Algae levels in Lake Balaton have continued to rise, with the highest concentration recorded in the Keszthely Basin. Experts are now urging swimmers in the area to take extra precautions and shower after bathing due to the growing dominance of filamentous cyanobacteria.

According to the latest measurements by the HUN-REN Balaton Limnological Research Institute, the increase in algal biomass that began in the second half of July has continued in recent weeks. However, significant differences remain between the various parts of the lake.

Algae levels highest in the Keszthely Basin

Measurements taken on 10 August 2026 found the highest a-chlorophyll concentration in the Keszthely Basin, at 80 micrograms per litre (µg/l), according to the institute.

The Szigliget Basin followed with 54 µg/l, while 32 µg/l was measured in the Szemes Basin. The water was considerably clearer in the Siófok Basin, with concentrations of 19 µg/l recorded near Tihany and 21 µg/l near Balatonfűzfő.

This means a clear west-to-east gradient in algal biomass continues to be visible along the lake. The pattern was also confirmed by the latest Sentinel-2 satellite imagery taken on 9 August.

Filamentous cyanobacteria are taking over

Researchers have also observed a significant change in the composition of the lake’s algal community.

The algae previously found in large quantities have declined considerably, while filamentous cyanobacteria have become dominant across the entire length of Lake Balaton.

One species in particular, Raphidiopsis raciborskii, has become widespread. The species is characteristic of late-summer conditions and is capable of fixing atmospheric nitrogen. Researchers have also noted the presence of Planktothrix, which cannot fix atmospheric nitrogen.

Despite the increased algal biomass, the situation may not look alarming to people on the beaches. Unlike earlier in the summer, the algae are not currently accumulating in large visible patches in sheltered areas. Instead, they are mostly distributed evenly throughout the water column.

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Experts advise swimmers to shower after bathing

Although there is currently no visible mass accumulation of algae, researchers are urging people swimming in the Keszthely Basin to take precautions.

Experts recommend showering after every swim in the Keszthely Basin.

Researchers stress that monitoring will remain particularly important for the rest of the summer, as a considerable part of the bathing season is still ahead.

It’s important to note that the latest findings do not mean that Lake Balaton’s beaches are unsafe across the board. However, the rising algal biomass and growing dominance of filamentous cyanobacteria mean that swimmers in the most affected areas should pay particular attention to the latest guidance from researchers.

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