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.
Highlights
New production, which is achieved from nitrogen inputs to an ecosystem and often taken as nitrate consumption, is used as a proxy for export production in steady-state ecosystems
The positive net ammonium production (NAP) (0.03 mmol m−2 d−1) in the CEP coincides to the highest ammonium content, suggesting that the subsurface surplus of ammonium regeneration is largely responsible for the observed deep ammonium maximum (DAM) in the equatorial Pacific Ocean
We have conducted a basin scale modeling study focusing on nitrogen uptake and regeneration in the equatorial Pacific
Summary
New production, which is achieved from nitrogen inputs to an ecosystem and often taken as nitrate consumption, is used as a proxy for export production in steady-state ecosystems. There are only a few studies addressing ammonium uptake and regeneration in the equatorial Pacific (e.g., Raimbault et al, 1999; Wang et al, 2005; Le Bouteiller et al, 2003; McCarthy et al, 1996), which clearly hampers our ability to understand the marine nitrogen cycle. Wang et al.: Nitrogen uptake and regeneration pathways does not vary much, approximately located at 100 m These observed features in ammonium dynamics, together with the relatively low concentrations of ammonium, may provide a sensitive and realistic check on the performance of biogeochemical/ecosystem models. Our approach includes (1) the use of field data collected from the equatorial Pacific for parameterization of nitrogen regeneration in our basin scale physicalbiogeochemical model, and (2) integrated model validations, for distributions of nitrate and ammonium, and for rates of nitrate uptake, ammonium uptake and regeneration
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