The Copernicus Sentinel-2 image above features swirls of vivid, emerald, green algal blooms in the Gulf of Finland.

The Gulf of Finland is the easternmost arm of the Baltic Sea. The area pictured shows the western part of the Gulf. Helsinki, Finland’s capital, and most populous city is visible in light brown on the coast in the top left corner of the image. The image also captures the Estonian coast at the bottom, with Tallin covered by clouds.

Every summer, swathes of algae bloom in the Baltic Sea. The blooms usually appear as green threads, as shown in this image, which is from July 2023. Streaks, eddies, and swirls, mixed by winds and currents, are clearly visible.

An algal bloom describes the rapid growth of phytoplankton—microscopic marine plants that drift on or near the sea surface. While individually microscopic, the chlorophyll they use for photosynthesis collectively tints the ocean waters, which allows them to be measured by satellites orbiting Earth.

These organisms are essential to life in the sea—they form the base of the marine food chain. Importantly, they also play a huge role in the removal of carbon dioxide from the atmosphere and the production of oxygen.

However, some phytoplankton and marine algae can be harmful to marine life and to humans. They can produce toxic substances, blooms can occur too often or last too long, depleting the concentration of oxygen in the water.

High water temperatures, slow circulation and excessive nutrients released into the ocean often lead to a rapid increase in the number of algae, and consequently large blooms, which can pose a threat to the natural ecosystem and aquaculture and tourism.

With its 13 spectral channels, Copernicus Sentinel-2’s imager can be used to detect algal blooms and measure aspects that define water quality, such as surface concentration of chlorophyll. Satellite data can be used to track the growth and spread of blooms and can help develop early warning systems to mitigate the impact on tourism and fishing industries.