MEPS 280:1-11 (2004)  -  doi:10.3354/meps280001

ABSTRACT: Northern Chile¹s oxygen minimum zone (OMZ) is considered to be an important site of N₂O production and efflux into the atmosphere, with a potentially global impact. Seawater samples from the OMZ core were used to determine how different O₂ levels and electron donor/acceptor availability affect N₂O cycling. N₂O production by denitrification (N₂Opd; acetylene treatment) and nitrification (N₂Opn; allylthiourea [ATU] treatment), and N₂O consumption by denitrification (N₂Ocd), were determined with in situ O₂, anoxic (0 µM O₂), hypoxic (~22.3 µM O₂), and potential (added substrate) experimental conditions. Under in situ O₂ levels (~4.6 µM), total N₂O production (N₂Opd + N₂Opn) was ~2.62 µM d^-1. Denitrification was responsible for over 92% of the total N₂Opd and nitrification for less than 8%. Nearly 100% of the N₂O produced was, however, consumed by denitrification. NO₃^- was reduced twice as rapidly as NO₂^-. Under anoxia, N₂Opd and N₂Ocd rates decreased by over 90%. The NO₃^- reduction was similar to that observed with in situ O₂, whereas a high rate of NO₂^- accumulation was observed. Conversely, increasing O₂ levels (~22.3 µM) doubled N₂Opd. Consequently, N₂Opd or NO₂^- reduction seems to be the process most sensitive to O₂ fluctuations. Adding organic carbon and NO₃^- increased N₂Opd and N₂Ocd slightly, whereas additional N₂O increased N₂Ocd abruptly. The fate of reduced NO₃^- in the OMZ core was controlled mainly by O₂ concentrations and indirectly by available organic carbon. Both variables are susceptible to the changes experienced in the eastern South Pacific during the El Niño Southern Oscillation cycle.

KEY WORDS: Nitrous oxide cycling · Oxygen minimum zone · Denitrification · O₂ · Organic matter regulation