Inorganic nitric nutrient salts in lakes and wetlands are found in the form of ammonium ions, nitrite ions and nitrate ions. Nitrite ions are known to be present in polluted, turbid waters, but usually not in appreciable quantities. Plant plankton generally use ammonium ions and nitrate ions as sources of nitrogen. While the use of nitrate ions depends on sunlight, ammonium ions are known to be taken up in the dark as well as in sunlight. Ammonium ions are also oxidized by bacteria to generate nitrite and nitrate ions. Furthermore, when DO in the water becomes low, nitrate ions are reduced by the action of bacteria to their final form as molecular nitrogen. This process eliminates nitrogen from the water, which is considered to be important because of its relation to the problem of europhication. The figure shows the vertical distribution of nitrate ions in Lake Biwa.

Vertical distribution of nitrate ions