotoxins are present, they can cause health problems if people swallow larger amounts of contaminated water or inhale it into the airways.
AGES lists possible symptoms, including nausea, fever, vomiting, diarrhoea, skin rashes, and, in more serious cases, collapse and paralysis. ORF Vienna also reported that some cyanobacteria can cause skin and eye irritation, nausea, vomiting or diarrhoea.
Absorption through the skin is considered unlikely, according to AGES, but skin irritation, inflammation or allergic reactions can still happen. More serious possible consequences include gastrointestinal inflammation, respiratory illness and liver damage.
Children need particular attention because they are more likely to swallow water while swimming or playing.
Why are dogs especially at risk?
Dog owners should be particularly careful around suspicious water or algae-like material at the shore.
AGES warns that cyanotoxins can be deadly for dogs – and the poisoning can be very quick. This can happen if dogs lick algal material from their fur or eat rotting material at the water’s edge. So the risk is not limited to swimming.
If a site has warning signs, barrier tape or a bathing ban, dogs should also be kept away from the water.
READ ALSO: Why Austria’s dry spring could affect farms, food and water this summer
What should you do after swimming?
If you have been in water where cyanobacteria may be present, AGES recommends showering and cleaning your swimwear.
Be cautious with water sports where people may swallow a lot of water. Also keep an eye on children and pets.
If you develop symptoms after swimming in suspicious water, contact a doctor and mention the possible exposure.
How quickly can the problem disappear?
One reason cyanobacteria can be confusing is that they may appear suddenly and then disappear again quickly.
In the Hirschstetten case, ORF Vienna reported that the visible streaks had already disappeared after lower temperatures, more cloud and rain. Gerald Loew, head of Vienna Waterways, said rain could help resolve the problem.
Still, authorities continued to monitor the water quality and take samples.
Key vocabulary
die Cyanobakterien – cyanobacteria
die Blaualgen – blue-green algae, the common name for cyanobacteria
die Algenblüte – algal bloom
die Badewasserqualität – bathing water quality
die Schlieren – streaks or cloudy trails in the water
die Giftstoffe – toxins
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