Beguiling Phytoplankton Bloom in the Baltic Sea Looks Incredible

Nearly every summer, colorful blooms of phytoplankton flourish in the Baltic Sea. And nearly every summer, satellite images detect art-like patterns as the phytoplankton trace the sea’s currents, eddies, and flows. But like the whorls of fingerprints, no two phytoplankton blooms are exactly alike.

These natural-color images, acquired on August 15, 2020, with the Operational Land Imager (OLI) on Landsat 8, show a late-summer phytoplankton bloom swirling in the Baltic Sea. The images feature part of a bloom located between Öland and Gotland, two islands off the coast of southeast Sweden. Note the dark, straight lines crossing the detailed image: these are ships cutting through the bloom.

Confirmation of the type of phytoplankton within this bloom would require the analysis of water samples. But experts familiar with blooms in this region say it is likely to be cyanobacteria—an ancient type of marine bacteria that captures and stores solar energy through photosynthesis. Large, late-summer blooms of cyanobacteria occur almost every year in the Baltic Sea.

Sediment cores extracted from the seafloor indicate that blooms of cyanobacteria have occurred in the Baltic Sea for thousands of years and they have played an important role in this aquatic ecosystem. And in the world’s oceans, cyanobacteria are important “nitrogen fixers” that can convert atmospheric nitrogen gas into an organic form that other phytoplankton can use as a nutrient to fuel growth.

Blooms in the Baltic Sea have intensified in recent decades, triggered in part by a rise in nutrient runoff from lands around the sea (particularly agricultural fertilizer and sewage). Excessive phytoplankton and algae growth can deplete the amount of oxygen in the water and cause dead zones.

The extent of this bloom spanned an area beyond the scope of these images; browse satellite images in Worldview to see it in context with the wider area. In some years, cyanobacteria blooms have covered as much as 200,000 square kilometers of the sea surface—slightly less than half the size of Sweden.

NASA Earth Observatory images by Joshua Stevens, using Landsat data from the U.S. Geological Survey. Story by Kathryn Hansen, with image interpretation by Norman Kuring/NASA GSFC and Ajit Subramaniam/LDEO/Columbia University.

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