The stable form of nitrogen in the presence of oxygen is NO3-, with N in the +V oxidation state. Phytoplankton must add 8 electrons to produce ammonia (NH3), with an oxidation state of -III, which is the form of nitrogen that they require.When organic matter is remineralized, bacteria will oxidize the NH3 to NO3-. In the presence of very low oxygen concentrations, bacteria may reduce NO3- to N2, in order to consume organic matter. During these processes, many nitrogen compounds with intermediate oxidation states are produced, including nitrite ( NO2-) and nitrous oxide (N2O).
Nitrate is generally well correlated with phosphate in the ocean. This correlation breaks down when oxygen concentrations drop below 20 µmol/kg and NO3- is consumed as a terminal electron acceptor to oxidize organic carbon.
Nitrate is typically determined colorimetrically by reduction to nitrite and then forming a pink azo dye with the nitrite. Nitrite and nitrate can be determined at low nanomolar levels by forming nitrogen oxides, which are determined by chemiluminescence in the gas phase. Nitrate can also be determined directly by measuring its absorption spectrum in the far ultraviolet.