The vulnerability of the Arctic to climate change has been realized due to disproportionately large increases in surface air temperatures which are not uniformly distributed over the seasonal cycle. Effects of this temperature shift are widespread in the Arctic but likely include changes to the hydrological cycle and permafrost thaw, which have implications for the mobilization of organic carbon into rivers. The focus of this research was to describe the seasonal variability of the chemical composition of dissolved organic matter (DOM) in the six largest Arctic rivers (Yukon, Mackenzie, Ob, Yenisei, Lena and Kolyma) using optical properties (UV-Vis Absorbance and Fluorescence) and lignin phenol analysis. We also investigated differences between rivers and how watershed characteristics influence DOM composition. Dissolved organic carbon (DOC) concentrations followed the hydrograph with highest concentrations measured during peak river flow. The chemical composition of peak-flow DOM indicates a dominance of freshly leached material with elevated aromaticity, larger molecular weight, and elevated lignin yields relative to base-flow DOM. During peak flow, soils in the watershed are still frozen and snowmelt water follows a lateral flow path to the river channels. As the soils thaw, surface water penetrates deeper into the soil horizons leading to lower DOC concentrations and likely altered composition of DOM due to sorption and microbial degradation processes. The six rivers studied here shared a similar seasonal pattern and chemical composition. There were, however, large differences between rivers in terms of total carbon discharge reflecting the differences in watershed characteristics such as climate, catchment size, river discharge, soil types, and permafrost distribution. The large rivers (Lena, Yenisei), with a greater proportion of permafrost, exported the greatest amount of carbon. The Kolyma and Mackenzie exported the smallest amount of carbon annually, however, the discharge weighted mean DOC concentration was almost 2-fold higher in the Kolyma, again, indicating the importance of continuous permafrost. The quality and quantity of DOM mobilized into Arctic rivers appears to depend on the relative importance of surface run-off and extent of soil percolation. The relative importance of these is ultimately determined by watershed characteristics.

Arctic regions are expected to be highly sensitive to climate change regarding the prediction of disproportionately large increases in surface temperatures and their related influence over the hydrological cycle and permafrost thaw. These modifications have the potential to impact biogeochemical cycles in these regions and in particular the mobilization of organic carbon into rivers. The most crucial period in arctic rivers hydrological cycle is the spring freshet that can account for most of the annual organic carbon transfers to the ocean in a very short window of time. The focus of this thesis was to monitor the dynamic of DOM in terms of quantity and quality along the hydrological cycle of the Yenisei river, through DOM characterization approaches to reveal seasonal variations in its composition, sources, age and degradation state. The first step was to make a review of the full range of existing DOM characterization approaches in worldwide river systems to identify the most widely used, the most relevant and reliable ones. Through the development of a DOM quality measurements database, we have been able to evaluate the geographical coverage of DOM characterization studies, to give estimates and ranges of values of the main reported DOM characterization variables and to observe global trends of DOM quality across latitudinal gradient. Second stage was to investigate DOC dynamic in the Yenisei river with regard to quantity and links with water chemistry and hydrology. We conducted sampling campaigns during three consecutive years (2014 to 2016), covering with a high sampling frequency the spring flood period to capture its very dynamic evolution. We reported DOC concentrations that followed the hydrograph with highest concentrations observed a few days before peak discharge. DOC concentration also responded to discharge variation (increase, likely due to higher precipitation) in early autumn. We reported average DOC flux over the three sampling years of about 4.53 Tg yr-1 which is within the range of values reported in the literature. We observed interannual variability with annual export estimates ranging from 5.45 Tg yr-1 in 2014 to 3.57 Tg yr-1 in 2016, likely driven by discharge amplitude. We confirmed the important role of spring freshet in DOC export with on average more than 65% occurring during this period (roughly May/June). Third point was to determine DOM quality combining characterization techniques. Combination of approaches helped to strengthen observations and cross validate interpretations. Most of the variables reported from the different characterization techniques confirmed one each other. The use of lignin biomarkers, optical properties and radiocarbon age of DOM allowed us to trace DOM main sources has primarily deriving from recently produced organic matter leached from boreal forest litter and top soil horizon during the spring flood and older organic matter derived from deeper soil horizons during low flow period.

Les projections concernant les changements climatiques dans les régions arctiques annoncent une augmentation disproportionnée des températures de surface pouvant influencer le cycle hydrologique et la fonte des pergélisols. Les hautes latitudes sont donc perçu comme des environnements potentiellement très sensibles aux modifications du climat. Ces modifications ont la capacité d’impacter les cycles biogéochimiques, en particulier la mobilisation du carbone vers les milieux aquatiques de la zone arctique. Dans les rivières arctiques, la période la plus importante du cycle hydrologique est la crue de printemps qui peut réaliser la majeure partie des transferts annuels de carbone organique vers l’océan dans un lapse de temps très court. L’objectif de cette thèse vise à suivre la dynamique des MOD en termes de quantité et de qualité au cours du cycle hydrologique du fleuve Ienisei, grâce à diverses techniques de caractérisation des MOD permettant de mettre en évidence les variations saisonniéres de leur composition, sources, âge et état de dégradation. La première étape a été d’examiner la large gamme de techniques de caractérisation des MOD dans les rivières du monde afin d’identifier les plus répendues,pértinentes et fiables. Grâce au développement d’une base de données des mesures des qualités des MOD, nous avons pu évaluer la couverture géographique des études de caractérisation des MOD, donner des estimations et gamme de valeurs des principales variables de caractérisation des MOD et observer des tendances générales des qualités des MOD selon un gradient latitudinal. La seconde étape a porté sur l’étude de la dynamique du COD dans le fleuve Ienisei concernant les variations de concentrations et les liens avec la chimie de l’eau et l’hydrologie. Nous avons mené des campagnes d’échantillonnage durant trois années consécutives (2014 à 2016), en couvrant avec une haute fréquence de prélèvement la période de la crue de printemps afin d’en capturer la dynamique rapide. Nous avons observé des concentrations en COD qui suivent l’hydrogramme avec les concentrations maximales mesurées quelques jours avant le pic de crue. Le COD a aussi répondu aux variations de débit (augmentation, probablement liée à des précipitations plus intenses du début d’automne. Nous avons enregistré un flux de COD moyen, durant les trois années de prélèvement, d’environ 4,53 Tg yr-1, ce qui est dans l’ordre de grandeur rapporté dans la littérature. Nous avons également observe ue variabilité interannuelle relativement importante avec les estimations d’exports annuels variant de 5,45 Tg yr-1 en 2014 à 3,57 Tg yr-1 en 2016, certainement influence par l’amplitude de débit. Nous avons confirmé le rôle prédominant de la crue de printemps dans les exports de COD avec en moyenne plus de 65% des exportations ayant lieu Durant cette période (approximatvement mai/juin). Le troisième point a consisté en l’étude de la qualité des MOD présentent dans les eaux du Ienisei et de son évolution durant le cycle hydrologique en combinant différentes outils de caractérisation. Associer les modes de caractérisation aide à renforcer les observations et à valider les interprétations. La plus part des variables mesurées avec les différentes techniques se sont confirmées les unes les autres. L’utilisation des biomarqueurs de la lignine, des propriétés optiques et de la datation 14C ont permis de tracer les sources prédominantes des MOD comme étant principalement dérivées de matières organiques récemment produites lixiviée depuis les litières forestières boréales et des horizons supèrieures pendant la crue de printemps et de matières organiques plus anciennes provenant des horizons plus profonds durant la période des basses eaux. / Arctic regions are expected to be highly sensitive to climate change regarding the prediction of disproportionately large increases in surface temperatures and their related influence over the hydrological cycle and permafrost thaw. These modifications have the potential to impact biogeochemical cycles in these regions and in particular the mobilization of organic carbon into rivers. The most crucial period in arctic rivers hydrological cycle is the spring freshet that can account for most of the annual organic carbon transfers to the ocean in a very short window of time. The focus of this thesis was to monitor the dynamic of DOM in terms of quantity and quality along the hydrological cycle of the Yenisei river, through DOM characterization approaches to reveal seasonal variations in its composition, sources, age and degradation state. The first step was to make a review of the full range of existing DOM characterization approaches in worldwide river systems to identify the most widely used, the most relevant and reliable ones. Through the development of a DOM quality measurements database, we have been able to evaluate the geographical coverage of DOM characterization studies, to give estimates and ranges of values of the main reported DOM characterization variables and to observe global trends of DOM quality across latitudinal gradient. Second stage was to investigate DOC dynamic in the Yenisei river with regard to quantity and links with water chemistry and hydrology. We conducted sampling campaigns during three consecutive years (2014 to 2016), covering with a high sampling frequency the spring flood period to capture its very dynamic evolution. We reported DOC concentrations that followed the hydrograph with highest concentrations observed a few days before peak discharge. DOC concentration also responded to discharge variation (increase, likely due to higher precipitation) in early autumn. We reported average DOC flux over the three sampling years of about 4.53 Tg yr-1 which is within the range of values reported in the literature. We observed interannual variability with annual export estimates ranging from 5.45 Tg yr-1 in 2014 to 3.57 Tg yr-1 in 2016, likely driven by discharge amplitude. We confirmed the important role of spring freshet in DOC export with on average more than 65% occurring during this period (roughly May/June). Third point was to determine DOM quality combining characterization techniques. Combination of approaches helped to strengthen observations and cross validate interpretations. Most of the variables reported from the different characterization techniques confirmed one each other. The use of lignin biomarkers, optical properties and radiocarbon age of DOM allowed us to trace DOM main sources has primarily deriving from recently produced organic matter leached from boreal forest litter and top soil horizon during the spring flood and older organic matter derived from deeper soil horizons during low flow period.