Reduced Sulfur Compounds in Bottom Sediments of Lakes at Different Stages of Separation from the Kandalaksha Bay of the White Sea

The paper presents the results of a study of bottom sediments of four lakes at different stages of separation from Kandalaksha Bay of the White Sea (Zelenyi Cape Lagoon, Kislo-Sladkoe Lake, Nizhnee Ershovskoe and Verkhnee Ershovskoe Lakes). The geochemical parameters of bacterial sulfate reduction are considered: the formation and accumulation of different forms of reduced sulfur with respect to the content of organic matter and reactive iron in sediments. The main sources of organic matter in sediments are identified: in the both freshwater Ershovskoe Lakes, allochthonous organic matter prevails, while in marine waterbodies (Kislo-Sladkoe Lake and Zelenyi Cape Lagoon), more labile autochthonous organic matter predominates. In the bottom sediments of lakes connected to the sea (Kislo-Sladkoe Lake and Zelenyi Cape Lagoon), the pyrite sulfur dominates in the composition of the total bacterial hydrogen sulfide derivatives ( $$\sum {{{{\text{S}}}_{{{{{\text{H}}}_{2}}{\text{S}}}}}} $$ ). In the both freshwater Ershovskoe Lakes, which are completely separated from the sea, the predominant form is organic sulfur.