Canton Zug, 08.03.2023

The Burgundy Blood algae has disappeared again

The Burgundy Blood algae have covered various parts of Lake Aegeri and Lake Zug over the past few

It has disappeared as quickly as it came: the Burgundy Blood algae. Only a few days ago, it was staining areas of Lake Zug and Lake Aegeri red. But it has now disappeared again. "The situation in Lake Zug and Lake Aegeri has eased again," writes Florian Weber, Director of Public Works, in response to an enquiry. At present, no "creaming" of burgundy blood algae can be detected on the lake surface.

The reason for this is the change in weather last week. "From autumn onwards, burgundy blood algae can rise to the surface of the lake, but, in order to bloom, they need a lot of light - in other words, good weather," explains Florian Weber. The algae has sunk back into deeper water by the end of February.

Burgundy blood algae belong to the genus of blue-green algae. They owe their name to a story about the Battle of Murten in 1476, when the Duke of Burgundy, Charles the Bold, went into battle against the Swiss Confederates. When the water of Lake Murten later turned red, a legend arose among the local inhabitants that it was the blood of the Burgundians who had died in the battle. It was, in fact, a mass occurrence of the Planktothrix rubescens algae.

Not algae, but a bacteria that can produce toxins
From a scientific point of view, however, blue-green algae are not algae at all, but bacteria, because they do not have a real cell nucleus - so-called cyanobacterial blooms. Mass reproduction requires calm, warm water, strong sunlight and sufficient nutrients, such as nitrogen and phosphorus. They form substances that can be toxic to animal organisms and, for this reason, the Cantonal Office for the Environment has issued a warning with the appearance of the red-coloured water.

The burgundy blood algae are basically free to move about in the lake. "In recent weeks, they have appeared on the lakeshore in sunny weather as a red surface film, or even in thicker layers, especially in the bays around  Lake Zug," says Florian Weber. It could be found distributed along the shores of Lake Aegeri as well. "Larger accumulations were found in Morgarten."

The Katastrophenbucht in Zug on 24 February 2023
The Katastrophenbucht in Zug a few days later       
Photos: Zug Department of Public Works
Director of Public Works Florian Weber                    
Photo: Matthias Jurt

Dogs are particularly at risk from blue-green algae. They often drink lake water near the shore, where the concentration of algae is high, or lick their fur after bathing. But the cantonal veterinarian Rainer Nussbaumer is not aware of any cases so far. He says: "A vet should be consulted immediately if a dog shows any signs of poisoning."

As a general rule, owners should not let their animals swim or drink in areas where the water is very green, or where a foam or algae carpet can be seen. If the animal does swim in dirty water, it should be hosed down immediately afterwards.

The Circulago as a driver of the Burgundy blood algae?
Burgundy blood algae will probably continue to discolour the water of Zug's lakes red in the future. Numerous studies indicate that global warming, over-fertilisation and rising CO2 levels are likely to increase the frequency, intensity and duration of cyanobacterial blooms worldwide.

But the construction director denies that agriculture or projects like the Circulago area heating system are responsible for an increased occurrence of burgundy blood algae: "Uses of the lake water, such as Circulago, for heat and cold production take the lake water from depths below the thermocline, and also return it there." The lake water used is heated or cooled by a maximum of three degrees Celsius. "It is therefore unlikely that the Circulago promotes the growth of burgundy blood algae or influences the location of blue-green algae in the lake," he adds.

Nor is agriculture responsible for their occurrence, he adds. "Burgundy algae can also occur in nutrient-poor lakes" - as the example of the Lake Ägeri shows.