Diatom taphocoenoses as a potential sink of mercury in the brackish water basin

Diatoms are microalgae that are employed as bioindicators for metal pollution. Their ability to accumulate metals, in addition to their role within the food web as primary producers, have been proposed to contribute to the bioaccumulation and biomagnification of pollutants such as mercury. The present study employs statistical analysis to identify diatom species that may exert a significant influence on the mercury cycle within the brackish, eutrophicated Outer Puck Bay. Concentrations of total mercury and its five fractions are analyzed using the thermodesorption method in surface sediments from the study area. The principal component analysis indicates that among the 175 diatom species identified in the sediments, planktic Coscinodiscus asteromphalus, Cyclotella choctawhatcheeana, Thalassiosira levanderi, and benthic Stephanocyclus meneghinianus can be associated with elevated sediment Hg concentrations. Variability in their relative abundance is proposed to be responsible for over 60 % of the variation in sediment Hg concentration. High concentrations of labile mercury fractions that can potentially biomagnify correlate with a high content of diatom valves preserved in the surface sediments. Principal component analysis and cluster analysis indicate that the planktic C. asteromphalus and T. levanderi are associated with elevated concentrations of organically bound Hg. Planktic Cyclostephanos dubius, on the other hand, is associated with elevated concentrations of loosely bound Hg. This work suggests that planktic diatoms enhance the flux of labile Hg from the water column to sediments. If the dominance of diatom flora in the brackish environment persists, the role of these microalgae in the Hg cycle will increase significantly.