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Abstract 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 () 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 PNMof the long‐term California Cooperative Oceanic Fisheries Investigation (CalCOFI) survey record. A multiple linear regression analysis of the CalCOFI PNMtime series identified two conditions that led to positive deviations in. Enhanced PNM, and potentially enhanced EZ nitrification, may be linked to (1) reduced phytoplankton competition for ammonium () andas interpreted from particulate organic carbon:chlorophyll ratios, and/or (2) to increased supply of(and thenoxidation to) from the degradation of organic nitrogen as interpreted from particulate organic nitrogen concentrations.