The biological process of the carbon cycle in the Antarctic Ocean is controlled by the photosynthetic activity of the primary producers. The amount of fixed carbon does not only depend on the photosynthetic activity but also on the carbon losses due to respiration. Thus, the ratio photosynthesis to respiration (rP/R) is an important parameter to predict the effect of climate change on the Antarctic ecosystem. Indeed, the ongoing changes in climate change are influencing the dynamics of environmental conditions, which has tremendous effects on the phytoplankton community. Therefore, two ecologically relevant species from the Southern Ocean were here investigated: the diatom Chaetoceros sp. and the prymnesiophyte Phaeocystis antarctica, studying the changes in the rP/R under global climate change conditions. Three main parameters were examined i.e temperature, salinity and iron limitation. The P/R ratio was significantly affected by temperature, while salinity had only a secondary importance, although with species-specific differences. More specifically, the values were ranging from 12.3 to 7.5 for Chaetoceros sp. and from 12.4 to 2.5 for P. antarctica. The changes in this ratio were principally due to variations in respiration, rather than in photosynthesis. Chaetoceros sp. appears to be less flexible in the regulation of the extent of photoprotective mechanisms (non-photochemical quenching and alternative electrons), but its photoprotective level was generally higher than in P. antarctica. Regarding iron limitation, data were successfully collected only for Chaetoceros sp.. The P/R ratio, equal to 2.8, did not change under iron limitation, with iron limited cells showing a very efficient acclimation to the lowered assimilatory metabolism by decreasing their respiratory losses.

Elemental stoichiometry defines a critical understanding of the relationship between nutrient availability and usage throughout different levels of the biological community. We found there is a link between available phosphorus (P), cellular phosphorus, and nematode development as postulated by the growth rate hypothesis (GRH). I predicted that in a P-poor environment, cellular RNA concentrations would be lower than they are in P-rich environment, and thus the 18s rRNA expression level will have reduced. To most efficiently regulate the uptake of limited P, I predicted that nematodes in P-poor environments would decrease the number of copies of the 18s rRNA gene in their genome. I measured life history traits as well as rRNA gene expression and gene copy number. We found that elemental stoichiometry predicts evolutionary changes consistent with the Growth Rate Hypothesis. We sequenced and assembled a draft genome of P. murrayi. Although we expected to find genes responsible for stress tolerance, we hypothesized that in response to strong selection pressure associated with living in a simplified ecosystem, over time the genome of P. murrayi should have undergone significant decay (gene loss) relative to species in ecosystems structured more strongly by biotic interactions. We found significantly fewer genes in P. murrayi. To compare patterns of gene expression between two highly divergent Antarctic nematode species, we sequenced and assembled the transcriptomes of S. lindsayae and P. murrayi. Under laboratory conditions at 4˚C, S. lindsayae had significantly lower rates of gene expression but expressed a significantly larger number of genes. We speculate that the differences in gene expression are correlated with life history traits (developmental rates) while the differences in the number of genes expressed can be explained by their different genetic systems (S. lindsayae is amphimictic, P. murrayi is parthenogenic) and the soil environments to which they are adapted. Since we previously showed that differences in available P content can influence the evolution of gene expression via gene copy number, and that this ultimately influences growth rate, we wondered how much of this response is driven by genetics versus how strongly these patterns are driven by temperature. To better understand this, we maintained wild type populations of P. murrayi in P-rich and P-poor conditions at 5˚C, 10˚C and 15˚C in the laboratory for over 40 generations and sequenced the transcriptomes prepared from each treatment group. We found that nutrient levels played an important role in gene expression when the temperature is optimal for P. murrayi culturing and that temperature is more important in gene expression when the available P is limited. This work underscores the utility of using principles of elemental stoichiometry coupled with genomic and transcriptomics research tools to make and test predictions about life history evolution. The results of my work also inform inferences about the ways in which nutrient availability also drives the organization of trophic interactions and ultimately ecosystems.

Der Einfluß eines Seherstroms auf die Ausbreitung langer barotroper Wellen wird untersucht. Der Antarktische Zirkumpolarstrom ist als eine zonale Scherströmung zu betrachten. Wir zeigen auf einer ß-Ebene, daß der Antarktische Zirkumpolarstrom für zonal sich ausbreitende Wellen als Wellenleiter wirkt. Für den linearen Fall des Scherstromes wird die analytische Lösung angegeben. Die gebundenen Wellen sind meridional modal strukturiert und bewegen sich mit einer Gruppengeschwindigkeit von rund 0.02ms·1 von West nach Ost (alle). / We study the influence of a shear current on barotropic long wave propagation. The Antarctic Circumpolar Current can be considered as a sheared zonal flow. We show tht on a ß-plane the Antarctic Circumpolar Current acts as a waveguide for zonally propagating waves. We give the analytical solution for the linear sheared case. The trapped waves are modal structured in the meridional direction and propagate from the West to the East (all) with a group velocity of nearly 0.02ms·1.

Le déphasage entre augmentation de température et augmentation de gaz à effet de serre (GES) dans l'atmosphère lors des grandes transitions climatiques passées est estimé grâce aux mesures effectuées dans les carottes de glace polaires dans la phase glace pour la température et dans la phase gaz (bulles d’air piégées) pour la concentration en GES. Ce déphasage est encore mal contraint et, pour résoudre ce problème, il est nécessaire de mieux comprendre le processus mécanique de transformation de neige en glace près de la surface de la calotte (centaine de mètres supérieurs, le névé). En l'absence de fusion, la transformation de la neige (matériau à porosité ouverte en contact avec l'atmosphère) en glace (matériau contenant des bulles d'air isolées) s'effectue progressivement sous l'action des gradients de température près de la surface, puis sous l'action du poids des couches de neige situées au-dessus. Selon les conditions de température et précipitation, ce processus peut prendre quelques décennies à plusieurs millénaires et s'étend sur une centaine de mètres de profondeur. Il contrôle la différence d'âge entre la glace et les gaz qu'elle renferme. La prédiction de la profondeur de piégeage des gaz représente un enjeu majeur pour la paléoclimatologie en particulier sur la séquence des changements relatifs de température et de concentration en gaz à effet de serre.Un modèle thermo-mécanique de densification de la neige a été conçu et développé au LGGE en intégrant la formulation des processus mécaniques, des propriétés thermiques, et des critères de piégeage des gaz. Les performances de ce modèle peuvent être testées et améliorées grâce à des études de structure de névés actuels (densité, rapport porosité ouverte/fermée, …). Pour les périodes plus anciennes, des mesures d’isotopes des gaz inertes d15N et d40Ar) dans l’air piégé dans les carottes de glace polaire permettent d’obtenir des informations directes sur les variations passées de la structure du névé (e.g. épaisseur de la zone diffusive). Les larges divergences observées en Antarctique entre les sorties de modèle de densification et les mesures isotopiques de gaz piégé dans la glace génèrent une grande incertitude sur les reconstructions climatiques passées et comprendre ce désaccord est un défi majeur de la paleoclimatologie actuelle.Dans le cadre de cette thèse, nous avons pris en compte les influences de la dépendance à la température des énergies d'activation et des impuretés (poussières) sur la vitesse de densification. Cela a permis de concilier les données et le modèle. Les résultats du modèle modifié sont globalement cohérents avec les profils de densité mesurés pour des névés actuels et les données d'isotopes de gaz inertes pour les déglaciations (aussi appelées terminaisons). Nous avons également présenté de nouvelles mesures à haute résolution de d15N et d40Ar pour les terminaisons 2 (129-138ka) et 3 (243-251ka) des carottes antarctiques de Dôme C et Vostok. Nous avons montré que les différentes évolutions de d15N entre les différents sites et différentes déglaciations s’expliquaient largement par les différences de taux d’accumulation qui contrôlent la profondeur de transition neige-glace. Nous avons aussi montré que l’utilisation des isotopes de l’air était un complément important à l’utilisation des isotopes de l’eau pour contraindre la dynamique climatique locale en Antarctique de l’Est lors des déglaciations. / The phasing between increases in temperature and greenhouse gas concentrations during large climatic variations in the past is classically estimated using analyses in polar ice cores, in the ice phase for the temperature and in the gas phase (trapped air bubbles) for the concentration of greenhouse gases. This phasing is still insufficiently constrained and solving this problem requires a better understanding of the mechanical process of snow to ice metamorphism near to the top of the ice sheet (i.e. the firn, about 100 m deep). In the absence of melting, the transformation of snow (a material with open porosity in contact with the atmosphere) into ice (a material containing isolated bubbles) occurs progressively as a response to temperature gradients near the surface, and the weight of overlying snow in deeper layers. Depending on temperature and precipitation conditions, this process occurs in a few decades to several millennia and a ~100 meters depth range. It controls the age difference between the ice and the entrapped gases. Predicting the gas trapping depth is a major issue in paleoclimatology, especially in order to understand the phasing between temperature changes and changes in greenhouse gas concentrations.A thermo-mechanical model of snow densification has been developed at LGGE, it includes the main mechanical processes, the thermal properties of ice, and gas trapping criteria. The model performances can be tested and improved using experimental studies of modern firns (density, open/closed porosity ratio, etc). For firnification under ancient climates, measurements of isotopes of inert gases (d15N et d40Ar) in the air trapped in ice cores provide direct informations about past variations of firn structure (e.g. diffusive zone thickness). Large differences between firn densification model outputs and gas isotopic data are obtained in Antarctica, and imply a large uncertainty on past climatic reconstructions. Understanding this discrepancy is a major issue in paleoclimatology.As part of this thesis work, we took into account the effects of the temperature dependence of activation energies and impurities (dust) on the firn densification speed. It allowed to reconcile the model results with available data. The modified model results show an overall consistency with measured density profiles of present-day polar firns and isotopes of inert gases over deglaciations (also called terminations). We also analyze new high resolution measurements of d15N and d40Ar over Terminations 2 (129-138ka) and 3 (243-251ka) on the Dome C and Vostok ice cores. We have shown that the different evolutions of d15N between different sites and different deglaciations are largely explained by differences in accumulation rates that control the snow/ice transition depth. We also showed that the use of air isotopes was an important complement to the use of water isotopes to constrain local climatic dynamics in eastern Antarctica during deglaciations.

Cellular_ polar research station Antarctica is the driest, the busiest and most extensive place on Earth, half-year-shrouded. People still live here. They set up the research stations in which they live and work. In these extreme climatic conditions, the station originates and disappears due to natural behaving. Stations provide researchers with safety and space for research, including comfort. They are built in different ways and provide different comforts, according to technological and transport options.

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Recent literature on Antarctica has suggested that China is becoming an increasingly crucial variable in defining governance structures on the continent. The thesis claims that China plays a role in restructuring relations and thus its Antarctic governance structures. It will therefore analyze the underlying causal role of China’s shifting identity, increasingly tense interactions with other treaty parties and changed interests in the Antarctic region. The analysis is based on theoretical assumptions of Conventional Constructivism which provides means to explain changes and the importance of state identities in international relations. Methodologically, the thesis employs a comparative research design to understand the cause-and-effect relationship between the independent and dependent variable. The thesis concludes that China is influencing governance structures by causing friction with other treaty parties and by exceeding the definitions of the Antarctic Treaty System based on its desire to become a ‘Polar Great Power’ and thus vast expansion.

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