Nitrification in the euphotic zone as evidenced by nitrate dual isotopic composition: Observations from Monterey Bay, California

Global Biogeochemical Cycles
By: , and 

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Abstract

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.

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Additional publication details

Publication type Article
Publication Subtype Journal Article
Title Nitrification in the euphotic zone as evidenced by nitrate dual isotopic composition: Observations from Monterey Bay, California
Series title Global Biogeochemical Cycles
DOI 10.1029/2006GB002723
Volume 21
Issue 2
Year Published 2007
Language English
Publisher AGU
Contributing office(s) Toxic Substances Hydrology Program, California Water Science Center
Description 13 p.
Country United States
State California
Other Geospatial Monterey Bay