The aim of this study was to investigate the importance of winter season for the production of carbon dioxide (CO2) and methane (CH4) in humic and clear-water boreal aquatic systems. The study was conducted in 16 experimental ponds in northern Sweden during the winter of 2013. Half of the ponds had a higher concentration of dissolved organic carbon (DOC). CO2, CH4, DOC and dissolved inorganic carbon (DIC) were measured repeatedly under the ice from January to April. The results show that CO2 was accumulated continually during winter. No difference in winter accumulation were found between humic and clear ponds. CH4 was rarely accumulated in neither humic nor clear ponds, and was not an important part of the gas flux at spring ice melt. At ice melt, the flux from humic ponds accounted for 1.6 g C m-2 and 1.7 g C m-2 from clear ponds, which was equivalent for 15.6% respective 100% of the annual gas emissions. On a whole-year basis humic ponds acted as a source of 10.3 g C m-2, while clear ponds acted as a sink of 14.7 g C m-2. 76 mg m-2 d-1 DOC was consumed in humic and 59 mg m-2 d-1 DOC in clear ponds while the DIC accumulation was 125 mg m-2 d-1 in humic and 118 mg m-2 d-1 in clear ponds. This study stresses the importance of ice-covered boreal aquatic systems as a significant parts of the global carbon cycling.

Dinitrous oxide (N2O) is a potent greenhouse gas some 354 times stronger than carbon dioxide (CO2) in the atmosphere. Recent studies show that lake denitrification contributes to a considerable part of the global N2O emissions. Despite this, lake-N2O emissions are not being accounted for in global greenhouse gas modeling because it has not yet been accurately understood and quantified. The aim of this study was to assess how denitrification varies between and within boreal lakes and how it is controlled by nitrate- (NO3) and carbon (C) availability and temperature. Studies on denitrification were performed using the acetylene inhibition technique on sediments from three lakes in northern Sweden (February to August, 2014). Results showed that denitrification was correlated (linear regression, r2=0.71) with NO3 concentrations in the hypolimnion water at ambient conditions and that additions of NO3 up to a concentration of 50 µg NO3-N L-1 increased denitrification. Temperature increased denitrification in all lakes, at all sites except in one lake in July, when nutrient concentrations were at its lowest. The spatial and temporal variation in denitrification was small at ambient conditions (1-3 µmol N2O m-2 h-1)but the variation in the response to nutrient additions and temperature increase was very high. This was in part attributed to differences in dissolved organic C (DOC). These findings have important implications for future denitrification research and how lake-N2O production is included in greenhouse gas modeling and contributes to our knowledge on how northern boreal lakes may respond to enhanced nutrient loadings and global warming.

Inland waters receive substantial amounts of organic carbon from adjacent watersheds. Only about half of the carbon exported from inland waters reaches the oceans, while the remainder is lost en route. This thesis identifies flocculation as an important and significant fate of carbon in the boreal landscape. Flocculation reallocates organic carbon from the dissolved state into particles which are prone to settle. Thus, flocculation relocates organic carbon from the water column to the sediment. The dissolved organic carbon (DOC), mainly originating from terrestrial sources, in a set of Swedish lakes was found to determine the extent of sedimentation of particulate organic carbon. A major fraction of the settling particles were of allochthonous origin. This implies that allochthonous DOC was the precursor of the settling matter in these lakes. The gross sedimentation was of the same magnitude as the evasion of carbon dioxide to the atmosphere. Sunlight, especially in the photosynthetically active region, stimulated flocculation of DOC. The effect of light appeared to involve a direct photochemical reaction. Iron was involved in the flocculation but it could not be unravelled whether the iron catalyzes the flocculation or just co-precipitates with the settling matter. Microbial activity was identified as the main regulator of the flocculation rates. Accordingly, alteration of temperature, oxygen concentration and pH did not affect flocculation only indirectly, via their effects on microbial metabolism. A comparison of fluorescence characteristics of organic matter collected in sediment trap and in the sediment surface layer revealed that autochthonous organic carbon was preferentially lost in the sediments while allochthonous matter increased. The recalcitrant nature of the flocculated matter could favour sequestration of this matter in the lake sediment. Hence, the lakes will act as sinks of organic carbon due to a slower mineralization of the flocculated matter in the sediments.

The global carbon cycle is intimately linked with the earth’s climate system. Knowledge about carbon cycling in the biosphere is therefore crucial for predictions of climate change. This thesis investigates the carbon dioxide balance of Swedish boreal lakes, its regulation, significance to the carbon budget of the boreal landscape, and sensitivity to climate change. Swedish boreal lakes were almost exclusively supersaturated in CO2 with respect to the atmosphere, resulting in an emission of CO2 from lakes to the atmosphere. Lake pCO2 was closely related to the concentration of terrigenous dissolved organic carbon (DOC), indicating that the utilization of terrigenous DOC by lake bacteria is a major source of CO2. This conclusion is supported by independent field studies, showing that net plankton respiration accounts for most of the CO2 emitted from Swedish boreal lakes, while photochemical mineralization and sediment respiration were less important. Mineralization of terrigenous DOC and subsequent emission of CO2 from lakes to the atmosphere was a major carbon loss factor in 21 major Swedish boreal catchments, removing 30-80% of the organic carbon exported from terrestrial soils to surface waters. Lake CO2 emission is in the same order of magnitude as organic carbon accumulation in boreal forest soils, and should therefore be included in the carbon budget of the boreal landscape. In a set of nearly 5000 global lakes, DOC concentration was a much more important regulator of lake pCO2 than temperature. Climate change will therefore affect the carbon balance of lakes primarily via alterations in terrestrial DOC export, rather than via changes in temperature per se. Both current observations and models of future climate suggest an increasing export of terrigenous DOC from many Scandinavian catchments. Hence, there probably is a current trend towards higher CO2 emission from Swedish boreal lakes, which is likely to continue in the future.

Inland waters have long been neglected in the global carbon cycle. They represent only 2,8% of the land area, but it has come clear that inland waters play a key role in the transformation of terrestrial fixed carbon to the atmosphere. Human activities do have an impact on the carbon cycling and it is important to understand how these changes affects natural biogeochemical and climatological processes. The purpose of this report was to investigate how forestry impacts the emission of carbon dioxide from boreal lakes and to evaluate which role lakes play in the global carbon cycle. The study was accomplished as a literature study and the search words that have been used are carbon cycling, carbon dioxide, forestry, boreal lakes, dissolved organic carbon and pCO2. The results show that in many studies does forestry increase the export of dissolved organic carbon from terrestrial environments to boreal lakes. This increase subsidies the net heterotrophy in boreal lakes, making them net sources of carbon dioxide to the atmosphere. The processes behind increased concentrations and emissions are however complex and factors like local topography, hydrology and climate are thought to have impacts on how much carbon dioxide that is produced at a given level of dissolved organic carbon. Forestry seems to have an increasing effect on the carbon dioxide emissions, but the key drivers behind this process are expressed differently between regions and the reasons underlying these differences remain to be explored in order to make precise global carbon models.

The aim of this report was to quantify the accumulation of carbon dioxide (CO2) and methane (CH4) in ice-covered lakes in winter (October-March) during three consecutive years, and to assess whether the concentrations of DIC and CH4, as well as the ratio DIC:CH4 differs between years and between lakes affected by clearcutting and untreated reference lakes. Water- and gas samples were collected from four boreal lakes (two affected by clearcutting and two untreated reference lakes) located in Västerbottens inland in spring. The lakes were found to accumulate on average 91, 55, and 84 mgCm-2d-1 during winters 2012-2013, 2013-2014, and 2014-2015, mostly originating from CO2. The concentrations of DIC (autumn and spring) were higher in the affected clear-cut lakes compared to reference lakes for all years, including the reference year (2012-2013) before clear-cutting. No such difference was found for CH4 or the ratio DIC:CH4. Accumulated concentrations of CO2 and CH4 varied between years while the ratio (accumulated) CO2:CH4 increased over time for all lakes but with no difference between the clear-cut lakes and the reference lakes. The differences between the clear-cut and the reference lakes, hence, are unclear with differences between years likely stemming from natural variations. The effects of clearcutting in catchment areas might be more visible at other times of year not seen in this study which focuses only on winter. The amount of CO2 and CH4 accumulated under ice in lakes was substantial i.e. high-lighting the importance of including winter accumulation of greenhouse-gases in estimations of yearly emissions from lakes.

No description available.

Movement increases the probability for an individual to find food resources, but also increase the metabolic costs and exposure to predators. Hence, swimming behavior of fish is strongly coupled to fitness. Even though swimming activity has been studied in numerous laboratory settings, less is known about in situ activity and its dependence on abiotic factors (temperature, light conditions and barometric pressure). In this study I hypothesized that the activity increases with 1) increasing temperature and decrease with 2) barometric pressure variability and 3) average light conditions (h/day). In order to test the universality of the three hypotheses I also searched for size dependent effects. Fish activity (km/day) was measured in three lakes on individual fish (N=14-21 per lake) using acoustic telemetry providing tracking of fish at a time resolution from seconds to hours. A positive correlation between temperature and swimming activity in line with my first hypothesis was only observed in one of the lakes. The activity decreased with increased variability in barometric pressure in two of the lakes, a finding supporting my second hypothesis. Meanwhile increased light conditions (h/day) decreased activity in one of the lakes, as predicted by my third hypothesis. Nevertheless, none of my hypotheses were valid in all three of the lakes and perch reacted differently to the abiotic factors. One of the possible explanations for this is the importance of size differences as I noticed that the swimming activity differed between bigger and smaller individuals. My findings suggest that not only the temperature, barometric pressure and light conditions alone predict the activity in perch, but also the fish individual size, predation and the metabolic costs linked to thermoregulation.

Inland waters process large amounts of organic carbon, contributing to CO2 and CH4 emissions, as well as storing organic carbon (OC) over geological timescales. Recently, it has been shown that the magnitude of these processes is of global significance. It is therefore important to understand what regulates OC cycling in inland waters and how is that affected by climate change. This thesis investigates the constraints on microbial processing of sediment OC, as a key factor of the carbon cycling in boreal lakes. Sediment bacterial metabolism was primarily controlled by temperature but also regulated by OC quality/origin. Temperature sensitivity of sediment OC mineralization was similar in contrasting lakes and over long-term. Allochthonous OC had a strong constraining effect on sediment bacterial metabolism and biomass, with increasingly allochthonous sediments supporting decreasing bacterial metabolism and biomass. The bacterial biomass followed the same pattern as bacterial activity and was largely regulated by similar factors. The rapid turnover of bacterial biomass as well as the positive correlation between sediment mineralization and bacterial biomass suggest a limited effect of bacterial grazing. Regardless of the OC source, the sediment microbial community was more similar within season than within lakes. A comparison of data from numerous soils as well as sediments on the temperature response of OC mineralization showed higher temperature sensitivity of the sediment mineralization. Furthermore, the low rates of areal OC mineralization in sediments compared to soils suggest that lakes sediments are hotspots of OC sequestration. Increased sediment mineralization due to increase in temperature in epilimnetic sediments can significantly reduce OC burial in boreal lakes. An increase of temperature, as predicted for Northern latitudes, under different climate warming scenarios by the end of the twenty-first century, resulted in 4–27% decrease in lake sediment OC burial for the entire boreal zone.

Une estimation des quantités de carbone organique dissous dans les millions de lacs boréaux est nécessaire pour améliorer notre connaissance du cycle global du carbone. Les teneurs en carbone organique dissous sont corrélées avec les quantités de matière organique dissoute colorée qui est visible depuis l’espace. Cependant, les capteurs actuels offrent une radiométrie et une résolution spatiale qui sont limitées par rapport à la taille et l’opacité des lacs boréaux. Landsat 8, lancé en février 2013, offrira une radiométrie et une résolution spatiale améliorées, et produira une couverture à grande échelle des régions boréales. Les limnologistes ont accumulé des années de campagnes de terrain dans les régions boréales pour lesquelles une image Landsat 8 sera disponible. Pourtant, la possibilité de combiner des données de terrain existantes avec une image satellite récente n'a pas encore été évaluée. En outre, les différentes stratégies envisageables pour sélectionner et combiner des mesures répétées au cours du temps, sur le terrain et depuis le satellite, n'ont pas été évaluées. Cette étude présente les possibilités et les limites d’utiliser des données de terrain existantes avec des images satellites récentes pour développer des modèles de prédiction du carbone organique dissous. Les méthodes se basent sur des données de terrain recueillies au Québec dans 53 lacs boréaux et 10 images satellites acquises par le capteur prototype de Landsat 8. Les délais entre les campagnes de terrain et les images satellites varient de 1 mois à 6 ans. Le modèle de prédiction obtenu se compare favorablement avec un modèle basé sur des campagnes de terrain synchronisées avec les images satellite. L’ajout de mesures répétées sur le terrain, sur le satellite, et les corrections atmosphériques des images, n’améliorent pas la qualité du modèle de prédiction. Deux images d’application montrent des distributions différentes de teneurs en carbone organique dissous et de volumes, mais les quantités de carbone organique dissous par surface de paysage restent de même ordre pour les deux sites. Des travaux additionnels pour intégrer les sédiments dans l’estimation sont nécessaires pour améliorer le bilan du carbone des régions boréales. / A remote sensing approach to estimate carbon stocks in the millions of boreal lakes is highly desirable to improve our understanding of carbon cycles. Lakes carbon content is often correlated to colored dissolved organic matter (CDOM) content, which is visible from space. Meanwhile, current sensors offer limited radiometry and spatial resolution in regard to boreal lakes opacity and size. Landsat 8, launched in February 2013, offers improved radiometry and spatial resolution, and will provide large-scale coverage of boreal regions. Limnologists gathered years of field campaigns in the boreal regions for which a clear Landsat 8 image will be available. Yet the possibility to combine legacy field data with new satellite imagery has not been assessed yet. Furthermore, the different strategies to select and combine timely repeated lakes measurements in the field and on the satellite have not been assessed either. In this study, we address the opportunities and limits to combine legacy field data with new satellite imagery to develop CDOM predictive models. Methods are based on field data from Quebec collected in 53 boreal lakes and 10 satellite images acquired with the prototype of Landsat 8. Delays between field campaigns and satellite overpasses varied from 1 month to 6 years. Results show that a CDOM predictive model based on existing field data compares favorably with models based on carefully coordinated field campaigns. The quality of the model does not improve by adding repeat measurements in the field and on the satellite, or by using atmospherically corrected images. Two images from different sites show different distributions of lakes dissolved organic carbon concentrations and volumes, but the total dissolved organic carbon storage per landscape unit in the two sites are in the same range. Additional work to link satellite data to lakes sediments carbon content is needed to refine the global carbon budget in the boreal regions.