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July 22, 2025 - Summer Bloom in the Baltic Sea
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Swirls of cloudy green covered the surface of the Baltic Sea near the Swedish island of Gotland in mid-July 2025. The Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Aqua satellite captured this true-color image of the widespread and dramatic phytoplankton bloom on July 19.
The Baltic Sea typically hosts two blooms of microscopic, plant-like organisms known as phytoplankton each year. The early spring bloom tends to be made up of dinoflagellates and diatoms, which thrive in the cooler waters chilled by ice melt and fed by the nutrient-rich spring runoff. These species are non-toxic and the pigments that they carry often colors the water in blue tones.
By mid-summer, the surface water in the Baltic Sea becomes quite warm. This spurs the explosive reproduction of a type of phytoplankton called blue-green algae. Different species of blue-green algae grow in these waters, at least one of which produces a potent toxin. Known as “microcystin”, this toxin can cause significant illness, including vomiting, diarrhea, skin rashes, and respiratory problems in humans or animals that have been exposed by touching, swimming in, or drinking water containing the toxin.
It's not possible to determine exactly what species are present in a bloom from satellite imagery, but the time of year and annual occurrence of blue-green algae in these waters strongly suggests that they are the primary cause of the color visible here. On July 18, the Swedish Herald quoted a biologist from the Swedish Meteorological and Hydrological Institute (SMHI) confirming that algal blooms have been seen approaching the beaches, especially on the east coast of Gotland.
Image Facts
Satellite:
Aqua
Date Acquired: 7/19/2025
Resolutions:
1km (73.6 KB), 500m (192.5 KB), 250m (387.9 KB)
Bands Used: 1,4,3
Image Credit:
MODIS Land Rapid Response Team, NASA GSFC
Swirls of cloudy green covered the surface of the Baltic Sea near the Swedish island of Gotland in mid-July 2025. The Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Aqua satellite captured this true-color image of the widespread and dramatic phytoplankton bloom on July 19.
The Baltic Sea typically hosts two blooms of microscopic, plant-like organisms known as phytoplankton each year. The early spring bloom tends to be made up of dinoflagellates and diatoms, which thrive in the cooler waters chilled by ice melt and fed by the nutrient-rich spring runoff. These species are non-toxic and the pigments that they carry often colors the water in blue tones.
By mid-summer, the surface water in the Baltic Sea becomes quite warm. This spurs the explosive reproduction of a type of phytoplankton called blue-green algae. Different species of blue-green algae grow in these waters, at least one of which produces a potent toxin. Known as “microcystin”, this toxin can cause significant illness, including vomiting, diarrhea, skin rashes, and respiratory problems in humans or animals that have been exposed by touching, swimming in, or drinking water containing the toxin.
It's not possible to determine exactly what species are present in a bloom from satellite imagery, but the time of year and annual occurrence of blue-green algae in these waters strongly suggests that they are the primary cause of the color visible here. On July 18, the Swedish Herald quoted a biologist from the Swedish Meteorological and Hydrological Institute (SMHI) confirming that algal blooms have been seen approaching the beaches, especially on the east coast of Gotland.
Image Facts
Satellite:
Aqua
Date Acquired: 7/19/2025
Resolutions:
1km (73.6 KB), 500m (192.5 KB), 250m (387.9 KB)
Bands Used: 1,4,3
Image Credit:
MODIS Land Rapid Response Team, NASA GSFC
