Rare winter phenomenon turns ice on Lipno Lake green
Visitors to the Lipno Reservoir in South Bohemia witnessed something unusual at the end of last year. The ice on the lake turned green, a striking sight in the middle of winter. The cause was an unexpected build-up of cyanobacteria trapped beneath the frozen surface.
Cyanobacteria are a common sight on Czech ponds and reservoirs, particularly in the summer months. They tend to appear when there are too many nutrients in the water, especially phosphorus, much of it linked to human activity.
Normally, these blooms fade with the arrival of autumn and disappear by the end of September. But Lipno is different. Hydrobiologist Petr Znachor from the Biology Centre of the Czech Academy of Sciences says a rare mix of conditions allowed the cyanobacteria to stay much longer than usual.
“It is a combination of several factors. To put it simply, the cyanobacteria that were present in large quantities at the end of summer and the beginning of autumn managed to persist into winter and then froze there.”
In simple terms, the algae stayed close to the surface for weeks, right up until the reservoir froze over. Calm weather, very little wind and plenty of sunshine helped keep them there. Once the ice formed, thin and in places very clear, the trapped cyanobacteria created vivid green patches, visible from the shore and even from the air.
“It is a common species of cyanobacterium, known by its Latin name Woronichinia naegeliana. It occurs in many bodies of water across the country and appears regularly at Lipno, especially in the second half of summer and in autumn. This species is also interesting because it can change colour. The bloom can be green, brown, and we have even seen yellow. It is something of a chameleon among cyanobacteria.”
The green ice was most noticeable during a brief warm spell around Christmas Eve. When freezing temperatures returned, scientists observed another unusual feature known as so-called cyanobacterial eyes.
These are clear spots in the ice sitting above darker clusters of cyanobacteria. They form because the organisms absorb sunlight differently from the surrounding ice. The phenomenon lasted until the end of the year and ended only after heavy snowfall reduced the amount of light under the ice.
Finding cyanobacteria beneath ice is rare worldwide, and the Lipno case is now one of the best-documented examples of this phenomenon. However, in connection with climate change, similar events are likely to occur more frequently in the future.
As for safety, the impact on water quality is much the same as in summer. Swimming is therefore not recommended, although in the middle of a snowy winter, that advice may be largely academic. The good news is that the cyanobacteria do not affect the ice itself, so skating is still fine.
