Nitrification in the euphotic zone as evidenced by nitrate dual isotopic composition: Observations from Monterey Bay, California
Article first published online: 4 MAY 2007
Copyright 2007 by the American Geophysical Union.
Global Biogeochemical Cycles
Volume 21, Issue 2, June 2007
How to Cite
2007), Nitrification in the euphotic zone as evidenced by nitrate dual isotopic composition: Observations from Monterey Bay, California, Global Biogeochem. Cycles, 21, GB2009, doi:10.1029/2006GB002723., , , , and (
- Issue published online: 4 MAY 2007
- Article first published online: 4 MAY 2007
- Manuscript Accepted: 11 JAN 2007
- Manuscript Revised: 4 DEC 2006
- Manuscript Received: 19 MAR 2006
- nitrate isotopes;
- Monterey Bay
 Coupled measurements of nitrate (NO3−), nitrogen (N), and oxygen (O) isotopic composition (δ15NNO3 and δ18ONO3) were made in surface waters of Monterey Bay to investigate multiple N cycling processes occurring within surface waters. Profiles collected throughout the year at three sites exhibit a wide range of values, suggesting simultaneous and variable influence of both phytoplankton NO3− assimilation and nitrification within the euphotic zone. Specifically, increases in δ18ONO3 were consistently greater than those in δ15NNO3. A coupled isotope steady state box model was used to estimate the amount of NO3− supplied by nitrification in surface waters relative to that supplied from deeper water. The model highlights the importance of the branching reaction during ammonium (NH4+) consumption, in which NH4+ either serves as a substrate for regenerated production or for nitrification. Our observations indicate that a previously unrecognized proportion of nitrate-based productivity, on average 15 to 27%, is supported by nitrification in surface waters and should not be considered new production. This work also highlights the need for a better understanding of isotope effects of NH4+ oxidation, NH4+ assimilation, and NO3− assimilation in marine environments.