Black Sea now has a turquoise hue; understand phenomenon

The Black Sea is located on the border between Europe and Asia and connects to the Mediterranean Sea through a chain of canals. Its surface often appears dark, but each spring and summer it transforms into a stunning expanse of vibrant turquoise waters.

The turquoise color is likely caused by coccolithophores, a type of phytoplankton covered in sheets of calcium carbonate that can give surface waters a milky blue appearance.

A NASA system captured an image of the phenomenon last Monday (22). See below:

Image captured - NASA
These types of phytoplankton tend to predominate in late spring and early summer. At other times of year, diatoms - a type of microscopic algae with silica shells - can become more prevalent and tend to darken the water rather than lighten it.

The Bosphorus, the strait that runs through Istanbul and connects the Black Sea to the Sea of Marmara, also turned turquoise.

An astronaut aboard the International Space Station photographed the strait on May 27, 2026, capturing the bloom of phytoplankton as it followed the currents on both sides of the channel.

Source: CNN