Simulated Future Trends in Marine Nitrogen Fixation Are Sensitive to Model Iron Implementation. Issue 3 (24th February 2022)

• Record Type: Journal Article
• Authors: Yao, Wanxuan; Kvale, Karin F.; Koeve, Wolfgang; Landolfi, Angela; Achterberg, Eric; Bertrand, Erin M.; Oschlies, Andreas
• Is Part Of: Global biogeochemical cycles. Volume 36:Issue 3(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-02-24
• Subjects: nitrogen fixation -- climate change -- biogeochemistry -- iron cycle -- nitrogen cycle; Biogeochemical cycles -- Periodicals; Electronic journals; 577.1405
• DOI: 10.1029/2020GB006851
• Languages: English
• ISSNs: 0886-6236

Abstract

Abstract: Biological nitrogen fixation is an important oceanic nitrogen source, potentially stabilizing marine fertility in an increasingly stratified and nutrient‐depleted ocean. Iron limitation of low latitude primary producers has been previously demonstrated to affect simulated regional ecosystem responses to climate warming or nitrogen cycle perturbation. Here we use three biogeochemical models that vary in their representation of the iron cycle to estimate change in the marine nitrogen cycle under a high CO2 emissions future scenario (RCP8.5). The first model neglects explicit iron effects on biology (NoFe), the second utilizes prescribed, seasonally cyclic iron concentrations and associated limitation factors (FeMask), and the third contains a fully dynamic iron cycle (FeDyn). Models were calibrated using observed fields to produce near‐equivalent nutrient and oxygen fits, with productivity ranging from 49 to 75 Pg C yr −1 . Global marine nitrogen fixation increases by 71.1% with respect to the preindustrial value by the year 2100 in NoFe, while it remains stable (0.7% decrease in FeMask and 0.3% increase in FeDyn) in explicit iron models. The mitigation of global nitrogen fixation trend in the models that include a representation of iron originates in the Eastern boundary upwelling zones, where the bottom‐up control of iron limitation reduces export production with warming, which shrinks the oxygen deficient volume, and reduces denitrification. Warming‐induced trends in the oxygen deficient volume in the upwelling zones have a cascading effect on the global nitrogen cycle, just as they have previously been shown to affect tropical net primary production. Plain Language Summary: Phytoplankton need nutrients to grow. Two of those nutrients are nitrogen and iron. Climate change projections suggest that in the future there could be less nitrogen supplied to the surface ocean, which might reduce phytoplankton growth. Less phytoplankton growth could impact a wide range of ocean services, like fishing and fossil carbon draw‐down. However, some phytoplankton have the ability to add "new" nitrogen to the surface ocean directly from the atmosphere. In this study, we explore how this biological fixation of new nitrogen might change into the future using three models. These models differ in how iron is represented, but all do equally well in representing the observed nutrient and oxygen distribution. Biological nitrogen fixation slightly decreases with climate change in the very complex iron model and the moderately complex iron model, but it increases strongly (by more than 70% by the year 2100) in the model that does not include iron effects on biology. Our study addresses the importance of iron models and how they can change our view of how the ocean responds to climate change. Key Points: Models performing similarly with respect to global NO3, PO4, and O2 distributions yield diverse responses in marine N2 fixation to warming Marine N2 fixation trends are sensitive to whether iron limits primary production in upwelling regions, for example, the Eastern Tropical Pacific