Variations de la structure trophique du lac-réservoir oligotrophe Bin El Ouidane (Maroc)

L'étude combinée des apports par les crues, du plancton, des contenus algaux du tractus digestif des Cladocères et du régime alimentaire de quelques espèces de poissons dans le lac de barrage oligotrophe Bin El Ouidane a permis de dégager les conclusions suivantes :- les apports particulaires et dissous par les crues durant la période d'étude 1995-1997 ont une influence marquée sur la dynamique saisonnière du zooplancton via les changements dans la dynamique du peuplement phytoplanctonique ;- durant les trois années d'étude, le maximum de biomasse zooplanctonique se situe généralement juste après les pics printanier et estival du phytoplancton mettant ainsi en évidence l'importance du zooplancton herbivore dans la chaîne alimentaire pélagique du lac ;- l'étude des contenus algaux du tractus digestif des Cladocères Daphnia longispina, Daphnia lumholtzi, Diaphanosoma brachyurum, Ceriodaphnia dubia, Ceriodaphnia reticulata et Bosmina longirostris a montré que l'essentiel des espèces sont des filtreuses herbivores qui adoptent un comportement nutritionnel variable en fonction du temps lié à la disponibilité en phytoplancton, à leur âge et aux caractéristiques biotiques et abiotiques de l'écosystème lacustre ; - l'étude préliminaire du régime alimentaire de trois espèces de poissons Tinca tinca, Eupomotis gibbosus et Cyprinus carpio montre que leur spectre alimentaire est nettement dominé par le plancton. La fraction faunistique au niveau de leur tube digestif est représentée essentiellement par les Cladocères de grande taille et les Chironomes. Ceci met en évidence l'importance de ces poissons, à régime essentiellement planctonophage, dans la régulation des peuplements planctoniques de la retenue Bin El Ouidane.The study of the flood transport, plankton and the stomach contents of Cladoceran species and some planktivorous and omnivorous fish of the reservoir-lake Bin El Ouidane has allowed to extract the following conclusions:- flood transport of dissolved and suspended matters during the period 1995-1997 has a net influence on the seasonal dynamics of the zooplancton via changes in the dynamics of the phytoplanktonic populations;- during the same period, the peak of zooplanktonic biomass appeared generally just after spring and summer phytoplanktonic peaks showing the importance of the herbivorous zooplankton in the food chain;- the study of algal stomach contents of Cladocerans Daphnia longispina, Daphnia lumholtzi, Diaphanosoma brachyurum, Ceriodaphnia dubia, Ceriodaphnia reticulata and Bosmina longirostris showed that all species are herbivorous filter-feeders that adopt a variable diet behavior linked to the availability of the phytoplankton in the lake, to their age and biotic and abiotic characteristics of the ecosystem;- the preliminary study of the stomach contents of three omnivorous and herbivorous fish Tinca tinca, Eupomotis gibbosus and Cyprinus carpio species showed also that their food spectre was clearly dominated by the plankton. The animal fraction in their digestive tube was represented by large Cladoceran species and Chironomes showing the importance of these fish, essentially planktivorous, in the regulation of planktonic populations in the reservoir-lake Bin El Ouidane

Cutting enumerative algorithm for the minimizing of energy function

We study the 1-D reconstruction problem of real-valued data . This problem can be formulated by minimizing the energy functio n or hamìltonien . To solve this problem, many methods have been proposed such as, simulated annealing (SA), the graduate d nonconvexiiy (GNC), and mean field annealing (MFA) . We propose a new algorithm called "Discontinuity Position Sweep " (in French Balayage par Position de Discontinuité : (BPD)) . This new algorithm comes from cutting enumerative methods i n combinatorial optimization and its main advantages over the aforementioned methods are that it is parallelisable together, it i s faster and it finds the better solution .Nous étudions le problème de reconstruction de données unidimensionnelle par minimisation de la fonction d'énergie ou hamiltonien. Plusieurs méthodes de relaxation, stochastique et déterministe, ont été proposées pour résoudre ce problème d'optimisation globale et non convexe, notamment le recuit simulé (SA), le recuit par champ moyen (MFA), et la non convexité graduelle (GNC). Nous proposons un nouvel algorithme déterministe de coupe énumérative en optimisation combinatoire. Cet algorithme que nous appelons Balayage par Position de Discontinuité (BPD) permet d'obtenir des gains significatifs en terme de temps de calcul et de qualité de la solution obtenue. De plus, il est de nature parallélisable

Structure and magnetism of self-organized Ge(1-x)Mn(x) nano-columns

We report on the structural and magnetic properties of thin Ge(1-x)Mn(x)films grown by molecular beam epitaxy (MBE) on Ge(001) substrates at temperatures (Tg) ranging from 80deg C to 200deg C, with average Mn contents between 1 % and 11 %. Their crystalline structure, morphology and composition have been investigated by transmission electron microscopy (TEM), electron energy loss spectroscopy and x-ray diffraction. In the whole range of growth temperatures and Mn concentrations, we observed the formation of manganese rich nanostructures embedded in a nearly pure germanium matrix. Growth temperature mostly determines the structural properties of Mn-rich nanostructures. For low growth temperatures (below 120deg C), we evidenced a two-dimensional spinodal decomposition resulting in the formation of vertical one-dimensional nanostructures (nanocolumns). Moreover we show in this paper the influence of growth parameters (Tg and Mn content) on this decomposition i.e. on nanocolumns size and density. For temperatures higher than 180deg C, we observed the formation of Ge3Mn5 clusters. For intermediate growth temperatures nanocolumns and nanoclusters coexist. Combining high resolution TEM and superconducting quantum interference device magnetometry, we could evidence at least four different magnetic phases in Ge(1-x)Mn(x) films: (i) paramagnetic diluted Mn atoms in the germanium matrix, (ii) superparamagnetic and ferromagnetic low-Tc nanocolumns (120 K 400 K) and (iv) Ge3Mn5 clusters.Comment: 10 pages 2 colonnes revTex formatte

Exchange bias in GeMn nanocolumns: the role of surface oxidation

We report on the exchange biasing of self-assembled ferromagnetic GeMn nanocolumns by GeMn-oxide caps. The x-ray absorption spectroscopy analysis of this surface oxide shows a multiplet fine structure that is typical of the Mn2+ valence state in MnO. A magnetization hysteresis shift |HE|~100 Oe and a coercivity enhancement of about 70 Oe have been obtained upon cooling (300-5 K) in a magnetic field as low as 0.25 T. This exchange bias is attributed to the interface coupling between the ferromagnetic nanocolumns and the antiferromagnetic MnO-like caps. The effect enhancement is achieved by depositing a MnO layer on the GeMn nanocolumns.Comment: 7 pages, 5 figure

Topological Monomodes in non-Hermitian Systems

Topological monomodes have been for long as elusive as magnetic monopoles. The latter was experimentally shown to emerge in effective descriptions of condensed-matter systems, while the experimental exploration of the former has largely been hindered by the complexity of the conceived setups. Here, we present a remarkably simple model and the experimental observation of topological monomodes generated dynamically. By focusing on non-Hermitian one-dimensional (1D) and 2D Su-Schrieffer-Heeger (SSH) models, we theoretically unveil the minimal configuration to realize a topological monomode upon engineering losses and breaking of lattice symmetries. Furthermore, we classify the systems in terms of the (non-Hermitian) symmetries that are present and calculate the corresponding topological invariants. To corroborate the theory, we present experiments in photonic lattices, in which a monomode is observed in the non-Hermitian 1D and 2D SSH models, thus breaking the paradigm that topological corner states should appear in pairs. Our findings might have profound implications for photonics and quantum optics because topological monomodes increase the robustness of corner states by preventing recombination.Comment: 30 (13+17) pages, 17 (4+13) figures, comments are welcom

Tuning the domain wall orientation in thin magnetic strips by induced anisotropy

We report on a method to tune the orientation of in-plane magnetic domains and domain walls in thin ferromagnetic strips by manipulating the magnetic anisotropy. Uniaxial in-plane anisotropy is induced in a controlled way by oblique evaporation of magnetic thin strips. A direct correlation between the magnetization direction and the domain wall orientation is found experimentally and confirmed by micromagnetic simulations. The domain walls in the strips are always oriented along the oblique evaporation-induced easy axis, in spite of the shape anisotropy. The controlled manipulation of domain wall orientations could open new possibilities for novel devices based on domain-wall propagation

First assessment of pollution impact at Essaouira coast (Morocco) using biotic and abiotic parameters and the red algae Ellisolandia elongata as potential bioindicator of organic pollution

Environmental pollutants might significantly affect the ecological integrity of coastal waters. Biological indicators like seaweeds have been used globally to assess water pollution. In the present work, the seaweed Ellisolandia elongata was used to evaluate the pollution status in coastal waters around Essaouira city. Three sites were chosen: One as reference station (S1) and two polluted ones (S2 and S3). Seaweed biodiversity, physiologic parameters of E. elongata, as well as, abiotic parameters were studied. Results showed that at the polluted stations, seaweed biodiversity was significantly lower than in S1. However, the concentrations of Total Suspended Solids, Electrical Conductivity, Biological Oxygen Demand, Chemical Oxygen Demand, Ammonium, and Orthophosphates at S2 were significantly higher than at S1. Metal content of E. elongata remained below detection limit at all stations except for Zn and Cu at S2 considered the most polluted station. With respect to the physiologic parameters, Proline, Glycine Betaine and Polyphenol contents at S2 were above, whereas, Chlorophyll a content and axis length were below those determined at the reference station. From the results it can be concluded that seaweed E. elongata could be a good indicator to determine organic pollution in marine ecosystem

Tunnel magnetoresistance and robust room temperature exchange bias with multiferroic BiFeO3 epitaxial thin films

We report on the functionalization of multiferroic BiFeO3 epitaxial films for spintronics. A first example is provided by the use of ultrathin layers of BiFeO3 as tunnel barriers in magnetic tunnel junctions with La2/3Sr1/3MnO3 and Co electrodes. In such structures, a positive tunnel magnetoresistance up to 30% is obtained at low temperature. A second example is the exploitation of the antiferromagnetic spin structure of a BiFeO3 film to induce a sizeable (~60 Oe) exchange bias on a ferromagnetic film of CoFeB, at room temperature. Remarkably, the exchange bias effect is robust upon magnetic field cycling, with no indications of training.Comment: 15 pages, 4 figure