Nitrous Oxide Production in the Eastern Tropical South Pacific Upwelling Zone
Abstract:The Eastern Tropical South Pacific upwelling zone, where low to undetectable oxygen concentrations exist in the water column, is a region of intense nitrous oxide (N2O) production in the ocean. N2O production is generally attributed to nitrification and denitrification in oxic and anoxic waters, respectively, with overlap under suboxic conditions. Seawater samples from different depths and in situ oxygen concentrations were incubated with 15N tracer labeled substrates (NH4+, NO2- and NO3-) to measure potential N2O production rates. These rates were used to determine contributions of nitrification and denitrification to total N2O production.
N2O reached maximum concentrations at the base of oxycline just above the oxygen deficient zone (ODZ) and nitrification was the major production pathway. The N2O yield from nitrification, i.e., the ratio of N2O to NO2- production from NH4+, increased from ~0.04% to ~1% as oxygen concentration decreased from 100% to ~1% saturation. This relationship is consistent with culture studies showing increased N2O yield from nitrification at low oxygen; and thus with high N2O production rate from nitrification in the oxycline. N2O production from NO3- was detected at the base of oxycline. Highest N2O production rates (up to 10 nM d-1) were detected at the top of the ODZ, with denitrification as the major pathway. At the secondary nitrite maximum within the core of the ODZ, rates of N2O production from denitrification were relatively high despite low N2O concentration, suggesting N2O reduction to N2 must be co-occurring. This implies rapid N2O turnover by denitrification within the ODZ and a close coupling between production and consumption. These results indicate that oxygen concentrations greatly influence both the magnitude of N2O production and the relative contributions of nitrification and denitrification. Because most N2O production occurred in the oxycline and in the uppermost layer of the ODZ, anticipated spatial expansion of these zones could increase the extent of N2O production and the marine N2O efflux to the atmosphere.