| Summary: | Mercury is a toxic metal that in excessive amounts negatively affects the human nervous system. It can biomagnify to enrich successive levels of the trophic web with higher and higher concentrations of Hg. Diatoms are cosmopolitan microalgae that represent the base of the trophic web. The objective of this study was to determine the importance of diatoms in the accumulation of Hg in surface sediments in the example of Puck Lagoon (southern Baltic Sea). The study verified the relationship of mercury forms with diatoms’ characteristic salinity groups and individual diatom species. The identified diatom flora permitted the distinguishment of three regions with different salinity preferences. The obtained results suggest that, together with organic matter and fine sediment fraction, diatoms are an important element of the marine environment in accumulating Hg. Area I, dominated by euhalobous and mesohalobous diatom species, was characterized by a high statistically significant correlation of mesohalobous with HgSO<sub>4</sub>, Hg in complexes with organic matter and HgS. In Area II, associated with the highest Hg concentrations (ca. 100 ng g<sup>−1</sup>), euhalobous showed a statistically significant linear correlation with HgSO<sub>4</sub>. In contrast, in Area III, the freshwater diatom community was associated with stable Hg forms. The identified species showed a connection with labile forms of Hg that can undergo biomagnification.
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