Preprints
https://doi.org/10.5194/bg-2021-40
https://doi.org/10.5194/bg-2021-40

  01 Mar 2021

01 Mar 2021

Review status: a revised version of this preprint was accepted for the journal BG and is expected to appear here in due course.

Cyanobacteria net community production in the Baltic Sea as inferred from profiling pCO2 measurements

Jens Daniel Müller1,2, Bernd Schneider1, Ulf Gräwe3, Peer Fietzek4, Marcus Bo Wallin5,6, Anna Rutgersson5, Norbert Wasmund7, Siegfried Krüger3, and Gregor Rehder1 Jens Daniel Müller et al.
  • 1Department of Marine Chemistry, Leibniz Institute for Baltic Sea Research Warnemünde, Rostock, Germany
  • 2Environmental Physics, Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich, Zurich, Switzerland
  • 3Department of Physical Oceanography and Instrumentation, Leibniz Institute for Baltic Sea Research Warnemünde, Rostock, Germany
  • 4Kongsberg Maritime Germany GmbH, Hamburg, Germany
  • 5Department of Earth Sciences, Uppsala University, Uppsala, Sweden
  • 6Department of Aquatic Sciences and Assessment, Swedish University of Agricultural Sciences, Uppsala, Sweden
  • 7Department of Biological Oceanography, Leibniz Institute for Baltic Sea Research Warnemünde, Rostock, Germany

Abstract. Organic matter production by cyanobacteria blooms is a major environmental concern for the Baltic Sea as it promotes thespread of anoxic zones. Partial pressure of carbon dioxide (pCO2) measurements carried out on Ships of Opportunity (SOOP) since 2003 have proven to be a powerful tool to resolve the carbon dynamics of the blooms in space and time. However, SOOP measurements lack the possibility to directly constrain the depth–integrated net community production (NCP) due to their restriction to the sea surface. This study tackles the resulting knowledge gap through (1) providing a best–guess NCP estimatefor an individual cyanobacteria bloom based on repeated profiling measurements of pCO2 and (2) establishing an algorithm to accurately reconstruct depth–integrated NCP from surface pCO2 observations in combination with modelled temperature profiles.

Goal (1) was achieved by deploying state–of–the–art sensor technology from a small–scale sailing vessel. The low–cost and flexible platform enabled observations covering an entire bloom event that occurred in July and August 2018 in the Eastern Gotland Sea. For the biogeochemical interpretation, recorded pCO2 profiles were converted to CT*, which is the dissolved inorganic carbon concentration normalised to alkalinity. We found that the investigated Nodularia–dominated bloom event had many biogeochemical characteristics in common with blooms in previous years. In particular, it lasted for about three weeks, caused a CT* drawdown of 80 μmol kg−1, and was accompanied by a sea surface temperature increase of 10 °C. The novel finding of this study is the vertical extension of the CT* drawdown up to 12 m water depth. Integration of the CT* drawdown across this depth and correction for vertical fluxes permit a best–guess NCP estimate of ~1.2 mol–C m−2.

Addressing goal (2), we combined modelled hydrographical profiles with surface pCO2 observations recorded by SOOP Finnmaid within the study area. Introducing the temperature penetration depth (TPD) as a new parameter to integrate SOOP observations across depth, we achieve a reconstructed NCP estimate that agrees to the best–guess within 10 %.

Applying the TPD approach to almost two decades of surface pCO2 observations available for the Baltic Sea bears the potential to provide new insights into the control and long–term trends of cyanobacteria NCP. This understanding is key for an effective design and monitoring of conservation measures aiming at a Good Environmental Status of the Baltic Sea.

Jens Daniel Müller et al.

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on bg-2021-40', Anonymous Referee #1, 24 Mar 2021
    • AC1: 'Reply on RC1', Jens Daniel Müller, 07 May 2021
  • RC2: 'Comment on bg-2021-40', Anonymous Referee #2, 17 Apr 2021
    • AC2: 'Reply on RC2', Jens Daniel Müller, 07 May 2021

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on bg-2021-40', Anonymous Referee #1, 24 Mar 2021
    • AC1: 'Reply on RC1', Jens Daniel Müller, 07 May 2021
  • RC2: 'Comment on bg-2021-40', Anonymous Referee #2, 17 Apr 2021
    • AC2: 'Reply on RC2', Jens Daniel Müller, 07 May 2021

Jens Daniel Müller et al.

Data sets

jens-daniel-mueller/BloomSail: os-2020-120_submission Jens Daniel Müller https://doi.org/10.5281/zenodo.4553314

Model code and software

jens-daniel-mueller/BloomSail: os-2020-120_submission Jens Daniel Müller https://doi.org/10.5281/zenodo.4553314

Jens Daniel Müller et al.

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Short summary
Based on months of profiling observations from a field campaign, we unravel how much biomass can sink out of a cyanobacteria bloom in the Baltic Sea and potentially contribute to the extension of death zones. More importantly, we show how this information can be accurately retrieved from long-term surface measurements made on cargo vessels, when combining them with modelled temperature data. This enables a better understanding of a severe concern for the Baltic’s good environmental status.
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