Response of freshwater diatoms to Early−Middle Holocene climate changes, SW Lithuania

Detailed diatom analysis, biodiversity analysis and non-metric multidimensional scaling were used to describe the response of diatom assemblages to a general climate change during the Early and Middle Holocene, with particular emphasis on the short-lived changes. Radiocarbon dating and diatom analysis of a sediment core from Lake Amalvas in SW Lithuania revealed that lacustrine sedimentation began before 10100 cal BP. The lake was shallow, oligotrophic−mesotrophic and enriched in calcium-bicarbonates until 9000 cal BP. A low trophic state and unstable environment, indicated by the variation in the B/P ratio and the gradual replacement of the planktonic taxa of Lindavia radiosa, L. ocellata by Aulacoseira ambigua and A. granulata, were established at the end of the Early Holocene (9000−8450 cal BP). The increasing abundance of planktonic Aulacoseira diatoms and the decreasing B/P ratio indicate a gradual increase in the trophic state and in the depth of the lake at the Early−Middle Holocene boundary (8450−7300 cal BP). Two periods of lower lake level, recognizable from an increased content of benthic taxa and a decreased overall number of taxa, may be associated with climate deterioration during the cold climate events known as the 9.3 ka and 8.2 ka events. However, the 9.3 ka cold event did not leave evidence of any significant changes in the diatom assemblage and can only be identified by the age scale that roughly corresponds to this event. The lack of a significant shift in the NMDS ordination around the 9.3 ka event supports the idea that there were no substantial climatic or environmental changes at that time. The 8.2 ka event is reflected by changes in the diatom assemblage around 8450−8300 cal BP, in particular a significant increase in the content of epiphytic Staurosira construens. Diversity metrics reveal significant shifts in community structure and diversity during the period, characterised by a declining species number, reduced evenness, and greater degrees of dominance. Samples corresponding to the 8.2 ka event shift markedly along the NMDS ordination gradient, suggesting that temperature, or a closely related climate variable, was likely the primary driver of the compositional change reflected in the ordination gradient.