Ammonium

Ammonium (NH4+) is the most reduced inorganic nitrogen species in seawater and the preferred nitrogen source for phytoplankton because its uptake requires no reduction. It is produced by heterotrophic remineralization of organic matter and zooplankton excretion, and it is consumed by both photosynthesis and nitrifying bacteria that oxidize it through nitrite to nitrate. Concentrations are usually low in the euphotic zone, often below 1 micromole per liter, since it is recycled fast. Ammonium-based uptake defines regenerated production, distinct from new production fueled by nitrate supplied from below the nutricline.