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GLOBAL BIOGEOCHEMICAL CYCLES,
VOL. 21,
GB2009,
doi:10.1029/2006GB002723,
2007
Nitrification in the euphotic zone as evidenced by nitrate dual isotopic composition: Observations from Monterey Bay, California
Scott D. Wankel
Department of Geological and Environmental Sciences, Stanford University, Stanford, California, USA
Carol Kendall
United States Geological Survey, Menlo Park, California, USA
J. Timothy Pennington
Monterey Bay Aquarium Research Institute, Moss Landing, California, USA
Francisco P. Chavez
Monterey Bay Aquarium Research Institute, Moss Landing, California, USA
Adina Paytan
Department of Geological and Environmental Sciences, Stanford University, Stanford, California, USA
Abstract
Coupled measurements of nitrate (NO3 −), nitrogen (N), and oxygen (O) isotopic composition (δ 15NNO3 and δ 18ONO3) 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 δ 18ONO3 were consistently greater than those in δ 15NNO3. 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.
Received 19
March
2006;
accepted 11
January
2007;
published 4
May
2007.
Keywords: nitrate isotopes;
nitrification;
Monterey Bay.
Index Terms: 0469 Biogeosciences: Nitrogen cycling; 0454 Biogeosciences: Isotopic composition and chemistry (1041, 4870); 0414 Biogeosciences: Biogeochemical cycles, processes, and modeling (0412, 0793, 1615, 4805, 4912); 0470 Biogeosciences: Nutrients and nutrient cycling (4845, 4850).
Read Full Article (file size: 858652 bytes) Cited by
Citation: Wankel, S. D., C. Kendall, J. T. Pennington, F. P. Chavez, and A. Paytan
(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.
This paper is not subject to U.S. copyright. Published in 2007 by the
American Geophysical Union.
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