According to NASA, which captured an aerial image of the phenomenon with its Aqua satellite on May 29, the cause is an ongoing bloom of phytoplankton.
The microscopic creatures make their own food from sunlight and dissolved nutrients in the water. At certain times, their populations explode in number, creating a bloom that can transform the water.
The current bloom in the Bosphorus (the waterway that runs through Istanbul, separating Europe and Asia) is a particularly bright one, according to Norman Kuring, an ocean scientist at NASA’s Goddard Space Flight Center. However, it is consistent with phytoplankton activity normally seen in May and June.