<p>The results of the study the distribution of sulfur and chlorine compounds in water and sediments of Mogilnoe Lake (Kildin Island, Barents Sea, Russian Federation) are presented. Mogilnoe Lake is a marine meromictic lake located in the high latitudes of the Russian Arctic. A unique feature of this lake is its subterranean connection with the sea, which makes it an analogue of tropical anchialine lakes. Research findings in 2019 confirmed the development of negative trends in the lake ecosystem, which have been recorded since the early twentieth century. It has been shown that over the past thirty years, hydrogen sulfide contamination of the monimolimnion has intensified: the concentrations of hydrogen sulfide have increased (up to 210&#xa0;mg L^–1), and the boundary of its distribution has risen to half the depth of the lake. The high level of accumulation of reduced sulfur compounds in lake sediments is comparable to data for surface sediments of marine reservoirs with permanent hydrogen sulfide contamination (e.g., the Black Sea). It has been established that the identified composition and concentrations of organochlorine compounds (primarily those from the list of persistent organic pollutants) in the sediments of Mogilnoe Lake are due to both inputs from various local anthropogenic sources and their atmospheric transport from low/mid-latitudes and nearby regions.</p>