Nitrogen fixation occurs only in a few coastal and estuarine locations worldwide, but prodigious amounts take place in the Baltic Sea. One reason for this is the intense denitrification in the deep waters of the Baltic Sea, removing nitrate from the water. The resulting low levels of fixed nitrogen stimulate N2-fixation.
Map of the Baltic Sea. The Baltic Sea is surrounded by (clockwise from left) Sweden, Finland, Russia, the Baltic States, Poland, Germany and Denmark.
Satellite image of a bloom of the N2-fixer Nodularia spumigena in the Baltic Sea, August 1983 (CZCS satellite).
A similar image from the SeaWiFS satellite, this time August 1999. Provided by the SeaWiFS Project, NASA/Goddard Space Flight Center and ORBIMAGE.