Nitrite cycling in the primary nitrite maxima of the eastern tropical North Pacific

[摘要] The primary nitrite maximum (PNM) is a ubiquitous feature of theupper ocean, where nitrite accumulates in a sharp peak at the base of theeuphotic zone. This feature is situated where many chemical and hydrographicproperties have strong gradients and the activities of several microbialprocesses overlap. Near the PNM, four major microbial processes are activein nitrite cycling: ammonia oxidation, nitrite oxidation, nitrate reductionand nitrite uptake. The first two processes are mediated by the nitrifyingarchaeal/bacterial community, while the second two processes are primarilyconducted by phytoplankton. The overlapping spatial habitats and substraterequirements for these microbes have made understanding the formation andmaintenance of the PNM difficult. In this work, we leverage high-resolutionnutrient and hydrographic data and direct rate measurements of the fourmicrobial processes to assess the controls on the PNM in the easterntropical North Pacific (ETNP). The depths of the nitrite maxima showed strongcorrelations with several water column features (e.g., top of thenitracline, top of the oxycline, depth of the chlorophyll maximum), whereasthe maximum concentration of nitrite correlated weakly with only a few watercolumn features (e.g., nitrate concentration at the nitrite maximum). Thebalance between microbial production and consumption of nitrite was a poorpredictor of the concentration of the nitrite maximum, but rate measurementsshowed that nitrification was a major source of nitrite in the ETNP, whilephytoplankton release occasionally accounted for large nitrite contributionsnear the coast. The temporal mismatch between rate measurements and nitritestanding stocks suggests that studies of the PNM across multiple timescalesare necessary.

[学科分类] 大气科学