Situated at the crossroads of Europe and Asia, the Black Sea typically maintains a dark appearance. However, during the spring and summer months, the basin transforms into a striking display of swirling turquoise. This phenomenon was captured on June 22, 2026, by the Ocean Color Instrument (OCI) aboard NASA’s Plankton, Aerosol, Cloud, ocean Ecosystem (PACE) satellite.
This milky-blue hue is typically caused by coccolithophores, a variety of phytoplankton encased in calcium carbonate plates. These organisms usually dominate the waters in late spring and early summer. In contrast, other seasons often see a prevalence of diatoms—microscopic algae with silica shells—which tend to darken the water’s color.
The phenomenon extends into the Bosphorus, the narrow strait in Istanbul connecting the Black Sea to the Sea of Marmara. On May 27, 2026, an astronaut aboard the International Space Station captured a photograph of the strait, documenting phytoplankton blooms tracing the currents along the waterway.
While coccolithophores are microscopic, their immense abundance during blooms makes them visible from space, providing researchers with a critical remote sensing tool to study bloom dynamics in areas where physical sampling is difficult. Beyond their aesthetic impact, these blooms play a vital role in the ocean’s carbon cycle; as the organisms die, they transport sequestered carbon to the seafloor, where it can be stored for long periods.
