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

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.