The siliceous microalgae Dictyocha speculum and Ebria tripartita as biomarkers and palaeoecological indicators in Holocene Baltic Sea sediments

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The relative abundances of the silicoflagellate Dictyocha speculum Ehrenberg and the ebridian Ebria tripartita (Schumann) Lemmermann within the total siliceous microfossil flora were determined in four sediment cores from the northwestern Baltic Sea covering the last c. 8,000 years, and that of Ebria tripartita in five short sediment cores from the southern Baltic Sea covering the last c. 100–200 years and in surface sediment samples from the northern and central Baltic Sea representing the years 1993 and 1997. In the present study it is shown that D. speculum has a well defined acme (peak) zone confined to c. 5,500–4,500 14C years BP, and it is suggested that this may represent the most saline phase in the Holocene history of the Baltic Sea. Ebria tripartita, on the other hand, has no distinct acme zone in the cores, although a slight increase in its relative abundance occurs c. 2,000–1,500 14C years BP in the three cores covering this period. It is also shown that there is no coherent trend in the abundance of E. tripartita during the most recent centuries, but that the variations seem to be confined to periods when there were major changes in diatom assemblages. It is concluded that E. tripartita is useful neither as a biomarker nor as a palaeoenvironmental indicator in the Baltic Sea. It is argued that the diatom assemblages, the resuspension effect, year to year fluctuations in bloom and the patchiness of the blooms are the main factors controlling the relative abundance of E. tripartita skeletons in Baltic Sea sediments.