Vienna warns urine‑driven algae risk in the Alte Donau

Gerald Loew, head of the Vienna water authority (MA45), has warned that the Alte Donau, a popular bathing area, is being polluted primarily by human waste. Recent measurements show that about 80 % of the contaminants entering the water come from bathers, chiefly urine and sunscreen, while only 20 % stem from natural sources such as leaves and waterfowl.

Loew cautioned that the high nutrient load could trigger a eutrophication event similar to the one that crippled the lake in the 1990s, leading to a dense algae layer that would suffocate aquatic life. Several swimmers have reported gastrointestinal problems after bathing there. To avert a “tipping point,” the city is continuously feeding fresh water from the Neue Donau into the Alte Donau, but low water levels and prolonged heat spells worsen the situation.

The authority urges the public to treat the natural water body as a lake, not a toilet, and to avoid applying sunscreen directly before entering the water.