English: On August 15, 2023, the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Aqua satellite captured a true-color image of spectacular colors filling Lake Sevan, Armenia.

The bright blue tones, which give an almost other-worldly tint to the largest freshwater lake in the South Caucasus, are the result of a bloom of small plant-like organisms called phytoplankton. Lake Sevan provides a home for several species of phytoplankton, but in recent years cyanobacteria have become increasingly dominant, especially in the warm summer months. Some species of Cyanobacteria (commonly called blue-green algae) can produce toxins that can make people and animals sick. While it is not possible to know the species creating the bright colors in Lake Sevan just from a satellite image, given the time of year and recent history of the lake, cyanobacteria are likely present in this bloom.

The bloom began in late July in the northwest corner of the lake, possibly fed by nutrient runoff carried in by the freshwater of the Hrazdan River which flows into Lake Sevan in this area. By early August, the colors had spread southward, finally encompassing the entire lake.