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General Session 2: Marine & estuarine biogeochemistry |
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Nitrification and nitrate assimilation in the subarctic North Atlantic
Wednesday 11th @ 1150-1210
Conference Hall
Xuefeng Peng* , Princeton University (Now at University of California, Santa Barbara)
Sarah Fawcett, Princeton University (Now at University of Cape Town)
Martin Wolf, Princeton University (Now at MIT)
Nicolas van Oostende, Princeton University
Dario Marconi, Princeton University
Daniel Sigman, Princeton University
Bess Ward, Princeton University
Presenter Email: xpeng@engineering.ucsb.edu
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| Nitrification, the two-step process oxidizing ammonium (NH4+) to nitrite (NO2-) and subsequently to nitrate (NO3-), is an essential part of the nitrogen (N) cycle. Nitrification as a source of nitrate within the sunlit surface ocean (the ˇ°euphotic zoneˇ±) has been largely neglected because it was thought to be completely inhibited by light. However, there is increasing evidence that nitrification is only partially or even not at all inhibited by light in the euphotic zone, and that it recovers in the dark. This implies that regenerated NO3- (from nitrification) may support primary production in the euphotic zone in addition to ˇ°newˇ± NO3- supplied via physical processes. As a result, the efficiency of the biological pump, usually approximated with the f-ratio, may be overestimated if regenerated NO3- is not considered.
In this study we investigated the significance of nitrification relative to NO3- assimilation in the subarctic North Atlantic during its most productive seasons: spring (May 2014) and autumn (September 2013). Nitrification rats and N assimilation rates were determined using tracer (15NH4+, 15NO2-, and 15NO3-) incubations. 15NO2- and 15NO3- were measured by isotope ratio mass spectrometry (IRMS) coupled with the azide and the denitrifier methods, respectively. The isotopic enrichment of particulate organic N was determined using IRMS. In addition, the natural abundance nitrogen and oxygen isotopes of NO3- (15N and 18O, respectively) were measured at the same stations using the denitrifier-IRMS method.
During both seasons, NH4+ and NO2- oxidation rates were low in both the mixed layer and euphotic zone (defined by the depth of 1% surface irradiance), with subsurface maxima evident below these surface layers. The euphotic zone integrated rate of nitrification amounted to 7 ¨C 8% of the NO3- assimilation rate in the spring and <5% in the autumn. The 15N and 18O of NO3- rose in concert from below the euphotic zone and mixed layer into surface waters. This pattern is characteristic of NO3- assimilation by phytoplankton, and confirms that the euphotic zone nitrification was much slower than the upward transport of ˇ°newˇ± NO3- into the subarctic North Atlantic surface. The mixed layer depth fluctuated significantly (by 17 - 40 m) over several days of sampling, such that NO3- regenerated just below this layer could have been incorporated into the surface NO3- pool during events of mixed layer deepening. In sum, nitrification contributed a small fraction of theNO3- available for assimilation by phytoplankton in the euphotic zone during spring and autumn in the subarctic North Atlantic. |
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