Nitrogen (N)

Speciation
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).   

Distribution in ocean
Dissolved nitrate NO₃^-  has a nutrient like vertical profile.  The tremendous variability in nitrate is shown by a north-south section through the Pacific Ocean.  Concentrations are depleted to values near zero (<10 nM in some areas) in the surface ocean (data).  This may limit primary production.  Infrequent, eddy events that inject nitrate into surface waters may be important controls on carbon cycling in the ocean.  They are best sampled with autonomous sensor systems on deep-sea moorings.  Temporal variability can also be large in the coastal zone.  The annual cycle of temperature, salinity and nutrients in the central California coastal region is available (600K download).

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.

The major form of nitrogen is nitrogen gas (N₂).  The nitrogen gas concentration is controlled by equilibrium with the partial pressure of N₂ in the atmosphere.  The vertical concentration profile of N₂ is determined by the solubility of the gas at the temperature and salinity found at each depth (N2data). 

The major intermediate of N reduction and oxidation is NO₂^-.  The vertical profile of  NO₂^- may show two concentration maxima.  One is near the base of the euphotic zone, where NO₂^- is produced as nitrate is converted to ammonia by phytoplankton and then back by bacteria.  The second maximum occurs if there is an oxygen minimum where concentrations are low enough to allow nitrate to be reduced.   

Residence time
The residence time is estimated from the rate of nitrate loss due to denitrification (Codispoti, 1989).

Analysis
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.

References