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Ice-covered periods might significantly contribute to lake emissions at ice-melt, yet a comprehensive understanding of under-ice carbon dioxide (CO 2 ) dynamics is still lacking. This study investigated the processes driving spatiotemporal patterns of under-ice CO 2 in large Lake Onego. In March 2015 and 2016, under-ice CO 2 , dissolved inorganic carbon (DIC), and dissolved organic carbon (DOC) distributions were measured along a river to an open-lake transect. CO 2 decreased from 120/129 μmol L −1 in the river to 51/98 μmol L −1 in the bay, and 34/36 μmol L −1 in the open lake, while DOC decreased from 1.18/1.55 mmol L −1 in the river to 0.67/1.04 mmol L −1 in the bay in 2015 and 2016, respectively. These decreases in concentrations with increasing distance from the river mouth indicate that river discharge modulates spatial patterns of underice CO 2 . The variability between the 2 years was mainly driven by river discharge and ice transparency affecting the extent of under-ice convection. Higher discharge during winter 2016 resulted in higher CO 2 concentrations in the bay. By contrast, intensive under-ice convection led to lower, more homogeneously distributed CO 2 in 2015. In conclusion, the river-to-bay transition zone is characterized by strong CO 2 variability and is therefore an important zone to consider when assessing the CO 2 budget of large lakes.

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