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

Abstract

Coupled measurements of nitrate (NO 3 − ), nitrogen (N), and oxygen (O) isotopic composition ( δ 15 N NO3 and δ 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 δ 18 O NO3 were consistently greater than those in δ 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.