Nitrogen uptake and regeneration pathways in the equatorial Pacific: a basin scale modeling study

Abstract

Abstract.It is well known that most primary production is fueled by regenerated nitrogen in the open ocean. Therefore, studying the nitrogen cycle by focusing on uptake and regeneration pathways would advance our understanding of nitrogen dynamics in the marine ecosystem. Here, we carry out a basin-scale modeling study, by assessing model simulations of nitrate and ammonium, and rates of nitrate uptake, ammonium uptake and regeneration in the equatorial Pacific. Model-data comparisons show that the model is able to reproduce many observed features of nitrate, ammonium, such as the deep ammonium maximum (DAM). The model also reproduces the observed de-coupling of ammonium uptake and regeneration, i.e., regeneration rate greater than uptake rate in the lower euphotic zone. The de-coupling largely explains the observed DAM in the equatorial Pacific Ocean. Our study indicates that zooplankton excretion and remineralization of organic nitrogen play a different role in nitrogen regeneration. Rates of zooplankton excretion vary from <0.01 mmol m −3 d −1 to 0.1 mmol m −3 d −1 in the upper euphotic zone while rates of remineralization fall within a narrow range (0.015-0.025 mmol m −3 d −1 ). Zooplankton excretion contributes up to 70% of total ammonium regeneration in the euphotic zone, and is largely responsible for the spatial variability of nitrogen regeneration. However, remineralization provides a steady supply of ammonium in the upper ocean, and is a major source of inorganic nitrogen for the oligotrophic regions. Overall, ammonium generation and removal are approximately balanced over the top 150 m in the equatorial Pacific.

Multiyear predictability of tropical marine productivity

“…The influence of these processes on NPP variability and the resulting spatial repartition of regenerated versus new production (respectively in the western and eastern part of the tropical Pacific) are in agreement with previous regional modeling studies forced by atmospheric reanalyses (37,38). Thus, the model predictive skill Fig.…”

Description and basic evaluation of Beijing Normal University Earth System Model (BNU-ESM) version 1

“…The soilcarbon scheme will be further improved to simulate the large carbon accumulations in organic peat soils, the stocks and dynamics of organic matter in permafrost. A dynamic marine ecosystem scheme will replace the current iBGC module, the new marine ecosystem scheme has improved parameterizations of dissolved organic materials and detritus , a phytoplankton dynamic module that produces a variable of carbon to chlorophyll ratio (Wang et al, 2009a), and refined nitrogen regeneration pathways (Wang et al, 2009b). Additionally, a three-dimensional canopy radiative transfer model (Yuan et al, 2014) will be adopted to replace the traditional one-dimensional two-stream approximation scheme in the land component to calculate terrestrial canopy radiation more realistically.…”

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

“…Figure 3 shows comparisons of the total nitrogen uptake, nitrate uptake, export production and net community production between the standard and NITR3 simulations for the equatorial Pacific. While the total nitrogen uptake was not expected to change (see details in [20]), our results also show little change in export production and net community production. However, there is a pronounced increase (20-50%) in the integrated nitrate uptake in the euphotic zone when nitrification rate is increased from 0 to 0.06 d −1 .…”

“…Ammonium, revealing considerable spatial and temporal variations with relatively low concentrations, may provide a sensitive and realistic check on the performance of biogeochemical models [30]. Moreover, an approach with integrated validations of concentrations and uptake rates of both nitrate and ammonium would provide an overall assessment on biogeochemical model skills [20], which requires integrated observations of different nitrogen forms and fluxes such as Raimbault et al [6].…”

“…Phytoplankton growth (thus nitrogen uptake) is a function of nitrogen, iron, temperature and light conditions. Parameterizations of nitrogen uptake and regeneration were described by Wang et al [20]. Our modeling effort has included extensive use of observational data for model calibration and validation; e.g., dissolved and particulate organic nitrogen [19], basinscale chlorophyll distribution [21], nitrate and ammonium uptake, and ammonium regeneration [20].…”