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

摘要

It is well known that most primary production is fueled by regeneratednitrogen in the open ocean. Therefore, studying the nitrogen cycle byfocusing on uptake and regeneration pathways would advance our understandingof nitrogen dynamics in the marine ecosystem. Here, we carry out abasin-scale modeling study, by assessing model simulations of nitrate andammonium, and rates of nitrate uptake, ammonium uptake and regeneration inthe equatorial Pacific. Model-data comparisons show that the model is ableto reproduce many observed features of nitrate, ammonium, such as the deepammonium maximum (DAM). The model also reproduces the observed de-couplingof ammonium uptake and regeneration, i.e., regeneration rate greater thanuptake rate in the lower euphotic zone. The de-coupling largely explains theobserved DAM in the equatorial Pacific Ocean. Our study indicates thatzooplankton excretion and remineralization of organic nitrogen play adifferent role in nitrogen regeneration. Rates of zooplankton excretion varyfrom <0.01 mmol m−3 d−1 to 0.1 mmol m−3 d−1 in theupper euphotic zone while rates of remineralization fall within a narrowrange (0.015–0.025 mmol m−3 d−1 . Zooplankton excretioncontributes up to 70% of total ammonium regeneration in the euphoticzone, and is largely responsible for the spatial variability of nitrogenregeneration. However, remineralization provides a steady supply of ammoniumin the upper ocean, and is a major source of inorganic nitrogen for theoligotrophic regions. Overall, ammonium generation and removal areapproximately balanced over the top 150 m in the equatorial Pacific.