Nitrite Cycling in the Primary Nitrite Maxima of the Eastern Tropical North Pacific
- 1Earth System Science, Stanford University, Stanford CA, 94305, USA
- 2Department of Ocean, Earth and Atmospheric Science, Old Dominion University, Norfolk VA, 23529, USA
Abstract. 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 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 Pacific. The depth 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 maxima), whereas the concentration of the nitrite maxima correlated weakly with fewer water column features (e.g. nitrate concentration at the nitrite maximum). The balance between microbial rate processes active in nitrite cycling 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 time scales are necessary.
Nicole Mayu Travis et al.
Status: final response (author comments only)