Abstract-Massive summer blooms of nitrogen-fixing cyanobacteria have been documented in the Baltic Sea since the 19th century, but are reported to have increased in frequency, biomass, and duration in recent decades-presumably in response to the well-documented anthropogenic eutrophication of the Baltic. Here, we present an 8,000-yr record of fossil cyanobacterial pigments, diatom microfossil assemblages, and ␦ 15

[1] The Baltic Sea became connected to the open sea about 8500 years B.P., and that marked the beginning of the still continuing Litorina Sea stage. There have since been significant long-term variations of the salinity from a maximum of some 10-15% to the present of 7-8%. The salinity variations have been known since the beginning of the twentieth century, and various mechanisms have been proposed to explain these. However, so far, no one has actually tried to quantify and thereby rank the relative importance of the different proposed mechanisms. In this paper we used a simple oceanographic model to quantify the impact of changes in crosssectional areas of the inlets to the Baltic. We found that such changes cannot explain the full variation of the salinity. We propose that the remaining salinity variations (0.5-5%) are explained by changes in net freshwater input in the range 15-60%. The calculated variations in the freshwater supply are in large coherent with known variations of the northern European climate. We show that other possible factors as increased mixing, frontal displacements, higher deep water salinity, and larger depths in Kattegat and larger oscillating flows across the sills due to increased sea level variability are less likely to have made substantial contributions to the salinity variations in the Baltic Sea.

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Selected geochemical parameters and siliceous microfossil assemblages in Baltic Sea sediments are presented which reflect past variations in redox conditions, salinity and primary production. The sediments were deposited during the freshwater Ancylus Lake (9500‐8000 14C BP) and brackish Litorina Sea (8000‐3000 BP) stages of the Baltic. The diatom record shows that surface‐water salinity increased further at c. 7000‐6500 BP, although smaller amounts of brackish water entered the basin from c. 8000 BP onwards. Attempts to use exchangeable Mg as a palaeosalinity indictor were not applicable. Gross primary productivity increased along with salinity, which has been interpreted as an effect of nutrient enrichment in the photic zone. This led between c. 6500 and 4500 BP to a high accumulation of organic carbon, anoxic or nearly anoxic bottom conditions and formation of laminated deposits. Certain laminae consist of alternating layers of organic and minerogenic material and were probably formed annually, i.e. in the manner of varves. The laminated successions are distinguished by enrichments of V, Cu and especially Mo. The highest Mo content occurs in the core from the greatest water depth, an effect of anoxic conditions during deposition. The Fe/Mn ratio was shown to be ambiguous as an indicator of past redox conditions. Since biogenic silica shows large variablity in contemporaneously deposited sediments, this parameter cannot be used as a proxy for the past production of siliceous algae in the Baltic Sea.

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