Features of sulfate reduction and accumulation of reduced sulfur compounds in freshwater shallow lake Nazarovskoe (Arkhangelsk region)

Investigations were carried out to study the process of sulfate reduction in freshwater lake Nazarovskoye (Konoshsky district, Arkhangelsk region). Field work was carried out in the winter (March) and summer (July) seasons from 2013 to 2017. Biogeochemical parameters (content of sulfates, iron, organic matter, sulfur forms) and the speed of the bacterial sulfate reduction process in the selected samples were determined using conventional or approved methods. The water in the lake was characterized as fresh, bicarbonate class of calcium group. In the water column, the maximum amounts of sulfates 107 mg/L and hydrogen sulfide 1355 μg / L were noted in the winter. In the bottom sediments of the lake, more intensive sulfate reduction occurred in the summer period (maximum process speed was 555 μg S/dm3 per day). In contrast to the majority of previously studied freshwater lakes in Arkhangelsk region, this reservoir had the highest concentrations of sulfates and hydrogen sulfide in water. At the same time, the lowest speed of the bacterial process of sulfate reduction both in water and bottom sediments was recorded. However, among the derivatives of hydrogen sulfide in bottom sediments, pyrite sulfur dominated among sulfide compounds in Lake Nazarovskoe, which may indirectly indicate a fairly active process of sulfate reduction in the sediments of this shallow reservoir. It can be assumed that despite the high speed of the process, in the summer the sulfide compounds formed in the upper layers of the sediments can be oxidized due to the penetration of oxygen to the bottom of the reservoir via the small depth of the lake; and in the winter compounds of reduced sulfur are accumulated in the bottom sediments (anaerobic conditions are noted even in the water column of the reservoir).