Preprints
https://doi.org/10.5194/egusphere-2024-3639
https://doi.org/10.5194/egusphere-2024-3639
13 Jan 2025
 | 13 Jan 2025

Oceanic enrichment of ammonium and its impacts on phytoplankton community composition under a high-emissions scenario

Pearse J. Buchanan, Juan J. Pierella Karlusich, Robyn E. Tuerena, Roxana Shafiee, E. Malcolm S. Woodward, Chris Bowler, and Alessandro Tagliabue

Abstract. Ammonium (NH4+) is an important component of the ocean’s dissolved inorganic nitrogen (DIN) pool, especially in stratified marine environments where intense recycling of organic matter elevates its supply over other forms. Using a global ocean biogeochemical model with good fidelity to the sparse NH4+ data that is available, we project increases in the NH4+:DIN ratio in over 98 % of the ocean by the end of the 21st century under a high-emission scenario. This relative enrichment of NH4+ is driven largely by circulation changes, and secondarily by warming-induced increases in microbial metabolism, as well as reduced nitrification rates due to pH decreases. Supplementing our model projections with geochemical measurements and phytoplankton abundance data from Tara Oceans, we demonstrate that shifts in the form of DIN to NH4+ may impact phytoplankton communities by disadvantaging nitrate-dependent taxa like diatoms while promoting taxa better adapted to NH4+. This could have cascading effects on marine food webs, carbon cycling, and fisheries productivity. Overall, the form of bioavailable nitrogen emerges as an potentially underappreciated driver of ecosystem structure and function in the changing ocean.

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Journal article(s) based on this preprint

23 Sep 2025
Relative enrichment of ammonium and its impacts on open-ocean phytoplankton community composition under a high-emissions scenario
Pearse J. Buchanan, Juan J. Pierella Karlusich, Robyn E. Tuerena, Roxana Shafiee, E. Malcolm S. Woodward, Chris Bowler, and Alessandro Tagliabue
Biogeosciences, 22, 4865–4883, https://doi.org/10.5194/bg-22-4865-2025,https://doi.org/10.5194/bg-22-4865-2025, 2025
Short summary
Pearse J. Buchanan, Juan J. Pierella Karlusich, Robyn E. Tuerena, Roxana Shafiee, E. Malcolm S. Woodward, Chris Bowler, and Alessandro Tagliabue

Interactive discussion

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2024-3639', Anonymous Referee #1, 26 Feb 2025
    • AC1: 'Reply on RC1', Pearse Buchanan, 19 May 2025
  • RC2: 'Comment on egusphere-2024-3639', Anonymous Referee #2, 24 Mar 2025
    • AC2: 'Reply on RC2', Pearse Buchanan, 19 May 2025
  • EC1: 'Comment on egusphere-2024-3639', Olivier Sulpis, 28 Apr 2025
    • AC3: 'Reply on EC1', Pearse Buchanan, 19 May 2025

Interactive discussion

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2024-3639', Anonymous Referee #1, 26 Feb 2025
    • AC1: 'Reply on RC1', Pearse Buchanan, 19 May 2025
  • RC2: 'Comment on egusphere-2024-3639', Anonymous Referee #2, 24 Mar 2025
    • AC2: 'Reply on RC2', Pearse Buchanan, 19 May 2025
  • EC1: 'Comment on egusphere-2024-3639', Olivier Sulpis, 28 Apr 2025
    • AC3: 'Reply on EC1', Pearse Buchanan, 19 May 2025

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload
ED: Reconsider after major revisions (21 May 2025) by Olivier Sulpis
AR by Pearse Buchanan on behalf of the Authors (07 Jun 2025)  Author's response   Author's tracked changes   Manuscript 
ED: Referee Nomination & Report Request started (17 Jun 2025) by Steven Bouillon
RR by Anonymous Referee #2 (30 Jun 2025)
ED: Publish as is (01 Jul 2025) by Steven Bouillon
AR by Pearse Buchanan on behalf of the Authors (03 Jul 2025)

Journal article(s) based on this preprint

23 Sep 2025
Relative enrichment of ammonium and its impacts on open-ocean phytoplankton community composition under a high-emissions scenario
Pearse J. Buchanan, Juan J. Pierella Karlusich, Robyn E. Tuerena, Roxana Shafiee, E. Malcolm S. Woodward, Chris Bowler, and Alessandro Tagliabue
Biogeosciences, 22, 4865–4883, https://doi.org/10.5194/bg-22-4865-2025,https://doi.org/10.5194/bg-22-4865-2025, 2025
Short summary
Pearse J. Buchanan, Juan J. Pierella Karlusich, Robyn E. Tuerena, Roxana Shafiee, E. Malcolm S. Woodward, Chris Bowler, and Alessandro Tagliabue

Data sets

Supplementary dataset 1: Nutrient concentration data Pearse J. Buchanan https://doi.org/10.5281/zenodo.14194938

Supplementary dataset 2: Ammonia oxidation rates Pearse J. Buchanan https://doi.org/10.5281/zenodo.14194938

Supplementary dataset 3: Coincident nutrient and regenerated to new primary production data Pearse J. Buchanan https://doi.org/10.5281/zenodo.14194938

Supplementary dataset 4: Variations in ammonia oxidation rates for pH changes. Rates normalized to a pH of 8. Pearse J. Buchanan https://doi.org/10.5281/zenodo.14194938

Pearse J. Buchanan, Juan J. Pierella Karlusich, Robyn E. Tuerena, Roxana Shafiee, E. Malcolm S. Woodward, Chris Bowler, and Alessandro Tagliabue

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Short summary
Ammonium is a form of nitrogen that may become more important for growth of marine primary producers (i.e., phytoplankton) in the future. Because some phytoplankton taxa have a greater affinity for ammonium than others, the relative increase in ammonium could cause shifts in community composition. We quantify ammonium enrichment, identify its drivers, and isolate the possible effect on phytoplankton community composition under a high emissions scenario.
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