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

The relative abundances of the silicoflagellate Dictyocha speculum Ehrenber g 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 Ebri a tripartita in five short sediment cores from the southern Baltic Sea cove ring the last c. 100-200 years and in surface sediment samples from the nor thern and central Baltic Sea representing the years 1993 and 1997. In the p resent study it is shown that D. speculum has a well defined acme (peak) zo ne confined to c. 5,500-4,500 C-14 years BP, and it is suggested that this may represent the most saline phase in the Holocene history of the Baltic S ea. Ebria tripartita, on the other hand, has no distinct acme zone in the c ores, although a slight increase in its relative abundance occurs c. 2.000- 1,500 C-14 years BP in the three cores covering this period. It is also sho wn 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 conclude d that E. tripartita is useful neither as a biomarker nor as a palaeoenviro nmental indicator in the Baltic Sea. It is argued that the diatom assemblag es, the resuspension effect, year to year fluctuations in bloom and the pat chiness of the blooms are the main factors controlling the relative abundan ce of E. tripartita skeletons in Baltic Sea sediments.