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Abstract. Summer cyanobacterial blooms represent a threat to theBaltic Sea ecosystem, causing deoxygenation of the bottom water and thespread of the so-called dead zones. The history of the Baltic Seacyanobacterial blooms is known from in situ and satellite observations sincethe early 1980s but is still not well understood. By comparing bothweekly resolved sediment trap material and a well-dated sediment core fromthe eastern Gotland Basin with monitoring and satellite cyanobacterial dataof the last ca. 35 years, it is shown here that 6- and 7-methylheptadecanelipids (expressed as 6+7Me-C17 : 0) may be potentially consideredsemiquantitative biomarkers for diazotrophic cyanobacteria, and more specifically for Nodularia spumigena. Using this organic proxy, it was thus possible to reconstructthe history of cyanobacterial blooms beyond the observational period with aresolution of 2–4 years since 1860. Cyanobacteria were constantly presentbut in relatively low abundance until 1920, when they started to alternatebetween periods with high and low abundance. Interestingly, there seems tobe no significant increase in cyanobacterial abundance in the 1950s, wheneutrophication and deoxygenation of the Baltic Sea increased considerably.While the early increase in cyanobacteria may be related to a small increasein phosphorus loading, decadal to multi-decadal fluctuations are likelyrelated to variability in the Baltic Sea surface temperature and,ultimately, to the Atlantic Multi-decadal Oscillation. A 7000-year 6+7Me-C17 : 0 record from the Bothnian Sea also suggests a relationshipwith the mean summer temperature in the Baltic Sea region but at amulti-centennial to multi-millennial timescale. The intensity of thecyanobacterial blooms in the Baltic Sea is thus likely mainly related tonatural processes such as temperature variability, at least at amulti-decadal to multi-millennial timescale.

biogeosciences

Reconstructing N&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;-fixing cyanobacterial blooms in the Baltic Sea beyond observations using 6- and 7-methylheptadecane in sediments as specific biomarkers