Variations écologiques et chorologiques de la végétation macrophytique des rivières acides du Massif armoricain et des Vosges du Nord (France)

La végétation macrophytique des rivières acides d'Europe occidentale présente une grande unité, marquée par la constance d'un ensemble floristique de base, dont certaines espèces (Callitriche hamulata, Myriophyllum alterniflorum, Potamogeton polygonifolius, P. alpinus, etc) sont propres à ces eaux acides, d'autres (Callitriche platycarpa, Sparganium emersum, Elodea canadensis, Potamogeton crispus, etc) étant communes avec les eaux calcaires.Cet ensemble subit des variations écologiques et chorologiques :L'augmentation du niveau trophique des eaux conduit d'une végétation oligotrophe (différenciée par P. polygonifolius, Juncus bulbosus, etc) à un groupement mésoeutrophe (caractérisé par Callitriche obtusangula, Nasturtium officinale, etc). Ainsi, quatre types de phytocénoses ont pu étre distingués dans les Vosges du Nord en fonction de ce paramètre. D'autre part, une pollution organique localisée, par exemple par une pisciculture en Bretagne, provoque une augmentation des espèces eutrophes (C. obtusangula) et une régression des taxons plus oligotrophes (Scapania undulata).L'alternance de biotopes courants et lents se traduit par un remplacement de phytocénoses : ainsi, en Bretagne, Oenanthe crocata et Ranunculus penicillafus sont associées sur les radiers, alors que Nuphar lutea et S. emersum caractérisent les mouilles. De plus, une diminution de l'éclairement permet la présence de groupements bryophytiques spécifiques.Les principales variations chorologiques entre les deux territoires correspondent à un enrichissement des zones atlantiques en macrophytes euatlantiques ou méditerranéoatiantiques, comme O. crocata ou Apium inundatum, qui manquent dans les régions médioeuropéennes. En outre, selon les territoires biogéographiques, l'écologie de certaines espèces peut varier (par exempte S. emersum, R. peltatus, P. polygonifolius).Les macrophytes permettent donc, à l'instar des invertébrés benthiques, l'élaboration de biodiagnostics des eaux courantes.For the study of the vegetation of acid rivers in France, two areas have been investigated by the authors : Armorican Massif and Northern Vosges (fig. 1).This paper has two aims :- assessing the general trends of macrophytic vegetation : floristic composition and plant communities, with special reference to the influence of physical and water quality parameters,- pointing out and interpreting the variations observed between the two regions (chorological differences), and in the watercourses (ecological variations).Floristic data and phytosociological surveys (from bryophytes to angiosperm macrophytes) were collected along 50 m. stretches (at least) by the stratified sampling method in the case of large watercourses, and by systematic surveys for smaller ones. Consecutive 50 m. long sketches were used to measure the effect of localized pollution. (The inventory is as precise as possible). A distinction was made between the aquatic communities and river-bankside ones. At the same time, physical parameters were measured or observed (depth, light conditions, width,...), certain water-quality parameters were measured (pH, conductivity,...) and water samples were collected for further analysis.The data collected are gathered and compared in a table (table 1) showing differences between aquatic and subaquatic species, and between sunlit and shaded stretches. Trophic tendencies of most species are given according to either our observations or the literature.The main ecological feature is the streaking longitudinal variation of aquatic species, which agrees with the zones described by ILLIES and BOTOSANEANU : crenon, rhitron and potamon. In sunlit areas the succession is dominated by the following species :Potamogeton polygonifolius, Juncus bulbosus, Glyceria fluitans,...-> Ranunculus spp., Callitriche hamulata, Myriophyllum alterniflorum,...     ->Potamogeton crispus, Elodea canadensis,...Areas in the shade are characterized by bryophyta; the succession is : Scapania ondulata->Rhynchostegim riparioïdes, Fontinalis antipyreticaAs for the aquatic species, a longitudinal succession of bankside species exists :Sparganium erectum, Carex paniculata, Carex rostrata->Phalaris arundinacea      ->Phalaris arundinacea, Typha latifoliaThe macrophytic vegetation in acid rivers of Western Europe shows a great homogeneity, marked by the constancy of a floristic list, in which some species (Callitriche hamulata, Myriophyllum alterniflorum, Potamogeton polygonifolius, Potamogeton alpinus, Scapania undulata, etc.) only grow into acid waters, while others species (Callitriche platycarpa, Sparganium emersum, Elodea canadensis, Potamogeton crispus, etc.) grow also in calcareous waters.This basic unit is submitted to ecological and chorological variations.Ecological variations are due mainly to changes of the water quality. They are illustrated by two examples :- two river systems in the Northern Vosges where a general survey shows longitudinal variations of communities without pollution or assessing different kinds of pollution,- a pisciculture pollution with observations of species distribution on consecutive sketches.An increase of water trophic level leads progressively from an oligotrophic vegetation (marked by Potamogeton polygonifolius, Juncus bulbosus, etc.) to a meso-eutrophic community (characterized by Callitfriche obtusangula, Nasturtium officinale, etc.). Four communities are distinguished in Northern Vosges (table 2) and characterize water quality (both pollution and trophic level) :A : Potamogeton polygonifolius, Juncus bulbosus, Sphagnum spp. community, with the following main water-quality parameters : conductivity : 40-80 µS, phosphates : 500 ppb N).Two vegetation maps show the distribution of these communities (fig. 2). The succession of communities is related to water quality and fastened sequences are compared to normal ones. Without any pollution, A and B communities caver more than 5 km along the upper streams; under these conditions, the entire sequence A to D develops to 15 km, while with much anthropic pollution, it may occur within less than 3 km. A heavy pollution leads to the leads of communities as observed with the change tram A to C (without any B) due to a camping-site.A localized organic pollution by a Breton pisciculture is responsible for water-quality changes (table 3): ammonia, nitrites, phosphates and conductivity increase. The succession of mineral nitrogen forms is assessed (fig. 3). In the N-NH4+ and N-NO2- peak areas, the eutrophic species increase (Callitriche spp., specially Callitriche obtusangula, Amblystegium riparium) and the oligotrophic ones decrease (Scapania undulata) (fig. 3). Tissue analysis of Ranunculus penicillatus and of Callitriche spp. does not show any enrichment in nitrogen in polluted areas: the ecological response is mainly an increase of nitrophilous macrophytes (Callitriche spp.).The influence of water quality is more obvious in the Northern Vosges for most of the oligotrophic communities grow in forested sketches, white in Brittany these conditions are rare and eutrophication occurs in all the streams, even in the source areas. Floristically, neutrophilous or ubiquitous species are numerous. Some of them indicate either eutrophication or pollution : Amblystegium riparium and perhaps Octodiceras fontanum for bryophyta, Callitriche obtusangula, Potamageton perfoliatus and Sparganium. emersum (long leaves form), for aquatic species, Nasturtium officinale, Glyceria maxima for helophytes.Physical parameters also lead to variations. The succession of swift and slow areas leads to changes in the communities. Thus, in Brittany, Oenanthe crocata and Ranunculus penicillatus are associated on the riffles, while Nuphar lutes and Sparganium emersum characterize the pools, even in the upper parts of the streams where there are watermill ponds.The relationships between local hydrodynamic features and species distribution are discussed with regards to the stretches around the pisciculture area.Main chorological variation is an enrichment of atlantic areas with numerous eu-atlantic or mediterraneo-atlantic species : Porella pinnata and Amblystegium fluviatile as bryophyla, Oenanthe crocata, Apium inundatum, Ranunculus omiophyllus as spermaphyta. As there are few or no species restricted to medio-european areas, Atlantic communities are often richer than eastern ones. But some invading species as Elodea nuttallii are spreading all over the country from North-Eastern France.Ecological trends of certain species (Sparganium emersum, Ranunculus peltatus, Potamogeton polygonifolius) can change between the two biogeographical areas. So, Sparganium emersum grows in the upper oligotrophic zones in Northern Vosges while it characterizes lower meso-eutrophic zones in Brittany. In Brittany, Ranunculus peltatus un exist in eutrophic areas when the stretches are far from the sea. Potamogeton polygonifolius is very rare in swift streams in Brittany.Therefore, macrophytes permit, as benthic invertebrates do, to get a river biodiagnosis. They are easily observed and mapped so they can be used for a general survey

Stability of trions in strongly spin-polarized two-dimensional electron gases

Low-temperature magneto-photoluminescence studies of negatively charged excitons (X- trions) are reported for n-type modulation-doped ZnSe/Zn(Cd,Mn)Se quantum wells over a wide range of Fermi energy and spin-splitting. The magnetic composition is chosen such that these magnetic two-dimensional electron gases (2DEGs) are highly spin-polarized even at low magnetic fields, throughout the entire range of electron densities studied (5e10 to 6.5e11 cm^-2). This spin polarization has a pronounced effect on the formation and energy of X-, with the striking result that the trion ionization energy (the energy separating X- from the neutral exciton) follows the temperature- and magnetic field-tunable Fermi energy. The large Zeeman energy destabilizes X- at the nu=1 quantum limit, beyond which a new PL peak appears and persists to 60 Tesla, suggesting the formation of spin-triplet charged excitons.Comment: 5 pages (RevTex), 4 embedded EPS figs. Submitted to PRB-R

Light and electric field control of ferromagnetism in magnetic quantum structures

A strong influence of illumination and electric bias on the Curie temperature and saturation value of the magnetization is demonstrated for semiconductor structures containing a modulation-doped p-type Cd0.96Mn0.04Te quantum well placed in various built-in electric fields. It is shown that both light beam and bias voltage generate an isothermal and reversible cross-over between the paramagnetic and ferromagnetic phases, in the way that is predetermined by the structure design. The observed behavior is in quantitative agreement with the expectations for systems, in which ferromagnetic interactions are mediated by the weakly disordered two-dimensional hole liquid.Comment: 4 pages and 3 figure

Interlayer coupling in ferromagnetic semiconductor superlattices

We develop a mean-field theory of carrier-induced ferromagnetism in diluted magnetic semiconductors. Our approach represents an improvement over standard RKKY model allowing spatial inhomogeneity of the system, free-carrier spin polarization, finite temperature, and free-carrier exchange and correlation to be accounted for self-consistently. As an example, we calculate the electronic structure of a Mn$_x$Ga$_{1-x}$As/GaAs superlattice with alternating ferromagnetic and paramagnetic layers and demonstrate the possibility of semiconductor magnetoresistance systems with designed properties.Comment: 4 pages, 4 figure

Observation of the spin-charge thermal isolation of ferromagnetic Ga_{0.94}Mn_{0.06}As by time-resolved magneto-optical measurement

The dynamics of magnetization under femtosecond optical excitation is studied in a ferromagnetic semiconductor Ga_{0.94}Mn_{0.06}As with a time-resolved magneto-optical Kerr effect measurement with two color probe beams. The transient reflectivity change indicates the rapid rise of the carrier temperature and relaxation to a quasi-thermal equilibrium within 1 ps, while a very slow rise of the spin temperature of the order of 500ps is observed. This anomalous behavior originates from the thermal isolation between the charge and spin systems due to the spin polarization of carriers (holes) contributing to ferromagnetism. This constitutes experimental proof of the half-metallic nature of ferromagnetic Ga_{0.94}Mn_{0.06}As arising from double exchange type mechanism originates from the d-band character of holes

Theory of Magnetic Properties and Spin-Wave Dispersion for Ferromagnetic (Ga,Mn)As

We present a microscopic theory of the long-wavelength magnetic properties of the ferromagnetic diluted magnetic semiconductor (Ga,Mn)As. Details of the host semiconductor band structure, described by a six-band Kohn-Luttinger Hamiltonian, are taken into account. We relate our quantum-mechanical calculation to the classical micromagnetic energy functional and determine anisotropy energies and exchange constants. We find that the exchange constant is substantially enhanced compared to the case of a parabolic heavy-hole-band model.Comment: 9 pages, 4 figure

A theory of ferromagnetism in planar heterostructures of (Mn,III)-V semiconductors

A density functional theory of ferromagnetism in heterostructures of compound semiconductors doped with magnetic impurities is presented. The variable functions in the density functional theory are the charge and spin densities of the itinerant carriers and the charge and localized spins of the impurities. The theory is applied to study the Curie temperature of planar heterostructures of III-V semiconductors doped with manganese atoms. The mean-field, virtual-crystal and effective-mass approximations are adopted to calculate the electronic structure, including the spin-orbit interaction, and the magnetic susceptibilities, leading to the Curie temperature. By means of these results, we attempt to understand the observed dependence of the Curie temperature of planar $\delta$-doped ferromagnetic structures on variation of their properties. We predict a large increase of the Curie Temperature by additional confinement of the holes in a $\delta$-doped layer of Mn by a quantum well.Comment: 8 pages, 7 figure

The influence of feature selection methods on accuracy, stability and interpretability of molecular signatures

Motivation: Biomarker discovery from high-dimensional data is a crucial problem with enormous applications in biology and medicine. It is also extremely challenging from a statistical viewpoint, but surprisingly few studies have investigated the relative strengths and weaknesses of the plethora of existing feature selection methods. Methods: We compare 32 feature selection methods on 4 public gene expression datasets for breast cancer prognosis, in terms of predictive performance, stability and functional interpretability of the signatures they produce. Results: We observe that the feature selection method has a significant influence on the accuracy, stability and interpretability of signatures. Simple filter methods generally outperform more complex embedded or wrapper methods, and ensemble feature selection has generally no positive effect. Overall a simple Student's t-test seems to provide the best results. Availability: Code and data are publicly available at http://cbio.ensmp.fr/~ahaury/

Mesoscale subduction at the Almeria-Oran front. Part 2: biophysical interactions.

This paper presents a detailed diagnostic analysis of hydrographic and current meter data from three, rapidly repeated, fine-scale surveys of the Almeria–Oran front. Instability of the frontal boundary, between surface waters of Atlantic and Mediterranean origin, is shown to provide a mechanism for significant heat transfer from the surface layers to the deep ocean in winter. The data were collected during the second observational phase of the EU funded OMEGA project on RRS Discovery cruise 224 during December 1996. High resolution hydrographic measurements using the towed undulating CTD vehicle, SeaSoar, traced the subduction of Mediterranean Surface Water across the Almeria–Oran front. This subduction is shown to result from a significant baroclinic component to the instability of the frontal jet. The Q-vector formulation of the omega equation is combined with a scale analysis to quantitatively diagnose vertical transport resulting from mesoscale ageostrophic circulation. The analyses are presented and discussed in the presence of satellite and airborne remotely sensed data; which provide the basis for a thorough and novel approach to the determination of observational error