[M-IS03] Wetland ecosystems under pressure, impacts on the environment and social activities

[MIS03-06] Spatial-temporal variations of dissolved organic carbon along a boreal water continuum (Western Siberia, Russia)

★Invited Papers

*Tatiana Raudina¹, Georgyi Istigechev¹, Danil Kurashev¹, Yulia Kharanzhevskaya^2,1 (1.National Research Tomsk State University, Tomsk, Russia, 2.Siberian Federal Scientific Centre of Agro-BioTechnologies of the RAS)

Keywords:dissolved organic matter, Western Siberia, wetlands , bog and river waters

Waters of the boreal landscapes of Western Siberia are distinguished by a high content of dissolved organic matter (DOM) quantitatively and qualitatively changing in space and time. This is due to the extensive distribution of wetlands in this environmentally important and highly vulnerable region (up to 70–80%), which are an important runoff-forming landscape and determine the hydrochemical parameters of rivers and lakes. During lateral transport along the aquatic continuum, i.e., from feeding sources, through headwater streams to large terminal water bodies, the modification or removal of DOM occurs due to biotic and abiotic processes. The change in the DOM composition during its migration is an important process that ensures the existence of biological cycles of elements in nature and the stability of aquatic ecosystems. This study aimed to seasonally quantify the magnitude of DOM, as well as its degradation (bio- and photo-) along the water continuum located in the southern taiga subzone of Western Siberia. In this work, we focused on a small wetland watershed (56°57′N; 82°30′E) located within the Great Vasyugan Mire. The surface and bog waters were collected during three field campaigns (May, August, and October) along the aquatic continuum which is represented by ridge-pool complexes (open fen), tall ryam (pine–shrub–sphagnum phytocenosis with high pine), waterlogged mixed forest, Klyuch stream, and Bakchar River. To assess the bio- and photodegradation of DOM, experiments were conducted in which water was incubated and exposed to direct sunlight, respectively. At each sampling point, the pH, water temperature, specific conductivity (Cond, μS cm⁻¹) and dissolved oxygen were measured in the field using a multiparameter instrument (MULTI 3430 SET, WTW, Germany). The dissolved organic carbon (DOC) content and absorbance (250 to 800 nm) were determined using a Shimadzu TOC-LCPN analyzer and Varicen, Cary 50 Scan, respectively. The pH values ranged from 3.7 (open fen) to 8.5 (Bakchar River), with the highest values in May. In river waters during the summer-autumn low water period, an increase in specific electrical conductivity was observed (from 334 to 580 μS cm⁻¹), which is due to the low water level and slow water exchange. On the contrary, in the spring, with an increase in water level, temperature, and oxygen saturation, a decrease in specific conductivity and pH occurred. The minimum DOC values found in the open fen and the Bakchar River (41.4–56.6 and 23.2–52.6 mg/l, respectively), and the highest are for the bog waters in forest and high ryam (> 100 mg/l). The DOC content in May was the lowest, which could be caused by dilution with melt water. SUVA₂₅₄ values ranged from 2.5 to 5.6, reaching a minimum in the bog waters of the fen, indicating the predominance of hydrophilic OM with the lowest degree of aromaticity. The biodegradable DOC fraction (BDOC) over the exposure incubation period ranged from 2% to 25%. The seasonal variability of bioavailable DOC concentrations with incubation time showed a significant increase in values in August and a decrease in October. The BDOC percentage increased to the main watercourses, indicating more active microbial metabolism in river waters during the study period. In addition, the photochemical pretreatment of stream DOM that occurs during export into rivers may impact the ability of microorganisms to mineralize DOM. The maximum photodegradation of DOC occurred in June (19–30%), which is consistent with a higher amount of total radiation. In general, there was a decrease in the percentage of DOC loss in the order stream > bog > ryam > forest, associated with the initial qualitative composition of DOC. This study highlights the importance of accounting for the large spatial heterogeneity of the aquatic environment which is necessary for modeling the carbon cycle in wetlands.
The reported study was supported by the Russian Science Foundation (project № 24-77-10049).