Brilliant swirls of iridescent green are adding color to the waters of the Baltic Sea, as its warm summer waters are being overrun with microorganisms, new satellite images reveal.
The images, obtained by the Landsat 9 satellite on July 20, 2025, reveal a massive bloom of phytoplankton that have recently taken up residence in the sea’s waters, unveiling the scale and intensity of the region’s seasonal surges in microscopic life.
In the images, major concentrations of microalgae are revealed in wisps of bright green that color the Baltic Sea’s waters south of Gotland and southeast of Stockholm. While the phytoplankton blooms create a very striking appearance, they are a regular occurrence in the highly nutrient-rich waters in summer months, where an abundance of sunlight and heat help these microscopic plant organisms thrive.
As with past years, 2025’s annual bloom is revealing the ecological complexities associated with photosynthetic cyanobacteria that dominate these events.
An Annual Bloom in the Baltic
Each summer, phytoplankton growth skyrockets in the Baltic’s waters, with ancient organisms known as cyanobacteria often leading the charge. These microorganisms, once more broadly called blue-green algae, tend to thrive in warm waters that are rich in sources of nutrients.
One of cyanobacteria’s hidden superpowers is their ability to leverage dissolved nitrogen and phosphorous, which are very common in the Baltic during peak summer conditions.
The ever-watchful eye of the Operational Land Imager-2 (OLI-2) onboard Landsat 9 obtained the recent imagery, which NASA made available at its Earth Observatory website. One might be inclined to think the vivid green swirls resemble a view of the Aurora Borealis dancing through night sky, if they weren’t already aware they were looking at microorganisms proliferating throughout the Baltic Sea’s warm summer waters.
However, a close look at some of the images will reveal boat wakes, visible as dark lines marking the vessels’ paths through the algae-dense waters.
Cyanobacteria Confirmed
The specific species involved with the bloom remains undetermined, although it was determined through separate observations by the Swedish Meteorological and Hydrological Institute (SMHI) that cyanobacteria was abundant in the Baltic’s waters the same day the Landsat 9 images were obtained.
While suspended sediment and pollen are likely also contributors to the colorful conditions present on the sea’s surface this summer, mapping by the SMHI appears to align with areas depicted in the satellite imagery and data NASA has made available.
In one example of the imagery obtained by OLI-2, Gotska Sandön, an uninhabited Swedish island north of Gotland that has been home to a national park for more than a century, can be seen surrounded by swirling areas of iridescent green (see below).
Key Players in the Baltic’s Summer Ecosystem
Cyanobacteria blooms do far more than just spike the Baltic Sea’s waters with iridescent green colors each summer. When they begin to emerge around late June or early July, these organisms also begin to play an important role in the broader marine ecosystem.
For instance, cyanobacteria’s influence on nitrogen cycling in the summer waters has a direct impact on the food web, which can be beneficial for other organisms thriving in this environment. However, these prolific microorganisms can also pose challenges as they begin to die off and decompose, a process which consumes oxygen to significant enough degrees that it can lead to oxygen-deprived areas that scientists refer to as “dead zones” close to the seafloor.
Although striking in appearance when observed from orbit, the annual summer microorganism blooms in the Baltic remind scientists of the fragile balance between life forms that exist there, as well as the broader implications of nutrient loading and the impact climate has on warming marine life.
Additional information and imagery can be found at NASA’s Earth Observatory website.
Micah Hanks is the Editor-in-Chief and Co-Founder of The Debrief. He can be reached by email at micah@thedebrief.org. Follow his work at micahhanks.com and on X: @MicahHanks.