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

Building Jobs in Iowa: New Deal Dams of the Wapsipinicon River Watershed in Northeast Iowa

https://ir.uiowa.edu/osa_pubs/1013/thumbnail.jp

Impact de pollutions ponctuelles sur les phytocénoses des rivières acides à neutres du Limousin (Massif Central, France)

L'impact des pollutions ponctuelles sur les phytocénoses aquatiques est étudié autour des rejets de 12 agglomérations dont 9 sont équipées d'une station d'épuration. Un échantillonnage systématique avec segmentation du cours d'eau autour de chaque rejet est réalisé. Sur chaque secteur, des relevés de végétation sont pratiqués au niveau de faciès d'écoulements homogènes dont on caractérise le milieu physique parallèlement à une analyse physicochimique de l'eau.L'ensemble des rejets provoque globalement une élévation de la conductivité, des teneurs en ammonium, nitrates et orthophosphates.Cela ce traduit par la régression de la phytocénose à Callitriche hamulata et Myriophyllum alterniflorum, par le développement de Ranunculus peltatus, Callitriche platycarpa et d'espèces cryptogames telles que Leptodyctium riparium, ou Melosira sp.Une Analyse en Composantes Principales menée sur l'ensemble des données permet d'opposer des phytocénoses propres aux secteurs amonts (Scapania undulata, Chiloscyphus polyanthus) à d'autres situées au niveau de rejets (Callitriche platycarpa, Leptodictyum riparium, Melosira sp.,).Une Analyse Canonique de Correspondances valide le déterminisme de la qualité physicochimique de l'eau sur la végétation. La conductivité, les teneurs en ammonium, nitrates et orthophosphates deviennent prépondérants par rapport aux facteurs du milieu physique classiquement discriminants dans l'installation des phytocénoses dans les rivières limousines.The impact of located pollution on aquatic phytocénoses is studied around 12 cities discharge. Nine of them are fitted out purification plant.The sampling method is based on consecutive segments from upstream to downstream. On each sector, vegetation records are realized in homogeneous water runoff facies, which are characterized by physical factors as well as water value measures.The whole discharge leads globally to an increase of conductivity, ammonium amount, nitrates and orthophosphates. The consequence of that is a decrease of Callitriche hamulata and Myriophyllum alterniflorum phytocénoses, a development of Ranunculus peltatus, Callitriche platycarpa and cryptogams species like Leptodictyum riparium or Melosira sp.A Component Principal Analysis applied on data, distinguishes phytocénoses belonging to upstream sectors (Scapania undulata, Chiloscyphus polyanthus) from the ones of discharges (Callitriche platycarpa, Leptodictyum riparium, Melosira sp.).A Canonical Correspondence Analysis validates the impact of physico-chemical water quality on vegetation. Conductivity, ammonium amount, nitrates and orthophosphates become more preponderant in comparison with physical environments usually discriminant for phytocénoses installation in Limousin rivers

Spatial patterns of grazing-related parameters in California coastal surface waters, July 1979

A surface survey using underway continuous mapping of temperature, chlorophyll, zooplankton (\u3e153 µm) wet weight density, and activity of the zooplankton digestive enzyme laminarinase was undertaken for a 20-day period during active upwelling in the California coastal region between Pt. Arena and Pt. Conception, California, U.S.A., during July, 1979...

Theory of Magnetic Anisotropy in III_{1-x}Mn_{x}V Ferromagnets

We present a theory of magnetic anisotropy in ${\rm III}_{1-x}{\rm Mn}_{x}{\rm V}$ diluted magnetic semiconductors with carrier-induced ferromagnetism. The theory is based on four and six band envelope functions models for the valence band holes and a mean-field treatment of their exchange interactions with ${\rm Mn}^{++}$ ions. We find that easy-axis reorientations can occur as a function of temperature, carrier density $p$, and strain. The magnetic anisotropy in strain-free samples is predicted to have a $p^{5/3}$ hole-density dependence at small $p$, a $p^{-1}$ dependence at large $p$, and remarkably large values at intermediate densities. An explicit expression, valid at small $p$, is given for the uniaxial contribution to the magnetic anisotropy due to unrelaxed epitaxial growth lattice-matching strains. Results of our numerical simulations are in agreement with magnetic anisotropy measurements on samples with both compressive and tensile strains. We predict that decreasing the hole density in current samples will lower the ferromagnetic transition temperature, but will increase the magnetic anisotropy energy and the coercivity.Comment: 15 pages, 15 figure

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

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

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