Cyanobacteria bloom. False-colour composite satellite image of what appears to be a large bloom of cyanobacteria (blue-green algae) swirling in the Baltic Sea. Satellite images alone cannot say definitively that whether or not this bloom contains cyanobacteria - analysis of ocean water samples would be required to confirm it. Cyanobacteria are photosynthetic marine bacteria and an important constituent of plankton. Blooms like this flourish in the Baltic Sea during summertime, when there is ample sunlight and high levels of nutrients. They can cause an oxygen-depleted 'dead zone' where other organisms cannot survive. Imaged by the Operational Land Imager (OLI) on the Landsat 8 satellite, on 11th August 2015.