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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-2019-455
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-2019-455
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 28 Nov 2019

Submitted as: research article | 28 Nov 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Biogeosciences (BG).

Reconstructing N2-fixing cyanobacterial blooms in the Baltic Sea beyond observations using 6- and 7-methylheptadecanes in sediments as specific biomarkers

Jérôme Kaiser1, Norbert Wasmund1, Mati Kahru2, Anna K. Wittenborn1, Regina Hansen1, Katharina Häusler1, Matthias Moros1, Detlef Schulz-Bull1, and Helge W. Arz1 Jérôme Kaiser et al.
  • 1Leibniz Institute for Baltic Sea Research (IOW), Seestrasse 15, 18119 Rostock-Warnemünde, Germany
  • 2Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA, 92093-0218, USA

Abstract. Summer cyanobacterial blooms represent a threat for the Baltic Sea ecosystem, causing deoxygenation of the bottom water and the spread of the so-called dead zones. The time history of the Baltic Sea cyanobacterial blooms is known from in situ and satellite observations since the early 1980s, but still not well understood. By comparing both weekly-resolved trap sediments and a well-dated sediment core from the Eastern Gotland Basin with monitoring and satellite cyanobacterial data of the last ca. 35 years, it is shown here that 6- and 7-methylheptadecane lipids (expressed as 6+7Me-C17:0) are robust semi-quantitative biomarkers for diazotrophic cyanobacteria, and likely mainly for Nodularia spumigena. Using this organic proxy, it was thus possible to reconstruct the history of cyanobacterial blooms beyond the observational period with a resolution of 2–4 years since 1860. Cyanobacteria were constantly present, but in relatively low abundance until 1920, when they started to alternate between periods with high and low abundance. Interestingly, there seems to be no significant increase in cyanobacterial abundance in the 1950s, when eutrophication and deoxygenation of the Baltic Sea increased considerably. Decadal to multi-decadal fluctuations are likely rather related to variability in the Baltic Sea surface temperature and, ultimately, to the Atlantic Multidecadal Oscillation. A 7000 years long 6+7Me-C17:0 record from the Bothnian Sea also suggests a relationship with the mean summer temperature in the Baltic Sea region, but at a multi-centennial to multi-millennial timescale. The intensity of the cyanobacterial blooms in the Baltic Sea is thus likely mainly related to natural processes such as temperature variability, at least at a multi-decadal to multi-millennial timescale.

Jérôme Kaiser et al.
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Short summary
Cyanobacterial blooms represent a threat for the Baltic Sea ecosystem, causing deoxygenation of the bottom water. In order to understand the natural versus anthropogenic factors driving these blooms, it is necessary to study long-term trends bexond observations. We have produced a record of cyanobacterial blooms since 1860 using organic molecules (biomarkers) preserved in sediments. Cyanobacterial blooms in the Baltic Sea are likely mainly related to temperature variability.
Cyanobacterial blooms represent a threat for the Baltic Sea ecosystem, causing deoxygenation of...
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