Whoa, Why Has This Frozen Lake Turned Green?

A frozen lake in Czechia turned an ominous green color, a warning sign of ecological changes due to a warming climate.

Researchers from the Biology Center of the Czech Academy of Sciences collected samples of a toxic blue-green algae bloom in Lake Lipno, which caused the ice to turn green in December 2025. Cyanobacteria blooms typically occur during the summer months and through autumn, but in this case, they seem to have persisted during the dead of winter, resulting in clear patches of ice absorbing the color of the algae.

The rare sight of the green ice is most likely due to weeks of sunshine, calm weather, and weak winds, according to the researchers.

A green christmas

Lake Lipno, the largest reservoir in Czechia (also known as the Czech Republic), is plagued by an excess of nutrients. The pollution is driven by industrial activities, agricultural runoff, municipal waste, and other human-led activities that result in unusual ecological phenomena such as cyanobacteria blooms.

Cyanobacteria blooms are rapid growths of bacteria known for having a foul smell and a greenish color. The blooms wreak havoc on the local ecology, poisoning nearby organisms in the water while also posing a risk to human health. They typically thrive in warm water, appearing predominantly during the summer months.

In Czechia, Lake Lipno has been experiencing longer algae seasons that can sometimes continue into December. By examining the samples from Lake Lipno, the researchers were able to confirm that it was in fact cyanobacteria blooms that were causing it to turn green.

Toward the end of last year, cyanobacteria remained near the surface for an unusually long time until the reservoir froze over. That resulted in areas of clear ice developing over dark cyanobacterial aggregates, forming the so-called cyanobacterial eyes.

The phenomenon was most pronounced around Christmas Eve, when the weather was warmer than usual during this time of the year. Some of the ice briefly melted and then refroze, allowing patches of the cyanobacterial eyes to form due to differences in the absorption of the Sun’s radiation.

Warmer weather ahead

The cyanobacteria blooms finally dissipated following heavy snowfall, which blocked the Sun’s light from reaching the algae beneath the ice.

The case of Lake Lipno is the most well-documented one worldwide and shows that regular cyanobacteria blooms can exhibit strange behavior under atypical circumstances. The researchers, however, predict the sight of green ice may become more common in the future as the effects of climate change continue to alter ecosystems.

“Green ice on Lake Lipno fits into the long-term changes we observe here in connection with eutrophication and ongoing climate change,†Petr Znachor, a hydrobiologist from the Biology Center of the Czech Academy of Sciences, said in a statement. “It suggests that we may witness similar surprises more frequently in the future.â€