| 1. |
- Lundin, Erik, 1982-
(författare)
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The role of inland waters in the carbon cycle at high latitudes
- 2014
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Understanding the drivers of climate change requires knowledge about the global carbon (C) cycle. Although inland waters play an important role in the C cycle by emitting and burying C, streams and lakes are in general overlooked in bottom-up approached C budgets. In this thesis I estimated emissions of carbon dioxide (CO2) and methane (CH4) from all lakes and streams in a 15 km2 subarctic catchment in northern Sweden, and put it in relation to the total catchment C exchange. I show that high-latitude aquatic systems in general and streams in particular are hotspots for C emission to the atmosphere. Annually, the aquatic systems surveyed in this study emitted about 10.8 ± 4.9 g C m-2 yr-1 (ca. 98 % as CO2) which is more than double the amount of the C laterally exported from the catchment. Although the streams only covered about 4% of the total aquatic area they emitted ca. 95% of the total aquatic C emission. For lake emissions, the ice break-ups were the most important annual events, counting for ca. 45% of the emissions. Overall, streams dominated the aquatic CO2 emission in the catchment while lakes dominated CH4 emission, 96 % and 62 % of the totals, respectively. When summing terrestrial and aquatic C fluxes together it showed that the aquatic emissions alone account for approximately two thirds of the total annual catchment C loss. The consequence of not including inland waters in bottom-up derived C budgets is therefore a risk of overestimating the sink capacity of the subarctic landscape. However, aquatic systems can also act as C sinks, by accumulating C in sediment and thereby storing C over geological time frames. Sediment C burial rates were estimated in six lakes from a chronology based on 210Pb dating of multiple sediment cores. The burial rate ranged between 5 - 25 g C m-2 yr-1, which is of the same magnitude as lake C emissions. I show that the emission:burial ratio is about ten times higher in boreal compared to in subarctic-arctic lakes. These results indicate that the balance between lakes C emission and burial is both directly and indirectly dependent on climate. This process will likely result in a future increase of C emissions from high-latitude lakes, while the C burial capacity of these same lakes sediments weaken.
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| 2. |
- Smedberg, Erik, 1957-
(författare)
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Linking landscape variables, hydrology and weathering regime in Taiga and Tundra ecoregions of Northern Sweden
- 2008
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- High-latitude watersheds have been regarded as a carbon sink with soil carbon accumulating at low temperature. This sink is now believed to turn into a source, acting as positive feedback to climate warming. However, thawing permafrost soils would allow more water to percolate down to deeper soil layers where some of the carbon could be “consumed” in weathering and exported as bicarbonate to the sea. Using a hydrological mixing model showed that this could counterbalance the predicted positive feedback resulting from thawing soils.Vegetation-covered riparian zones in headwater areas appear to have a significant role for the dissolved constituent fluxes. Higher concentrations of weathering products are found in taiga and tundra rivers with larger areas of forest and peat cover in the watershed. These landscape elements can thus be regarded as “hot spots” of river loading with dissolved constituents.Comparing a regulated and an unregulated river tested the hypothesis that damming leads to a depletion of major elements also in oligotrophic river systems as a consequence of changes in landscape elements. A loss of upper soils and vegetation through inundation prevents the contact of surface waters with vegetated soil, and consequently reduces weathering fluxes. The hypothesis that the lower fluxes of dissolved silica (DSi) in the regulated river could also be explained by biological uptake was then tested using a model, and budget calculations indicate a significant reduction as a result of regulation. About 10% of this reduction can be attributed to the flooding of the fluvial corridor and the rest to diatom blooms in the reservoirs. A more detailed study of landscape elements for the headwaters of the river Luleälven showed that only 3% of the surface area has been inundated by reservoirs but ca. 37% of the deciduous forest. Such a significant loss of hot spots may indeed explain the observed lower DSi fluxes in the regulated watersheds of northern Sweden.
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| 3. |
- Wallin, Marcus
(författare)
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Evasion of CO₂ from streams : quantifying a carbon component of the aquatic conduit in the boreal landscape
- 2011
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Lateral export of carbon (C) from soils to running waters is a persistent pathway for C with terrestrial origin. This "aquatic conduit" might be especially important in boreal regions where a significant part of the global C stock is stored in the soil. Even though the awareness of the fate of terrestrially derived C is increasing in regional and global C budgets, the scarcity of data on the contribution of streams is widely acknowledged. In particular, the evasion (degassing) of gaseous C (i.e. CO₂ and CH₄) from the water surface of streams requires better characterization. This thesis aims to quantify the evasion of CO₂ from boreal streams within the 67 km2 Krycklan catchment, and explore the factors controlling this diffuse flux. All streams in the Krycklan catchment were consistently supersaturated in CO₂ and were hence a source for atmospheric CO₂ all year around. The source for this supersaturation of CO₂ was to a great extent explained by the export of respired C from the catchment soils. This was shown by exploring the export of dissolved inorganic carbon (DIC) across the soil/stream/atmosphere interfaces in a headwater catchment. The study also found that CO₂ evasion from the stream surface is a rapid process, and that much of the DIC leaving the soils is returned to the atmosphere as CO2 before leaving the headwaters. Evasion of CO₂ is dependent on the water-atmosphere concentration gradient, but also the gas exchange ability across the water-atmosphere interface (the gas transfer coefficient). The spatiotemporal variability of the gas transfer coefficient for carbon dioxide (KCO2) was found to be large, but the slope of the stream can be used to predict the spatial component of this variability. The positive relationship between KCO2 and stream section steepness was used to determine the spatial distribution of gas exchange ability for the entire stream network of forested Sweden. By combining concentration measurements and field-determined relationships with a high resolution digital elevation model (DEM) we were able to model the CO₂ evasion for each grid-cell of stream in the Krycklan catchment. Evasion of CO₂ from the entire stream network constituted a major component (<69 %) of the entire aquatic C flux. This study highlights the importance of including CO₂ evasion from streams in estimates of the aquatic conduit for carbon in boreal regions.
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