1. Euphotic zone: primary nitrite maximum (PNM) at the base of the euphotic zone, coincident with a nitracline (high nitrate gradient). Created mainly by oxidation of ammonium, plus some phytoplankton reduction, and limited phytoplankton uptake due to lig
2. Nearly anoxic regions, mainly in the eastern tropical regions, such as in the Pacific. Created by reduction of nitrate by denitrifying bacteria, part of process to complete denitrification (to N2).
3. Occurrence near sediments not so often described.
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