Traits of peat- and upland-derived stream dissolved organic carbon in the permafrost region around Abisko, northern Sweden

Abstract

Climate change-induced thawing of permafrost mobilizes previously frozen carbon of high potential reactivity, thereby, fuelling microbiological production of greenhouse gases. For this work, an analysis of the quantity, quality, and bioreactivity of DOC in streams from the Stordalen discontinuous permafrost catchment was carried out with stream water samples during the relatively warm summer of 2018.

In the study area permafrost is limited to peatland which is present in lowland areas. We hypothesized that (i) peatland-influenced streams have a higher dissolved organic carbon (DOC) concentration than streams in upland areas of the Stordalen catchment. However, no strong differences in the quantity of DOC between peaty lowland and well-drained upland areas were found. In general, DOC quantity was lower compared to levels obtained by previous studies conducted in the area but due to the lack of discharge measurements for this study, a comparison with other studies is difficult. Furthermore, no clear pattern was detected regarding the extent of carbon degradation, represented by a small range of the ratio between the isotopically stable carbon-12 and carbon-13. However, a slightly enhanced degradation in areas located in upland areas was detected.

Additionally, it was predicted that (ii) DOC from upland areas was associated with low molecular weight and relatively high bioreactivity compared to DOC derived from peaty lowland areas. The DOC from all sampled sub-catchments, including the lowland sampling points in the peat, showed surprisingly high bioreactivity, although the upland catchments, where peat was not present, were generally associated with the lowest molecular weights and highest bioreactivity of the DOC.

Upland areas are characterised by a faster turnover of carbon, as oxygen can diffuse into the shallow organic layer. Thus, (iii) the share of labile C was higher in better-drained upland areas than in peatland areas. This work showed how quantifiable amounts of low molecular weight DOC can reach streams from permafrost catchments during a warm and dry summer, due to the increasing thickness of the active layer.

Popular Abstract

The current increase and prediction in air temperature for northern latitudes have a decreasing influence on the permafrost extent. Permafrost is characterized as a frozen ground with a temperature ≤ 0°C for at least two consecutive years. Previously frozen and thus unavailable organic carbon is becoming bioavailable, i.e. microorganisms utilize the freshly thrown carbon and metabolize it into greenhouse gases such as carbon dioxide or methane. Furthermore, dissolved organic carbon (DOC), which is carbon broken down into very small particles, becomes flushed out from the thawing permafrost into the surrounding inland waters. The DOC can be further metabolized by microorganisms and transported further with the water flow into lakes, rivers, and oceans. The properties of DOC depends on its degradation extent. If oxygen is available DOC can easier be degraded compared to oxygen-free environments. To understand the carbon dynamic in a discontinuous permafrost region, DOC properties and quantity were analysed based on its location of sample extraction (well-drained upland vs. peaty lowland areas). The analysis was performed on DOC sampled in streams from the Stordalen discontinuous permafrost catchment in the Abisko region, northern Sweden, during the relatively warm summer of 2018.

In the study area permafrost is limited to peatland which is present in lowland areas. However, no strong differences in the quantity of DOC between peaty lowland and well-drained upland areas were found. In general, DOC quantity was lower compared to levels obtained by previous studies conducted in the area but due to the lack of discharge measurements for this study, a comparison with other studies is difficult. Furthermore, regarding the stable carbon isotope ratio of carbon-12 and carbon-13, a slightly enhanced degradation in areas located in upland areas was detected, which was further supported by smaller molecules, meaning more broken-down carbon. However, the analysis of the molecular structure of the DOC revealed that the discharged DOC from all sampled sub-catchments, including the lowland sampling points in the peat, showed surprisingly low molecular weight DOC. But the upland catchments, where peat was not present, were generally associated with the lowest molecular weights and highest bioreactivity of the DOC. The systematically observed low molecular weight DOC may indicate that significant amounts of low molecular weight DOC could reach streams from permafrost catchments during a warm summer.

• author: Kainz, Nora
• supervisor: Martin Berggren; Andreas Persson
• organization: Dept of Physical Geography and Ecosystem Science
• course: NGEM01 20211
• year: 2021
• type: H2 - Master's Degree (Two Years)
• subject: Earth and Environmental Sciences
• keywords: physical geography, ecosystem analysis, permafrost degradation, DOC, DOC concentration, stable isotope carbon signature, DOC quality, DOC degradation, Atmospheric Sciences and Biogeochemical Cycles
• publication/series: Student thesis series INES
• report number: 555
• language: English
• id: 9062278
• date added to LUP: 2021-08-03 17:56:21
• date last changed: 2021-08-09 20:20:40