Freshwater Carbon Across Arctic Lowland Tundra Ecosystems

Arctic lowland tundra landscapes are low-relief regions located in the high latitudes of the Northern Hemisphere and are underlain by permafrost, a defining feature that regulates their hydrological, ecological, and biogeochemical processes. Within these landscapes, freshwater ecosystems, including lakes, ponds, streams, and seasonally flooded areas, are widespread and form important connections between terrestrial environments, downstream aquatic systems, and the atmosphere. These systems receive carbon from surrounding soils and vegetation while also generating and transforming carbon through aquatic primary production and biogeochemical processes, resulting in strong spatial heterogeneity in carbon concentrations and composition. Permafrost underlies approximately 14 % of the Northern Hemisphere land surface and stores an estimated 1056.8 ± 167.2 Pg of carbon in the upper three meters, while freshwater ecosystems can cover up to 40 % of Arctic lowland landscapes. Despite their extensive coverage and connectivity, freshwater ecosystems remain underrepresented in Arctic carbon assessments, particularly with respect to their variability across ecosystem types and spatial scales. This thesis investigates freshwater carbon across Arctic lowland tundra ecosystems using field observations from two regions: the northeast Siberian Arctic tundra and the Hudson Bay Lowlands (Canada). Measurements were conducted during the ice-free season and focus on dissolved organic carbon (DOC), dissolved inorganic carbon (DIC), dissolved organic matter (DOM) characteristics, and greenhouse gas (CO2, CH4, N2O) concentrations. The thesis addresses three main questions: (1) how freshwater greenhouse gas emissions compare to the terrestrial carbon sink, (2) how DOM composition relates to greenhouse gas concentrations, and (3) how lateral carbon transport varies along hydrological connections. Overall, this thesis evaluates how carbon in freshwater ecosystems varies among ecosystem types and along hydrological gradients in Arctic lowland landscapes. In the Indigirka River tundra lowlands of northeastern Siberia, a landscape characterized by a mosaic of peat-forming polygonal wetlands, Yedoma uplands, and river floodplains, all freshwater systems were supersaturated in CO2 (27–1333 µmol L−1) and CH4 (0.04–251 µmol L−1), with ponds consistently showing the highest concentrations. Upscaled inland water emissions to an area of approximately 18 km2 (7.03 ± 1.30 to 9.63 ± 1.24 Mg C d-1) offset approximately 9 % to 13 % of the terrestrial carbon sink. Relationships between DOM characteristics and dissolved greenhouse gas concentrations differed among Arctic freshwater ecosystem types. In a small coastal catchment in the Hudson Bay Lowlands, dissolved carbon transport varied along the terrestrial–freshwater continuum, with headwater peatland and coastal sandy heathland landscapes exhibiting contrasting relationships between soil pore water and surface water carbon. In surface waters, DOC concentrations remained within a relatively narrow range across the catchment, whereas DIC concentrations increased downstream, accompanied by changes in δ13C-DIC and DOM characteristics. Differences between soil pore and surface waters were greater in the headwater peatland than in the coastal sandy heathland, highlighting the influence of landscape characteristics and hydrological connectivity on terrestrial–freshwater carbon transport. Late-summer rainfall further altered dissolved carbon transport by increasing DOC concentrations and aromaticity while reducing DIC concentrations in surface waters, demonstrating the sensitivity of carbon export to episodic hydrological events. Overall, this thesis demonstrates that freshwater carbon cycling in Arctic lowland tundra ecosystems is shaped by interactions among hydrological connectivity, carbon composition, and ecosystem-specific processes. Freshwater emissions, DOM–greenhouse gas relationships, and lateral carbon transport varied across freshwater ecosystems highlighting the importance of landscape and ecosystem characteristics in regulating carbon transfer and transformation across the terrestrial–freshwater continuum. Episodic hydrological events, including flooding and rainfall, further altered carbon fluxes by changing water extent and carbon transport. Together, these findings demonstrate that Arctic freshwater ecosystems play an important role in regulating the transfer, transformation, and loss of carbon across the terrestrial–aquatic– atmospheric continuum.