The Bosphorus is a strait that separates Europe from Asia — and in recent days, its normally dark blue waters have turned a remarkable turquoise.
Some residents of Istanbul, noticing the suddenly bright and milky waters, wondered on social media whether the hue was caused by pollution or an earthquake that shook the region on Monday, AFP reported.
But the cause of the jewel-toned waters is a bloom of phytoplankton, according to NASA.
Phytoplankton are microscopic organisms that live in watery environments – and they make their own food, using sunlight and dissolved nutrients.
"When conditions are right, phytoplankton populations can grow explosively, a phenomenon known as a bloom," says NASA. "Blooms in the ocean may cover hundreds of square kilometers and are easily visible in satellite images. A bloom may last several weeks, but the life span of any individual phytoplankton is rarely more than a few days."
The agency says that a type of phytoplankton common in the Black Sea (to which the Bosphorus connects) are coccolithophores, which are plated with white calcium carbonate: "When aggregated in large numbers, these reflective plates are easily visible from space as bright, milky water."
"The May ramp-up in reflectivity in the Black Sea, with peak brightness in June, seems consistent with results from other years," said Norman Kuring, an ocean scientist at NASA's Goddard Space Flight Center.
But not all phytoplankton blooms make the water brighter, Kuring explains. "Diatoms, which also bloom in the Black Sea, tend to darken water more than they brighten it."
Ahmet Cemal Saydam, professor of environmental science at Hacettepe University, told the Dogan news agency that the coccolithophore in question is Emiliania huxleyi, according to AFP.
"This has nothing to do with pollution," he said, and added that the organism is good for anchovies, popular fare in Istanbul.
"Across the Black Sea there is an explosion of Emiliania huxleyi," he said. "This is a blessing for the Black Sea."