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

Kai   G Schulz; E. P. Achterberg; J. Arístegui; L. T. Bach; I. Baños; T. Boxhammer; Dirk Erler; M. Igarza; V. Kalter; A. Ludwig; C. Löscher; Jana Meyer; Judith Meyer; F. Minutolo; E. von der Esch; B. B. Ward; U. Riebesell

doi:10.5194/bg-18-4305-2021

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

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

Kai G Schulz, E. P. Achterberg, J. Arístegui, L. T. Bach, I. Baños, T. Boxhammer, Dirk Erler, M. Igarza, V. Kalter, A. Ludwig, …

Biogeosciences, Vol.18(14), pp.4305-4320

23/07/2021

DOI: https://doi.org/10.5194/bg-18-4305-2021

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Abstract

Upwelling of nutrient-rich deep waters make eastern boundary upwelling systems (EBUSs), such as the Humboldt Current system, hot spots of marine productivity. Associated settling of organic matter to depth and consecutive aerobic decomposition results in large subsurface water volumes being oxygen depleted. Under these circumstances, organic matter remineralisation can continue via denitrification, which represents a major loss pathway for bioavailable nitrogen. Additionally, anaerobic ammonium oxidation can remove significant amounts of nitrogen in these areas. Here we assess the interplay of suboxic water upwelling and nitrogen cycling in a manipulative offshore mesocosm experiment. Measured denitrification rates in incubations with water from the oxygen-depleted bottom layer of the mesocosms (via 15 N label incubations) mostly ranged between 5.5 and 20 nmol N 2 L −1 h −1 (interquartile range), reaching up to 80 nmol N 2 L −1 h −1 . However, actual in situ rates in the mesocosms, estimated via Michaelis–Menten kinetic scaling, did most likely not exceed 0.2–4.2 nmol N 2 L −1 h −1 (interquartile range) due to substrate limitation. In the surrounding Pacific, measured denitrification rates were similar, although indications of substrate limitation were detected only once. In contrast, anammox (anaerobic ammonium oxidation) made only a minor contribution to the overall nitrogen loss when encountered in both the mesocosms and the Pacific Ocean. This was potentially related to organic matter C / <svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="8pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="165b352473919034209a9d51d0eaf41d"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-18-4305-2021-ie00001.svg" width="8pt" height="14pt" src="bg-18-4305-2021-ie00001.png"/></svg:svg> N stoichiometry and/or process-specific oxygen and hydrogen sulfide sensitivities. Over the first 38 d of the experiment, total nitrogen loss calculated from in situ rates of denitrification and anammox was comparable to estimates from a full nitrogen budget in the mesocosms and ranged between ∼ 1 and 5.5 µ mol 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 at about 20 m 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 N 2 in a relatively shallow layer of the surface ocean, provided that there are oxygen-deficient conditions like those during coastal upwelling in our study. Published data for primary productivity and nitrogen loss in all EBUSs reinforce such conclusion.

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Title: Nitrogen loss processes in response to upwelling in a Peruvian coastal setting dominated by denitrification – a mesocosm approach
Creators: Kai G Schulz - Southern Cross University
E. P. Achterberg - GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany
J. Arístegui - Instituto de Oceanografía y Cambio Global, IOCAG, Universidad de Las Palmas de Gran Canaria ULPGC, Las Palmas, Spain
L. T. Bach - Institute for Marine and Antarctic Studies, University of Tasmania, Hobart, Tasmania, Australia
I. Baños - Instituto de Oceanografía y Cambio Global, IOCAG, Universidad de Las Palmas de Gran Canaria ULPGC, Las Palmas, Spain
T. Boxhammer - GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany
Dirk Erler (Author) - Southern Cross University
M. Igarza - Instituto del Mar del Perú (IMARPE), Dirección General de Investigaciones en Oceanografía y Cambio Climático, Callao, Peru
V. Kalter - GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany
A. Ludwig - GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany
C. Löscher - Department of Biology, Nordcee, Danish Institute for Advanced Study (DIAS), University of Southern Denmark, Campusvej 55, Odense M, Denmark
Jana Meyer - GEOMAR Helmholtz Centre for Ocean Research Kiel
Judith Meyer - GEOMAR Helmholtz Centre for Ocean Research
F. Minutolo - GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany
E. von der Esch - Institute of Hydrochemistry, Chair of Analytical Chemistry and Water Chemistry, Technical University of Munich, Munich, Germany
B. B. Ward - Department of Geosciences, Princeton University, Princeton, New Jersey 08544, USA
U. Riebesell - GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany
Publication Details: Biogeosciences, Vol.18(14), pp.4305-4320
Publisher: Copernicus Publications
Grant note: This research has been supported by the Collaborative Research Center (SFB 754), “Climate–Biogeochemistry Interactions in the Tropical Ocean”, financed by the German Research Foundation (DFG), the Leibniz Award 2012 (granted to Ulf Riebesell), the Helmholtz International Fellow Award 2015 (granted to Javier Arístegui), and EU funding by AQUACOSM (granted to a number of participants, including Javier Arístegui and Kai G.Schulz).
Identifiers: 991012961498102368
Academic Unit: Centre for Coastal Biogeochemistry; Faculty of Science and Engineering; Science
Language: English
Resource Type: Journal article

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

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