Every spring and summer, the Black Sea goes through a brief but spectacular transformation — its waters take on a vivid turquoise hue, visible even from space. This year the phenomenon was captured twice: on 27 May 2026, an astronaut aboard the International Space Station photographed the characteristic turquoise swirls flowing through the Bosphorus toward the Sea of Marmara, and on 22 June the Ocean Color Instrument aboard NASA’s PACE satellite documented a large-scale bloom covering much of the sea’s surface.
What colours the water
The cause isn’t pollution but a natural biological process — a mass bloom of coccolithophores, microscopic single-celled algae covered in calcium carbonate plates. These plates scatter sunlight in a way that gives the water a milky-blue, almost turquoise appearance. Coccolithophores typically dominate in late spring and early summer, in contrast to diatoms, another major type of phytoplankton, which tend to darken the water rather than brighten it.
A role in the carbon cycle
Beyond the visual effect, coccolithophores play a genuine role in the marine ecosystem: as they grow, they absorb carbon, and when they die, some of that carbon sinks to the seafloor, where it can remain stored for long periods of time — one of the natural mechanisms by which oceans help regulate the planet’s carbon cycle.
Are the blooms changing?
A recent study by scientists at the Shirshov Institute of Oceanology, published in the journal Diversity, notes something worth watching: in 2022 and 2023, coccolithophore blooms in the northwestern Black Sea lasted unusually long — persisting into July instead of fading by mid-June as is typical. One or two years of observations are too few to call this a lasting trend, but this is exactly the kind of shift that long-term monitoring of the sea is meant to catch.
That is precisely why we track similar signals ourselves — water quality, temperature anomalies, and the spread of invasive species are among the indicators we include in our analysis of the Black Sea. Read more on the topic →
Source: NASA Earth Observatory