Document Type


Publication Date




Publication Title









The primary nitrite maximum (PNM) is a ubiquitous feature of the upper ocean, where nitrite accumulates in a sharp peak at the base of the euphotic zone. This feature is situated where many chemical and hydrographic properties have strong gradients and the activities of several microbial processes overlap. Near the PNM, four major microbial processes are active in nitrite cycling: ammonia oxidation, nitrite oxidation, nitrate reduction and nitrite uptake. The first two processes are mediated by the nitrifying archaeal/bacterial community, while the second two processes are primarily conducted by phytoplankton. The overlapping spatial habitats and substrate requirements for these microbes have made understanding the formation and maintenance of the PNM difficult. In this work, we leverage high-resolution nutrient and hydrographic data and direct rate measurements of the four microbial processes to assess the controls on the PNM in the eastern tropical North Pacific (ETNP). The depths of the nitrite maxima showed strong correlations with several water column features (e.g., top of the nitracline, top of the oxycline, depth of the chlorophyll maximum), whereas the maximum concentration of nitrite correlated weakly with only a few water column features (e.g., nitrate concentration at the nitrite maximum). The balance between microbial production and consumption of nitrite was a poor predictor of the concentration of the nitrite maximum, but rate measurements showed that nitrification was a major source of nitrite in the ETNP, while phytoplankton release occasionally accounted for large nitrite contributions near the coast. The temporal mismatch between rate measurements and nitrite standing stocks suggests that studies of the PNM across multiple timescales are necessary.


© Authors 2023.

This work is distributed under the Creative Commons Attribution 4.0 (CC BY 4.0) License.

Data Availability

Article states: "Cruise data from RB1603 can be accessed on bco-dmo (, last access: 1 December 2017, Mulholland and Jayakumar, 2017), and corresponding pump profiler data can be accessed on the Stanford Digital Repository (, Travis et al., 2023)."

"Code availability: The multiple linear regression model analyses were based on best subset selection using the leaps package in R (Lumley, 2020). Specific code as applied to the 2016 ETNP dataset is available on the Stanford Digital Repository (".

Original Publication Citation

Travis, N. M., Kelly, C. L., Mulholland, M. R., & Casciotti, K. L. (2023). Nitrite cycling in the primary nitrite maxima of the eastern tropical North Pacific. Biogeosciences, 20(2), 325-347.


0000-0001-8819-189X (Mulholland)


Article Location