An unusual natural phenomenon appeared on Lake Lipno in South Bohemia, the Czech Republic, at the end of 2025. Large amounts of accumulated cyanobacteria in the water caused the ice to turn green. The phenomenon was thoroughly documented by hydrobiologists from the Biology Center of the Czech Academy of Sciences, who also collected and analyzed water samples.
Green ice on Lake Lipno caused by cyanobacteria in December 2025. Credit: Petr Znachor, BC CAS
As there are very few scientific records of this phenomenon worldwide, the green ice on Lake Lipno will rank among the best-documented cases globally. The event confirms that Lake Lipno has long been burdened by an excess of nutrients in the water (so-called eutrophication) and is also affected by ongoing climate warming, with ecological changes occurring even outside the traditional summer season.
Microscopic analysis of the collected samples confirmed that the organism responsible was the common cyanobacterium Woronichinia naegeliana, which also dominates Lake Lipno during summer and autumn. The primary driver of cyanobacterial growth is an excess of nutrients in the water, especially phosphorus, which largely results from human activities.
While on most Czech reservoirs cyanobacterial blooms gradually disappear with the onset of autumn and usually vanish by the end of September, Lake Lipno is an exception in this respect. Long-term data show that cyanobacteria often dominate here until November and can occur in smaller amounts even during December and January.
Green ice on Lake Lipno caused by cyanobacteria. Credit: Petr Znachor, BC CAS
Green ice reflects long-term environmental change
At the end of 2025, cyanobacterial biomass remained near the water surface for an unusually long time, from autumn until the reservoir froze over, most likely due to a combination of calm weather, weak winds, and a long duration of sunshine. Beneath the thin and locally very transparent ice cover, the accumulated cyanobacteria formed striking green patches visible both from the shore and in aerial photographs.
The green ice was most pronounced during a temporary warming around Christmas Eve. After refreezing, a particularly distinctive feature was the formation of so-called "cyanobacterial eyes"—areas of clear ice above dark cyanobacterial aggregates caused by differences in the absorption of solar radiation. The phenomenon persisted until the end of the year and likely ended only after heavy snowfall reduced light penetration beneath the ice.
"Green ice on Lake Lipno fits into the long-term changes we observe here in connection with eutrophication and ongoing climate change. It suggests that we may witness similar surprises more frequently in the future," says hydrobiologist Petr Znachor from the Biology Center of the Czech Academy of Sciences.
A close-up view of cyanobacteria frozen in ice with bubbles of produced oxygen confirms that photosynthesis was taking place. Credit: Petr Znachor, BC CAS
Under-ice occurrences of cyanobacteria are rare worldwide and have so far been documented only in a limited number of cases. The recent observations at Lake Lipno, therefore, rank among the best-described records and also show that even common cyanobacterial species can exhibit atypical behavior under certain conditions.
Green ice on Lake Lipno caused by cyanobacteria. Credit: Petr Znachor, BC CAS
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