Nitrification in the euphotic zone as evidenced by nitrate dual isotopic composition: Observations from Monterey Bay, California
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Coupled measurements of nitrate (NO(3)(-)), nitrogen (N), and oxygen (O) isotopic composition (delta(15)N(NO3) and delta(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)(-) assimilation and nitrification within the euphotic zone. Specifically, increases in delta(18)O(NO3) were consistently greater than those in delta(15)N(NO3). A coupled isotope steady state box model was used to estimate the amount of NO(3)(-) 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)(-) assimilation in marine environments.