Distributions of inorganic nitrogen and biological production in the equatorial Pacific: a basin-scale model sensitivity study of nitrification

Xiujun Wang,Raghu Murtugudde

doi:10.1186/s40562-015-0022-3

Abstract

Recent evidence indicates that there is stronger nitrification in the euphotic zone than previously thought. We employ a physical-biogeochemical model to study the implications of nitrification for basin-scale distributions of nitrate, ammonium, and biological production in the equatorial Pacific. The model can faithfully reproduce observed features in nitrate distribution, with or without photoinhibition of nitrification in the euphotic zone. In addition, new production, net community production and export production are not very sensitive to the parameterization of nitrification in this model. However, simulated ammonium distribution, nitrate uptake and ammonium uptake are sensitive to this parameterization. High nitrification results in low ammonium concentration, low ammonium uptake rate, and high nitrate uptake rate in the euphotic zone. This study suggests that nitrification may be responsible for up to 40% of nitrate uptake in the equatorial Pacific. This modeling study also demonstrates large differences (in terms of the magnitude and spatial distribution) between nitrate uptake, new production and export production, reflecting decoupling of upward nutrient supply, biological uptake and downward export.

Highlights

The equatorial Pacific Ocean is the largest oceanic source of carbon dioxide (CO2) to the atmosphere [1]
There were only minor differences in the surface nitrate among the four simulations (Figure 2). These results indicated that modeled nitrate was not highly sensitive to the parameterization of nitrification commonly used in the biogeochemical models, which was noted by Mongin et al [23]
While this study shows that such a low rate may be a reasonable average over large scales, it is questionable whether measured nitrification rate at a specific time from the standard and NITR3 simulations

Summary

Introduction

The equatorial Pacific Ocean is the largest oceanic source of carbon dioxide (CO2) to the atmosphere [1]. The strong outgassing is partly attributable to the low efficiency of the “biological pump”, i.e., the export of particulate organic materials from surface waters to the deep ocean. Studying the “biological pump” is critical to our understanding of the carbon cycle. Due to the difficulty in measuring export production, an alternative approach, measuring new production, has been employed to determine the magnitude of the “biological pump”. New production, defined as uptake of preformed nitrogen (primarily nitrate) that is transported from deep waters into the euphotic zone, should equal to export production [2,3,4]. New production is often determined by measuring nitrate consumption by phytoplankton.

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Conclusion