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Biogeosciences An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/bg-2020-35
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-2020-35
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 18 Feb 2020

Submitted as: research article | 18 Feb 2020

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A revised version of this preprint is currently under review for the journal BG.

Factors controlling plankton productivity, particulate matter stoichiometry, and export fluxin the coastal upwelling system off Peru

Lennart Thomas Bach1, Allanah Joy Paul2, Tim Boxhammer2, Elisabeth von der Esch3, Michelle Graco4, Kai Georg Schulz5, Eric Achterberg2, Paulina Aguayo6, Javier Aristegui7, Patrizia Ayon4, Isabel Banos7, Avy Bernales4, Anne Sophie Boegeholz8, Francisco Chavez9, Shao-Min Chen2,10, Kristin Doering2,10, Alba Filella2, Martin Fischer8, Patricia Grasse2, Mathias Haunost2, Jan Hennke2, Nauzet Hernandez-Hernandez7, Mark Hopwood2, Maricarmen Igarza11, Verena Kalter2,12, Leila Kittu2, Peter Kohnert2, Jesus Ledesma4, Christian Lieberum2, Silke Lischka2, Carolin Loescher13, Andrea Ludwig2, Ursula Mendoza4, Jana Meyer2, Judith Meyer2, Fabrizio Minutolo2, Joaquin Ortiz Cortes2, Jonna Piiparinen12, Claudia Sforna2, Kristian Spilling14,15, Sonia Sanchez4, Carsten Spisla2, Michael Sswat2, Mabel Zavala Moreira16, and Ulf Riebesell2 Lennart Thomas Bach et al.
  • 1Institute for Marine and Antarctic Studies, University of Tasmania, Hobart, Tasmania, Australia
  • 2GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany
  • 3Institute of Hydrochemistry, Chair of Analytical Chemistry and Water Chemistry, Technical University of Munich, Munich, Germany
  • 4Dirección General de Investigaciones Oceanográficas y cambio Climático, Instituto del Mar del Peru (IMARPE), Callao, Perú
  • 5Centre for CoastalBiogeochemistry, School of Environment, Science and Engineering, Southern Cross University, Lismore, NSW, Australia
  • 6Millennium Institute of Oceanography (IMO), Universidad de Concepción, Concepción, Chille
  • 7Instituto de Oceanografía y Cambio Global, IOCAG, Universidad de Las Palmas de Gran Canaria ULPGC, Las Palmas, Spain
  • 8Department of Biology, Institute for General Microbiology, Christian-Albrechts-Universität zu Kiel, Kiel, Germany
  • 9Monterey Bay Aquarium Research Institute, Moss Landing, United States of America
  • 10Department of Earth Sciences, Dalhousie University, Halifax, Canada
  • 11Programa de Maestría en Ciencias del Mar, Universidad Peruana Cayetano Heredia, Lima, Peru
  • 12Memorial University of Newfoundland, Department of Ocean Sciences, Logy Bay, Newfoundland, Canada
  • 13University of Southern Denmark, Odense, Denmark
  • 14Finnish Environment Institute, Marine Research Centre, Helsinki, Finland
  • 15Faculty of Engineering and Science, University of Agder, Kristiansand, Norway
  • 16Escuela Superior Politécnica del Litoral, Guayaquil, Ecuador

Abstract. Eastern boundary upwelling systems (EBUS) are among the most productive marine ecosystems on Earth. The high productivity in surface waters is facilitated by upwelling of nutrient-rich deep waters, with high light availability enabling fast phytoplankton growth and nutrient utilization. However, there are numerous biotic and abiotic factors modifying productivity and biogeochemical processes. Determining these factors is important because EBUS are considered hotspots of climate change, and reliable predictions on their future functioning requires understanding of the mechanisms driving biogeochemical cycles therein. In this study, we used in situ mesocosms to obtain mechanistic understanding of processes controlling productivity, organic matter export, and particulate matter stoichiometry in the coastal Peruvian upwelling system. Therefore, eight mesocosm units with a volume of ~50 m3 were deployed for 50 days ~6 km off Callao during austral summer 2017, coinciding with a coastal El Niño event. To compare how upwelling of different water bodies influences plankton succession patterns, we collected two subsurface waters at different locations in the regional oxygen minimum zone (OMZ) and injected these into four replicate mesocosms, respectively (mixing ratio ≈ 1.5:1 mesocosm: OMZ water). The differences in nutrient concentrations between the collected water bodies were relatively small, and therefore we do not consider treatment differences in the present paper. The phytoplankton communities were initially dominated by diatoms but shifted towards a pronounced dominance of the mixotrophic harmful dinoflagellate (Akashiwo sanguinea) when inorganic nitrogen was exhausted in surface layers. The community shift resulted in a major short-term increase in productivity during A. sanguinea growth which left a pronounced imprint on organic matter C:N:P stoichiometry. However, C, N, and P export fluxes were not affected by this ecological regime shift because A. sanguinea persisted in the water column and did not sink out during the experiment. Accordingly, ongoing export fluxes during the study were maintained mainly by a remaining “background” plankton community. Overall, biogeochemical pools and fluxes were surprisingly constant in between the ecological regime shifts. We explain this constancy by light limitation through self-shading by phytoplankton and inorganic nitrogen limitation which constrained phytoplankton growth. Thus, gain and loss processes seemed to be relatively well balanced and there was little opportunity for blooms, which represents an event where the system becomes unbalanced. The mesocosm study revealed key links between ecological and biogeochemical processes for one of the economically most important regions in the oceans.

Lennart Thomas Bach et al.

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Lennart Thomas Bach et al.

Lennart Thomas Bach et al.

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Short summary
The eastern boundary upwelling system off Peru is among the most productive ecosystems in the oceans but the factors that control its functioning are poorly constrained. Here we used mesocosms, moored ~6 km offshore Peru, to investigate how processes in plankton communities drive key biogeochemical processes. We show that nutrient and light co-limitation keep productivity and export at a remarkably constant level while stoichiometry changes strongly with shifts in plankton community structure.
The eastern boundary upwelling system off Peru is among the most productive ecosystems in the...
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