The Black Sea Turned Turquoise, Thanks to a Phytoplankton Bloom

The Black Sea isn’t black, and it’s not usually turquoise either. But a huge bloom of phytoplankton has illuminated it — and the connected Bosporus and the Golden Horn of Istanbul — with beautiful swirls of milky blue-green. This aquatic artwork appears every summer, but this year’s bloom is one of the brightest since 2012, according to Norman Kuring, a NASA scientist. It’s so bright, it can be seen from space.

NASA created this composite image of the bloom with data and satellite images on May 29. It has gotten brighter since and appears to be nearing its expected peak, around the summer solstice. After a particularly luminous weekend, thousands of people began talking about the opaque, jewel-toned water on social media.

The marine artists responsible are most likely phytoplankton, teeny organisms that live off energy derived from a combination of dissolved nutrients and their ability, like plants, to break down sunlight. In the Black Sea, and the eastern European rivers Danube and Dnieper that feed it, live a common group of phytoplankton called coccolithophores.

Coccolithophores do well in warm, stratified waters, where they often dominate, surviving even with few nutrients. They multiply asexually, and boom — what a pretty picture.

These microscopic algae build their own armor by surrounding themselves with dozens of limestone, or calcium carbonate, scales. All bundled up, they look like tiny, crocheted marbles under a microscope. But gathered in the trillions, they cloud up the bath of blue-green water as they die, dropping their shells, which scatter sunlight. It’s like a painter’s palette: Drop a dab of white paint into a greenish blue, and you get turquoise.

“The optical effect is striking,” said William M. Balch, a researcher at Bigelow Laboratory for Ocean Sciences in Maine. “If you are in a boat, the turquoise water goes from horizon to horizon.”

The algae live only a few days, but the turquoise water can last weeks longer as the reflective, chalky armor slowly sinks at a rate of nearly four inches a day.

“They’re sort of the dandruff of the ocean,” Dr. Balch said.

These types of phytoplankton blooms are common in the Black Sea, but scientists can’t say for certain why this year’s bloom burst so brilliantly. Perhaps warmer, more stratified water, increased light availability, the absence of the specific nutrients that their main competitors eat or a reduction in other species that can eat them allowed the coccolithophores to thrive, Dr. Balch said. It’s possible that heavy rains brought in nutrients — like iron, which is important for coccolithophores — from the Sahara, according to The Associated Press.

This isn’t the only place to find milky coccolithophore creations. Blooms occur in waters all over the world, including those around Iceland, New Zealand, Chile and South Africa.