Euphotic Zone Nitrification in the California Current Ecosystem

Nitrification, the microbial conversion of ammonium to nitrite then to nitrate, occurs throughout the oceanic water column, yet the environmental factors influencing the production of nitrate in the euphotic zone (EZ) remain unclear. In this study, the natural abundances of N and O isotopes (δ¹⁵N and δ¹⁸O, respectively) in nitrate were used in an existing model framework to quantify nitrate contributed by EZ nitrification in the California Current Ecosystem (CCE) during two anomalously warm years. Model data estimated that between 6% and 36% of the EZ nitrate reservoirs were derived from the combined steps of nitrification within the EZ. The CCE data set found nitrification contributions to EZ nitrate to be positively correlated with nitrite concentrations ([NO2-] at the depth of the primary nitrite maximum (PNM). Building on this correlation, EZ nitrification in the southern California Current was estimated to contribute on average 20% ± 6% to EZ nitrate as inferred using the PNM [NO2-] of the long‐term California Cooperative Oceanic Fisheries Investigation (CalCOFI) survey record. A multiple linear regression analysis of the CalCOFI PNM NO2- time series identified two conditions that led to positive deviations in [NO2-]. Enhanced PNM [NO2-], and potentially enhanced EZ nitrification, may be linked to (1) reduced phytoplankton competition for ammonium (NH4+) and NO2- as interpreted from particulate organic carbon:chlorophyll ratios, and/or (2) to increased supply of NH4+ (and then NH4+ oxidation to NO2-) from the degradation of organic nitrogen as interpreted from particulate organic nitrogen concentrations.