This Envisat image captures blue-green algae blooms filling the Baltic Sea. 'Algae bloom' is the term used to describe the rapid multiplying of phytoplankton, microscopic marine plants that drift on or near the surface of the sea. While individually microscopic, the chlorophyll that phytoplankton use for photosynthesis collectively tints the surrounding ocean waters, providing a means of detecting these tiny organisms from space with dedicated 'ocean colour' sensors, like Envisat's Medium Resolution Imaging Spectrometer (MERIS) that acquired this image on 11 July 2010.