Diatoms are regarded as reliable bioindicators for assessing heavy metal contamination in marine ecosystem including mercury. In the present study diatoms were used as bioindicators to reconstruct changes in the concentration of total mercury and its fractions in the Middle and Late-Holocene in the southern Baltic based on a 450 cm long sediment core. The goal of the study was to reconstruct changes in the concentration of Hg _tot and its fractions in relation to biodiversity of the diatom flora, climatic fluctuations occurring inferred from pollen and Pediastrum records and in relative abundance of sediment fractions. Hg _tot concentration increased from 4.6 ng g ⁻¹ (6300 cal. yr BP) to 353 ng g ⁻¹ (2021). Among the analyzed fractions HgS (⩽86.6%) dominated until ca. 4200 cal. yr BP after which a gradual increase in the proportion of Hg _F2 (⩽69.8%), was observed. The current study showed that over the last 6300 cal. yr BP the increase in Hg concentrations in the sediments of the Puck Bay has been influenced by human activities in addition to natural sources (atmospheric input, climatic fluctuations, storm events, diatoms productivity). About 4200–200 cal. yr BP the diatoms Schionodiscus oestrupii and Rhizosolenia hebetata f. hebetata enabled the identification eight time intervals during which climatic fluctuations influenced changes in sedimentary Hg _tot concentrations. Over the last ca. 200 cal. yr BP the highest Hg concentrations were associated with intensive human activity as evidenced by the anthropogenic diatom flora present at that time ( Cyclotella choctawhatcheeana , Thalassiosira levanderi ).

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