Researchers have unraveled the mystery of the greening of the frozen Czech lake and believe that climate change and pollution will make such events more common.
In Czechia, the ice of a frozen lake unexpectedly turned emerald green, providing biologists with an unprecedented opportunity to study a strange and rather sinister natural phenomenon, writes Popular Science, reports Focus.
At the end of last year, scientists from the Czech Academy of Sciences traveled to Lake Lipno in South Bohemia to collect and study samples of a rare cyanobacterial bloom in the midst of winter. The results of this work are believed to help better understand the problem threatening local marine life and nearby settlements.
Like other bodies of water, Lake Lipno is familiar with cyanobacteria. These blue-green photosynthetic algae typically thrive in warm summer and autumn months, especially in nutrient-rich environments — a process known as eutrophication.
It is worth noting that cyanobacterial blooms are known to be accompanied by unpleasant odors, but the real problem lies in the damage they cause to the local ecology. Previous studies have already shown that each algal bloom leads to an exponential increase in the amount of cyanotoxins, which can poison and even kill nearby aquatic organisms. Unfortunately, observations show that climate change and human pollution are leading to more frequent algal blooms.
According to scientists, in most freshwater bodies in Czechia, cyanobacterial blooms usually cease by the end of September. However, observations show that in Lake Lipno, the algal bloom season has been prolonged for quite some time. Marine biologists have repeatedly recorded significant populations of cyanobacteria throughout November, and sometimes even in December and January.
Similar conditions at the end of 2025 allowed the algal biomass to remain at the surface of the lake until the water began to freeze. Scientists believe this was made possible by weeks of sunny, calm weather and favorable winds. Analysis results also confirmed that their field samples contained the widespread cyanobacterial species Woronichinia naegeliana.
Researchers found that the thin ice cover itself was transparent, and the cyanobacteria retained their characteristic green color, which was easily noticeable from the shore. A brief warming around December 24 melted some of the ice, which then froze again. The bloom ceased only after a heavy snowfall finally blocked enough light from reaching it under the ice.
Researchers are also concerned that these icy winter blooms will impact ecosystems and are likely to become more common as the planet warms.
