Photosynthetic organisms inhabiting the water column are called phytoplankton; they include microscopic unicellular algae and photosynthetic bacteria. There are a great number of them in the water.
When the surface water of Black Sea is the warmest, there can be between thousands to tens of millions of phytoplankton cells in 1 liter of seawater near the eastern coast. There can be ten, even hundred times more phytoplankton cells in water in the Western part of the Black Sea, which is well fertilized by large rivers, such as the Danube (whose mouth is visible at the top left in the full image).
As one can observe in this image, at the moment, the tiny organisms, usually invisible to the naked eye, are so concentrated in the waters of the sea that they are visible “en mass” as a blue and green phytoplankton bloom (click here for previous images).