Reconstructing N<sub>2</sub>-fixing cyanobacterial blooms in the Baltic Sea beyond observations using 6- and 7-methylheptadecane in sediments as specific biomarkers
<p>Summer cyanobacterial blooms represent a threat to the Baltic Sea ecosystem, causing deoxygenation of the bottom water and the spread of the so-called dead zones. The history of the Baltic Sea cyanobacterial blooms is known from in situ and satellite observations since the early 1980s but i...
| Main Authors: | , , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2020-05-01
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| Series: | Biogeosciences |
| Online Access: | https://www.biogeosciences.net/17/2579/2020/bg-17-2579-2020.pdf |
| Summary: | <p>Summer cyanobacterial blooms represent a threat to the
Baltic Sea ecosystem, causing deoxygenation of the bottom water and the
spread of the so-called dead zones. The history of the Baltic Sea
cyanobacterial blooms is known from in situ and satellite observations since
the early 1980s but is still not well understood. By comparing both
weekly resolved sediment trap material 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<span class="inline-formula">+</span>7Me-C<span class="inline-formula"><sub>17 : 0</sub></span>) may be potentially considered
semiquantitative biomarkers for diazotrophic cyanobacteria, and more specifically for <i>Nodularia spumigena</i>. 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.
While the early increase in cyanobacteria may be related to a small increase
in phosphorus loading, decadal to multi-decadal fluctuations are likely
related to variability in the Baltic Sea surface temperature and,
ultimately, to the Atlantic Multi-decadal Oscillation. A 7000-year 6<span class="inline-formula">+</span>7Me-C<span class="inline-formula"><sub>17 : 0</sub></span> 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.</p> |
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| ISSN: | 1726-4170 1726-4189 |