Frozen algal bloom in Lipno Reservoir, Czech Republic

Lipno Reservoir, the largest reservoir in the Czech Republic, lies on the upper course of the Vltava River close to the Austrian border and is framed by the forests of the Bohemian Forest region. 

In winter, the reservoir often freezes, but this winter the lake surface took on an unusual colouration. This image, acquired by one of the Copernicus Sentinel-2 satellites on 18 December 2025, shows green-tinted ice caused by a high concentration of cyanobacteria. In the central part of the reservoir, the frozen bloom appears as a vivid green area contrasting with adjacent darker water. In the north-western sector bloom-free ice displays pale, whitish tones, while in the south-eastern section, unfrozen water reveals an active algal bloom extending along the reservoir’s narrow basin.

Copernicus data support the monitoring of algal bloom extent and persistence, helping inform water-quality management and timely public advisories.