Seasonal and spatial variation in hydrochemical parameters of Lake Onego (Russia): insights from 2016 field monitoring

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=7μgL−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 О2 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.

Dissolved Organic Matter and Associated Trace Metal Dynamics from River to Lake, Under Ice-Covered and Ice-Free Conditions

Hannah Elisa Chmiel, Natacha Tofield-Pasche

The present study investigates the changes in dissolved organic matter (DOM) composition and its influences on trace metal dispersion from the Shuya River (SR) in the Petrozavodsk Bay of Lake Onega during ice covered and ice-free periods. Humic substances (HS) found in the SR dominated the composition of DOM through the river-bay-lake continuum in both periods. When the bay was ice-covered, both the aromaticity and the size of HS varied in the water column according to a horizontal stratification and decreased in the bay, while under ice-free conditions, they decreased along the river-lake gradient, suggesting in both cases a decrease in the proportion of HS with high aromatic character. These findings were associated with an overall decrease in the proportion of HS components that have the highest molecular masses. The quantification of metal bound to HS revealed that these characteristics were associated with a decrease in the binding capacity of the HS for Fe and Al but not Cu while dispersing in the bay to the lake. Pb was found to bind on HS, but its behavior in the bay could not be related to the HS dispersion nor to the changes in HS properties.

2019

Lateral variations and vertical structure of the microbial methane cycle in the sediment of Lake Onego (Russia)

Nathalie Dubois, Natacha Tofield-Pasche

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 ice-covered 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.

2019

Implications of river intrusion and convective mixing on the spatial and temporal variability of under-ice CO2

Damien Bouffard, Natacha Tofield-Pasche

Ice-covered periods might significantly contribute to lake emissions at ice-melt, yet a comprehensive understanding of under-ice carbon dioxide (CO2) dynamics is still lacking. This study investigated the processes driving spatiotemporal patterns of under-ice CO2 in large Lake Onego. In March 2015 and 2016, under-ice CO2, dissolved inorganic carbon (DIC), and dissolved organic carbon (DOC) distributions were measured along a river to an open-lake transect. CO2 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 CO2. 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 CO2 concentrations in the bay. By contrast, intensive under-ice convection led to lower, more homogeneously distributed CO2 in 2015. In conclusion, the river-to-bay transition zone is characterized by strong CO2 variability and is therefore an important zone to consider when assessing the CO2 budget of large lakes.

2019