Effect of light and substrate availability on the primary nitrite maximum in the Gulf of Aqaba, Red Sea

The primary nitrite maximum (PNM) is a common feature in seasonally stratified seas; however, there is debate over which processes are chiefly responsible for the formation of PNM. Two general mechanisms have been proposed: (1) spatial segregation of ammonium oxidizers and nitrite oxidizers at different depths in the water column, and (2) exudation of nitrite by phytoplankton following incomplete nitrate assimilation. We show that in the seasonally stratified Gulf of Aqaba, Red Sea, both processes contribute to PNM formation at different times of year and depths in the water column. During the days immediately following stratification, nitrite formation was strongly correlated with decreasing irradiance and chlorophyll, suggesting that incomplete nitrate reduction by light limited phytoplankton was a major source of nitrite. However, as stratification progressed, nitrite continued to be generated below the euphotic depth by ammonium oxidation, possibly due to differential photoinhibition of ammonium and nitrite oxidizing populations. Natural abundance stable nitrogen isotope analyses revealed a decoupling of isotopic signatures of the nitrogen and oxygen in the combined nitrate and nitrite pool. This decoupling became stronger as stratification progressed, and suggests assimilation and nitrification were co-occurring. Nitrogen tracer experiments also show that the organic nitrogen pool was rapidly recycled, serving as an important source for both nitrification and assimilation. Our results demonstrate that nitrogen transformation rates throughout the water column are controlled by light and substrate availability over diel and seasonal cycles and allow phytoplankton and nitrifying microbes to contribute jointly to PNM formation.