Every spring, the waters of the Black Sea undergo a spectacular transformation, turning from a deep indigo to a bright turquoise hue. The natural phenomenon, visible even from space, is caused by a massive bloom of coccolithophores, microscopic single-celled algae covered in calcium carbonate plates that scatter sunlight and give the sea its characteristic blue-green color.
On June 22, 2026, NASA's PACE satellite photographed the Black Sea at the height of this bloom, while an astronaut aboard the International Space Station had documented the same phenomenon on May 27, with turquoise stripes stretching across the Bosphorus and following sea currents. Beyond the stunning view, the phenomenon has attracted the interest of scientists. A study published in the scientific journal Diversity by the Shirshov Institute of Oceanology finds that in 2022 and 2023 the coccolithophore bloom lasted much longer than usual, continuing into July, while it normally ends in mid-June.
According to the researchers, this was caused by a combination of factors, including weak winds, a shallower thermocline, and an imbalance of nutrients in the water. The lack of strong winds meant that the surface layer of the water remained calm and illuminated, creating ideal conditions for algae growth. The chemical composition of the water also played an important role. In July 2022, heavy rainfall transported additional amounts of phosphorus to coastal areas, while low nitrogen levels favored the growth of the species Gephyrocapsa huxleyi, the dominant coccolithophore in the Black Sea. At the peak of the bloom, over nine million cells per liter of water were recorded.
Scientists say these organisms play an important role in the global carbon cycle. Coccolithophores absorb carbon from the water to form their calcium carbonate plates, and some of this carbon is deposited on the sea floor when the organisms die. However, the process of forming these plates also releases carbon dioxide, which can reduce the ocean's ability to absorb CO₂ from the atmosphere. Researchers consider the Black Sea a natural laboratory for understanding the impact of climate change on the carbon cycle. Analysis of data from the past 25 years shows that such long blooms have only been recorded in 2022 and 2023.
Although the authors do not call this a permanent trend, they warn that climate change could affect the frequency of such phenomena. If global warming favors more stable surface layers of water, coccolithophore blooms could become more common, changing the way the oceans absorb carbon dioxide from the atmosphere. The scientists emphasize that long-term monitoring remains essential to understand whether these changes are temporary or signal a broader transformation of marine ecosystems.
