This image of the Black Sea, acquired on June 4, 2008, by the MODIS on the Aqua satellite, shows swirling blooms of phytoplankton coloring the surface waters blue and green. The large volume of freshwater flowing into the Black Sea from large European rivers, including the Danube and Dnieper Rivers, makes the sea much less salty than open oceans.
The freshwater flow also delivers many nutrients, which are washed into the sea from land. These nutrients support large blooms of phytoplankton: microscopic photosynthetic organisms (algae and bacteria). The chlorophyll and other pigments the organisms use for photosynthesis change the way light reflects off the surface, and these changes are visible in satellite imagery.