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.
Date Published:
July 28, 2015
A. Paytan, C. Kendall, F. P. Chavez, J. T. Pennington, S. Wankel,

Coupled measurements of nitrate (NO 3 À), nitrogen (N), and oxygen (O) isotopic composition (d 15 N NO3 and d 18 O NO3) 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 NO 3 À assimilation and nitrification within the euphotic zone. Specifically, increases in d 18 O NO3 were consistently greater than those in d 15 N NO3 . A coupled isotope steady state box model was used to estimate the amount of NO 3 À supplied by nitrification in surface waters relative to that supplied from deeper water. The model highlights the importance of the branching reaction during ammonium (NH 4 +) consumption, in which NH 4 + 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 NH 4 + oxidation, NH 4 + assimilation, and NO 3 À assimilation in marine environments.

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