Preprints
https://doi.org/10.5194/bg-2021-6
https://doi.org/10.5194/bg-2021-6

  18 Jan 2021

18 Jan 2021

Review status: this preprint is currently under review for the journal BG.

Nitrogen loss processes in response to upwelling in a Peruvian coastal setting dominated by denitrification

Kai G. Schulz1, Eric P. Achterberg2, Javier Arístegui3, Lennart T. Bach4, Isabel Baños3, Tim Boxhammer2, Dirk Erler1, Maricarmen Igarza5, Verena Kalter2,6, Andrea Ludwig2, Carolin Löscher7, Jana Meyer2, Judith Meyer2, Fabrizio Minutolo2, Elisabeth von der Esch8, Bess B. Ward9, and Ulf Riebesell2 Kai G. Schulz et al.
  • 1Centre for Coastal Biogeochemistry, School of Environment, Science and Engineering, Southern Cross University, Lismore, NSW, Australia
  • 2GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany
  • 3Instituto de Oceanografía y Cambio Global, IOCAG, Universidad de Las Palmas de Gran Canaria ULPGC, Las Palmas, Spain
  • 4Institute for Marine and Antarctic Studies, University of Tasmania, Hobart, Tasmania, Australia
  • 5Instituto del Mar del Perú (IMARPE), Dirección General de Investigaciones en Oceanografía y Cambio Climático, Callao, Peru
  • 6Memorial University of Newfoundland, Department of Ocean Sciences, Logy Bay, Newfoundland, Canada
  • 7Nordcee, DIAS, Department of Biology, University of Southern Denmark, Campusvej 55, Odense M-DK
  • 8Institute of Hydrochemistry, Chair of Analytical Chemistry and Water Chemistry, Technical University of Munich, Munich, Germany
  • 9Department of Geosciences, Princeton University, Princeton, New Jersey 08544

Abstract. Upwelling of nutrient-rich deep waters make Eastern Boundary upwelling systems (EBUS), such as the Humboldt Current System, hotspots of marine productivity. Associated settling of organic matter to depth and consecutive aerobic de composition results in large sub-surface water volumes being oxygen-depleted. Under these circumstances organic matter remineralisation can continue via denitrification which represents a major loss pathway of bioavailable nitrogen. Another process removing significant amounts of nitrogen in these areas is anaerobic ammonium oxidation. Here we assess the interplay of suboxic water upwelling and nitrogen cycling in a manipulative off-shore mesocosm experiment. Measured denitrification rates in the oxygen-depleted bottom layer of the mesocosms mostly ranged between 5.5–20 (interquartile range), reaching up to 80 nmol N2 L−1 h−1. However, realised in-situ rates did most likely not exceed 0.6–1.6 nmol N2 L−1 h−1 (interquartile range), due to substrate limitation in the mesocosms. This was in contrast to realised rates in the surrounding Pacific. Both in the mesocosms and the Pacific Ocean anammox made only a minor contribution to overall nitrogen loss when encountered. Over the first 38 days of the experiment, total nitrogen loss calculated from denitrification and anammox rates was comparable to estimates from a full nitrogen budget in the mesocosms and ranged between ∼ 2–6 μmol N L−1. This represents up to ∼ 20 % of the initially bioavailable inorganic and organic nitrogen standing stocks. Interestingly, this loss is comparable to the total amount of particulate organic nitrogen that was exported into the sediment traps at the bottom of the mesocosms in about 20 metres depth. Altogether, this suggests that a significant portion, if not the majority of nitrogen that could be exported to depth, is already lost, i.e. converted to N2 in a relatively shallow layer of the surface ocean, provided oxygen-deficient conditions like during coastal upwelling in our study.

Kai G. Schulz et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on bg-2021-6', Annie Bourbonnais, 05 Feb 2021
  • RC2: 'Comment on bg-2021-6', Anonymous Referee #2, 15 Feb 2021

Kai G. Schulz et al.

Kai G. Schulz et al.

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Short summary
Upwelling of nutrient-rich deep waters to the surface make Eastern Boundary upwelling systems hotspots of marine productivity. This leads to sub-surface oxygen-depletion and transformation of bio-available nitrogen into inert N2. Here we quantify nitrogen loss processes following a simulated deep-water upwelling. Denitrification was the dominant process and budget calculations suggest that a significant portion of nitrogen that could be exported to depth is already lost in the surface ocean.
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