Nitrite occupies an intermediate oxidation state in the marine inorganic nitrogen cycle between the most reduced form, ammonia, and the most oxidised form, nitrate. Nitrite accumulation therefore identifies either loci of importation across the ocean boundaries (allocthonous sources) or of water column nitrogen fluxes where nitrite produce exceeds nitrite utilisation (autochthonous sources). The most significant biological processes contributing to nitrite production are denitrification including assimilatory and dissimilatory nitrate reduction and nitrification dominated by ammonia oxidation. Nitrite accumulation occurs when at least one of these processes exceeds the uptake capabilities of phytoplankton and/or bacteria.