The significance of methane production by lakes to the global production of greenhouse gas is well acknowledged while underlying processes sustaining the lacustrine methane budget remain largely unknown. We coupled biogeochemical data to functional and phylogenetic analyses to understand how sedimentary parameters characterize the methane cycle vertically and horizontally in the icecovered bay of the second largest lake in Europe, Lake Onego, Russia. Our results support a heterogeneous winter methane cycle, with higher production and oxidation closest to riverine inputs. Close to the river mouth, the largest numbers of copies of methane-related functional genes pmoA and mcrA were associated with a specific functional community, and methane production potential exceeded oxidation, resulting in 6-10 times higher methane fluxes than in the rest of the bay. The elevated fluxes arise from the spatial differences in quantity and type (lacustrine versus riverine sources) of organic matter. More homogeneity is found toward the open lake, where the sediment is vertically structured into 3 zones: a shallow zone of methane oxidation; a transitional zone (5-10 cm) where anaerobic methane oxidation is dominant; and a methane production zone below. This vertical pattern is structured by the redox gradient and human-induced changes in sedimentary inputs to the bay. Retrieved 16S rRNA gene sequences from Candidatus Methanoperedens and Cand. Methylomirabilis suggest that anaerobic oxidation of methane occurs in these freshwater lake sediments.

This study describes seasonal water quality parameters measured in Petrozavodsk Bay, Lake Onego, Russia. Petrozavodsk Bay (PB) lies in the north of Lake Onego and adjacent to the city of Petrozavodsk. PB water quality is controlled by anthropogenic input, inflow of the Shuya River, and water exchange with open areas of the greater Lake Onego. We measured ion composition, organic matter, nutrients, gas composition, trace elements, and mercury throughout 2016, to evaluate PB water quality. Elevated humic content and organic matter, including total organic carbon (TOC = 17.0 mg L −1 ), total phosphorus (TP = 36 µg L −1 ), and water color (134 mg Pt L −1 ), demonstrated the eutrophic character of Shuya River input. Low humic and organic matter content (TOC = 6.5 mg L −1 , TP = 7 µg L −1 , water color = 26 mg Pt L −1 ) indicated the oligotrophic character of open lake waters. During winter, the PB hydrochemical regime was primarily controlled by Shuya River inflow because water exchange between the bay and open lake was restricted during its ice-covered period. As a result, PB chemical indices varied considerably. TOC varied from 7.6 to 17.8 mg L −1 , TP from 7 to 55 µg L −1 , and О 2 from 69% to 87% saturation during this period. Total filterable mercury concentrations (THg = inorganic mercury plus methylmercury) at all measurement sites remained low. Overall, these results help constrain understanding of lake dynamics, anthropogenic influence, and river input to the lake during icecovered periods. ARTICLE HISTORY

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