Turquoise phytoplankton bloom transforms Black Sea waters
Images from spring and summer 2026 reveal how microscopic organisms alter the colour of the region’s surface waters and influence the ocean’s carbon cycle.
The Black Sea, situated at the boundary between Europe and Asia, underwent a distinct seasonal transformation in spring and summer 2026, shifting from its usual dark surface to a swirling expanse of turquoise. This change was captured by the Ocean Color Instrument on NASA’s PACE satellite on June 22, 2026, and by an astronaut aboard the International Space Station on May 27, 2026. The images document a widespread bloom of phytoplankton that has altered the visual appearance of the sea and the nearby Bosphorus Strait.
The milky-blue hue is likely caused by an abundance of coccolithophores, a type of phytoplankton covered in calcium carbonate plates. These microscopic organisms tend to dominate the Black Sea during late spring and early summer. Their high concentration during a bloom makes them visible from space, allowing remote sensing to serve as a valuable tool for researchers studying bloom dynamics in areas where direct water sampling is limited.
This seasonal event contrasts with other times of the year when diatoms, a different type of microscopic algae with silica shells, become more prevalent. While coccolithophores brighten the water, diatoms tend to darken it. The Bosphorus Strait, which runs through Istanbul and connects the Black Sea to the Sea of Marmara, also exhibited the turquoise colouring. The ISS photograph captured phytoplankton tracing currents on both sides of the waterway, with north oriented toward the bottom of the frame.
Beyond their visual impact, these blooms play a role in the ocean’s carbon cycle. When coccolithophores die, some of the carbon they have absorbed sinks to the seafloor, where it can remain stored for extended periods. The International Space Station Program supports the ISS National Lab in facilitating such Earth observations, ensuring that images taken by astronauts are made freely available to scientists and the public to aid in understanding planetary systems.
The imagery was processed using data from NASA EOSDIS LANCE and GIBS/Worldview, with the astronaut photograph acquired using a Nikon Z9 digital camera. The ISS image was provided by the ISS Crew Earth Observations Facility and the Earth Science and Remote Sensing Unit at NASA Johnson Space Center. The visual record highlights the dynamic nature of the Black Sea’s ecosystem and the utility of space-based monitoring in tracking these biological events.
