Transport and Processing of Dissolved Organic Carbon in Peatland-Dominated Catchments in Subarctic Scandinavia

Summary

Northern subarctic peatland landscapes contain some of the largest terrestrial carbon reservoirs on Earth. Over thousands of years, cold climates, waterlogged soils, and slow decomposition have allowed plant material to accumulate and be stored as peat. These carbon stores, however, are not stable under a warming climate. In Arctic and subarctic regions, temperatures are rising faster than the global average, altering hydrology, ecosystem structure, and, where present, the stability of permafrost in many peatland systems. As permafrost thaws, it not only changes the ground surface, but also the pathways through which carbon moves in the landscape. The collapse of ice-rich permafrost in peat can create ponds, wetlands, and new drainage features, forming a dynamic network of aquatic environments. These waters can act both as sites where carbon is stored and transformed, and as conduits that transport carbon further through streams and rivers toward downstream and coastal ecosystems. Inland waters therefore play an important role by linking terrestrial carbon sources to aquatic systems while simultaneously transforming dissolved organic carbon (DOC) through microbial mineralization, photochemical reactions, and gas exchange with the atmosphere. In peatland-dominated landscapes, large quantities of DOC can be mobilized when hydrological flow paths connect organic soils to aquatic networks. Whether this carbon is retained, transformed, or transported down stream depends strongly on how water moves through the landscape and how connected different parts of the system are. This thesis investigates how permafrost degradation, hydrologic connectivity, and landscape composition interact to regulate DOC dynamics in peatland-dominated ecosystems of subarctic Scandinavia. The work combines detailed field investigations in a thawing peatland headwater catchment with regional analyses of DOC patterns across the Tana River basin. By linking observations from small-scale peatland systems to patterns at the river-basin scale, this thesis aims to improve understanding of how carbon moves through northern latitude landscapes. Investigations of the processes governing carbon dynamics were conducted in the Iškoras peatland complex on the Finnmarksvidda plateau in northern Norway, a degrading permafrost palsa mire within the sporadic permafrost zone. As one of the southernmost and most rapidly degrading permafrost peatland systems in Fennoscandia, Iškoras represents a landscape where thaw-driven hydrological and biogeochemical changes are already well advanced. This makes it a natural setting for studying processes that are likely to become more widespread in currently more stable permafrost regions as permafrost continues to thaw. Repeated measurements of water chemistry, dissolved greenhouse gases (GHG), and hydrological conditions were used to examine how carbon is mobilized, transformed, and transported across different parts of the system. The results show that permafrost degradation creates a mosaic of aquatic environments with distinct hydrochemical characteristics. Small ponds formed by thaw contain elevated DOC concentrations and high supersaturation of carbon dioxide (CO2) and methane(CH4), reflecting strong inputs of organic matter from thawing peat and active GHG production. In contrast, downstream stream waters show lower DOC concentrations and a stronger integration of ground water and catchment scale inputs. Hydrological conditions play a key role: when waters are isolated, DOC concentrations can increase, whereas in creased connectivity promotes dilution and downstream trans port. At the regional scale, analyses across 90 catchments of the Tana River basin show that wetland and peatland coverage strongly predicts DOC as well as iron concentrations, highlighting peatlands as major sources of DOC. The findings in this thesis demonstrate that DOC dynamics in subarctic peatland landscapes are governed by interactions between landscape structure, hydrological connectivity, and temperature-sensitive biogeochemical processes associated with permafrost thaw. As northern latitude regions continue to warm, these interactions will be crucial in determining whether carbon stored in peat soils is retained locally, released to the atmosphere, or transported through river networks.