Coupled measurements of nitrate (NO3–), nitrogen (N), and oxygen (O) isotopic composition (δ15N-NO3 and δ18O-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 NO3– assimilation and nitrification within the euphotic zone. Specifically, increases in δ18O-NO3 were consistently greater than those in δ15N-NO3 . 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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