Microbial nitrification in the marine euphotic zone: Rates and relationships with nitrite distributions, recycled production and nitrous oxide generation

Rising levels of carbon dioxide and other greenhouse gases in the atmosphere may lead to global warming, while chlorofluorocarbons and other trace gases are depleting Earth's protective stratospheric ozone layer. Microbial nitrification, the biologically mediated oxidation of ammonium to nitrite to nitrate, plays a key role in both of these issues. Nitrification complicates measurements of new production, resulting in inaccurate estimates of atmospheric carbon dioxide removal by the ocean's natural biological pump, and nitrification produces the ozone-depleting greenhouse gas nitrous oxide as a by-product.; I have utilized sensitive chemical and radioisotopic techniques to measure nitrification rates and concentrations of relevant nitrogen compounds at the oligotrophic North Pacific benchmark Station ALOHA. My goals were to assess the role of nitrification in the recycling of nitrogen within the euphotic zone, to investigate the influence of nitrification on the structure and variability of nitrite distributions and to examine the relationship between nitrification and the production of nitrous oxide.; Nitrification rates in the euphotic zone were highly variable, ranging from {dollar}<{dollar}2 to 123 {dollar}mu{dollar}mol m{dollar}sp{lcub}-3{rcub}{dollar} d{dollar}sp{lcub}-1{rcub}{dollar}. Nitrate production via nitrification within the euphotic zone was several times greater than the estimated eddy-diffusive flux of nitrate from below, indicating that nitrate-based production cannot be equated with new production.; Nitrite measurements revealed that the primary nitrite maximum has a double-peaked structure, with a large upper maximum and a lower maximum of lesser magnitude. The vertical distributions of nitrogenous compounds and nitrification rates suggest that the upper peak is produced by the incomplete assimilatory reduction of nitrate by phytoplankton, while the lower peak is maintained through the differential photoinhibition of nitrifying bacteria. A light-dependent computer model gave results consistent with this hypothesis.; Nitrous oxide exhibited supersaturation, even within the surface mixed layer. A mass balance of nitrous oxide within the euphotic zone suggests that in situ production of nitrous oxide, rather than a flux from below the euphotic zone, supports the sea-air flux of this trace gas at Station ALOHA. However, much of this in situ nitrous oxide generation appears to be fueled by a process other than nitrification which remains uncharacterized.