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.

No description available.

Methane (CH4) and carbon dioxide (CO2) are two greenhouse gases and main drivers of global climate change. Lakes are known to be a source of CH4 and CO2 to the atmosphere. While the importance of these emissions is clear, their magnitudes and regulation are still uncertain due to the scarcity of flux measurement data from lakes. Most previous flux measurements have been carried out on lakes <10 km2 and the extrapolations are not representative of large lakes directly. Recent research has led to a growing recognition of the great importance of lakes as a source of emissions. Still, the relationship between environmental variables, lake properties and seasonal changes and the variability between and within lakes raises several question marks. Larger scale studies of greenhouse gases are needed to determine the spatial and temporal dynamics that exist. In this study, a floating chamber method and manual sampling was used to investigate the spatiotemporal variability and influencing variables of CH4 flux and concentration, as well as dissolved inorganic carbon (DIC) and pCO2aq (partial pressure of CO2 in the water). The sampling was conducted during five weeks in September and October 2020 in three large Swedish lakes. Our results generally showed varying CH4 values between the three lakes, indicating that nutrients affect the amount and emission of CH4. A pattern was found where the CH4 was higher near the shore and at a shallower depth. There was a correlation between CH4 concentration and the environmental variables wind speed and air- and water temperatures. Our DIC values were high in two of the lakes and low in one, all lakes’ DIC differed significantly from each other. The pCO2 did not have any difference within the lakes, and there was no difference between the lakes except in one case. Both DIC and pCO2 correlated with air- and water temperature. This study displays the large spatiotemporal variability within and between large lakes and that representative values for large lakes require more measurements under different conditions to distinguish how greenhouse gases emit and flux between lakes and atmosphere. / Metan (CH4) och koldioxid (CO2) är två växthusgaser och stora drivkrafter för globala klimatförändringar. Sjöar är kända för att vara en källa för CH4 och CO2 till atmosfären. Trots att betydelsen av dessa utsläpp är tydlig är deras storlek och reglering fortfarande osäker på grund av brist på flödesmätdata från sjöar. De flesta tidigare flödesmätningarna har utförts på sjöar <10 km2 och det har påvisats att extrapoleringar inte är direkt representativa för stora sjöar. Ny forskning har lett till ett mer allmänt erkännande av sjöars stora betydelse som källa till utsläpp. Trots detta väcker förhållandet mellan miljövariabler, sjöegenskaper, säsongsförändringar och variationen mellan och inom sjöar flera frågetecken. Storskaliga studier om växthusgaser behövs för att bestämma den rumsliga och tidsmässiga dynamiken som finns. I denna studie användes en kammarmetod och manuell provtagning för att undersöka spatiotemporal variabilitet och miljövariabler som kan påverka CH4 flöde och koncentration, samt upplöst oorganiskt kol (DIC) och pCO2aq (partial trycket av CO2 i vattnet). Provtagningen genomfördes under fem veckor i september och oktober 2020 i tre stora svenska sjöar. Våra resultat visade generellt varierande CH4 värden mellan de tre sjöarna, vilket indikerade att näringsämnen påverkar mängd och utsläpp av CH4. Ett mönster noterades där CH4 var högre nära stranden och på ett grundare djup. Det fanns ett samband mellan CH4 koncentration och miljövariablerna vindhastighet och luft- och vattentemperatur. DIC-värdena var höga i två av sjöarna och låga i en, alla sjöarnas DIC skiljde sig signifikant från varandra. pCO2 hade ingen skillnad inom sjöarna, och det fanns ingen skillnad mellan sjöarna utom i ett fall. Både DIC och pCO2 korrelerade med luft- och vattentemperatur. Studien visar den stora spatiotemporala variationen inom och mellan stora sjöar och att representativa värden för stora sjöar kräver fler mätningar under olika förhållanden för att urskilja hur växthusgaser emitterar och flödar mellan sjöar och atmosfär.

Over the past decades, evidence show that the anthropogenetic greenhouse gases (GHG) emissions of carbon dioxide (CO₂) and methane (CH₄) are the main drivers behind global warming and are becoming stronger. Globally, hydropower is among the main sources of renewable energy and the popular notion that hydropower electricity is carbon neutral has been under debate as evidence from measurements in different regions of the globe show significant and highly variable carbon emissions from hydropower reservoirs. But these global estimates are still highly uncertain since they are restricted to a few locations in the south of Europe, North America, and South America, and lack both the temporal and spatial variability in addition to some of the flux pathways (often downstream emission and degassing). This study assesses the CH4 and CO₂ emissions from reservoirs associated to three small hydropower stations in the south of Sweden and aims to understand potential spatial and temporal variability in the temperate region. The study performed flux measurements of CH4 and CO₂, an analysis of CH4 and DIC concentration in the water, and a depth profile of temperature, DO, CH4 and DIC at the hydropower station’s reservoirs. In summation this study finds significant CH4 and DIC concentrations, as well as CH4 and CO₂emissions from the studied reservoirs. The findings of this study underline the notion that hydropower might be a `blind spot` in the Swedish GHG budget report, and if so, the carbon emissions from hydropower electricity need to be re-evaluated.