544 research outputs found
The mycorrhizal status of Phragmites australis in several polluted soils and sediments of an industrialised region of Northern Portugal
Roots of Phragmites australis from three polluted soils and sediments (a periodically flooded stream bank containing organic pollutants, a high-pH drying sedimentation pond and an acidic, periodically flooded sand polluted by industrial effluents) were sampled over a 1-year cycle of plant growth to assess the degree of colonisation by arbuscular mycorrhizal fungi (AMF). At the dry sedimentation pond, root samples of Juncus effusus and Salix atrocinerea were also taken to assess the presence of AMF throughout the year. Root colonisation was low (<5% root length colonised) but arbuscule presence peaked in P. australis during the spring and autumn prior to flowering. These changes in arbuscule abundance were also seen in a parallel greenhouse trial using seed taken from one of the sites. Roots of J. effusus contained mainly vesicular colonisation but arbuscule activity peaked during the winter months (December–March). S. atrocinerea roots were found to be ectomycorrhizal throughout the year but the fine feeder roots were colonised by AMF. The results confirm that semi-aquatics, like P. australis, can become arbuscular mycorrhizal but that this status changes during the year depending on soil moisture content and plant phenology. The influence of AMF in these polluted soils is uncertain but the potential exists to establish a more diverse plant ecosystem during the landscaping of these areas (phytostabilisation) by management of adapted plant and AMF ecotypes
Generalised Elliptic Functions
We consider multiply periodic functions, sometimes called Abelian functions,
defined with respect to the period matrices associated with classes of
algebraic curves. We realise them as generalisations of the Weierstras
P-function using two different approaches. These functions arise naturally as
solutions to some of the important equations of mathematical physics and their
differential equations, addition formulae, and applications have all been
recent topics of study.
The first approach discussed sees the functions defined as logarithmic
derivatives of the sigma-function, a modified Riemann theta-function. We can
make use of known properties of the sigma function to derive power series
expansions and in turn the properties mentioned above. This approach has been
extended to a wide range of non hyperelliptic and higher genus curves and an
overview of recent results is given.
The second approach defines the functions algebraically, after first
modifying the curve into its equivariant form. This approach allows the use of
representation theory to derive a range of results at lower computational cost.
We discuss the development of this theory for hyperelliptic curves and how it
may be extended in the future.Comment: 16 page
Threat of Predation Does Not Affect Crassostrea virginica Filtration
Biotic interactions can structure ecological communities and influence ecosystem functioning. As ecosystem engineers and filter feeders, bivalves often have disproportionately large effects on ecosystem functioning. They also utilize numerous morphological and behavioral responses to reduce predation, which can include changes in their filtration rates. To test the response of Crassostrea virginica filtration rates to the presence of predators, juvenile and adult oysters were separately exposed to varying types of predation risk from Callinectes sapidus and Panopeus herbstii in outdoor mesocosms. Water column chlorophyll a concentrations and crab behavior were measured over the duration of the experiment. Predation risk had no effect on oyster reef drawdown of chlorophyll a, which suggests that this important ecosystem function of oyster reefs is not mediated by behaviorally induced predator effects. Therefore, efforts to model how oyster predators influence filtration rates and associated ecosystem services should focus primarily on the factors that influence oyster mortality rather than predator effects on oyster behavior
Zurek-Kibble domain structures: The Dynamics of Spontaneous Vortex formation in Annular Josephson Tunnel Junctions
Phase transitions executed in a finite time show a domain structure with
defects, that has been argued by Zurek and Kibble to depend in a characteristic
way on the quench rate. In this letter we present an experiment to measure the
Zurek-Kibble scaling exponent sigma. Using symmetric and long Josephson Tunnel
Junctions, for which the predicted index is sigma = 0.25, we find sigma = 0.27
+/- 0.05. Further, there is agreement with the ZK prediction for the overall
normalisation.Comment: To be published in Phys. Rev. Lett
Collective excitations of trapped Bose condensates in the energy and time domains
A time-dependent method for calculating the collective excitation frequencies
and densities of a trapped, inhomogeneous Bose-Einstein condensate with
circulation is presented. The results are compared with time-independent
solutions of the Bogoliubov-deGennes equations. The method is based on
time-dependent linear-response theory combined with spectral analysis of
moments of the excitation modes of interest. The technique is straightforward
to apply, is extremely efficient in our implementation with parallel FFT
methods, and produces highly accurate results. The method is suitable for
general trap geometries, condensate flows and condensates permeated with vortex
structures.Comment: 6 pages, 3 figures small typos fixe
Variations in water use by a mature mangrove of Avicennia germinans, French Guiana
In the tropical intertidal zones, little is known on water uptake by mangroves. Transpiration rates are generally measured at leaf level, but few studies exist on water use at tree or stand levels. The objective of this study was to measure sap flow in trees of different sizes to appreciate the range of variation in water use that may exist in a site dominated by 80% mature Avicennia germinans. The results showed that from the dry to the wet season the mean water use increased from 3.2 to 5.3 dm3 d−1 in small trees (DBH ∼ 13 cm), from 11.5 to 30.8 dm3 d−1 in medium trees (∼24 cm) and from 40.8 to 64.1 dm3 d−1 in large ones (∼45 cm). Sapwood remained active up to a depth of 8 cm with radial variations within the stem. Weak correlations were obtained with VPD and net radiation. This study confirmed that transpiration was larger under low levels of salinity. Water use at stand level (∼1900 living stems ha−1) was estimated to be in the range of 5.8 to 11.8 m3 ha−1 d−1 according to the season
Recommended from our members
Synthesis of a base-stock for electrical insulating fluid based on palm kernel oil
This report presents a method for synthesizing base-stock for green industrial product from a vegetable oil with a high composition of unsaturated fatty acids. Epoxy methyl ester of palm kernel oil was synthesized from laboratory purified palm kernel oil using a two-step reaction and the products were used as a base-stock for green electrical insulation fluid. Epoxidized palm kernel oil was first prepared through epoxidation reaction involving purified palm kernel oil, acetic acid and hydrogen peroxide in the presence of amberlite as catalyst which lasted for 4 h. It was then followed by transesterification reaction involving the epoxidized product and methanol in the presence of sodium hydroxide as catalyst to synthesize the corresponding epoxy methyl ester. The thermal and electrical breakdown properties of the epoxy methyl ester demonstrated significantly improved properties for its use as raw material for bio-based industrial products such as electrical insulation fluids
Dark soliton states of Bose-Einstein condensates in anisotropic traps
Dark soliton states of Bose-Einstein condensates in harmonic traps are
studied both analytically and computationally by the direct solution of the
Gross-Pitaevskii equation in three dimensions. The ground and self-consistent
excited states are found numerically by relaxation in imaginary time. The
energy of a stationary soliton in a harmonic trap is shown to be independent of
density and geometry for large numbers of atoms. Large amplitude field
modulation at a frequency resonant with the energy of a dark soliton is found
to give rise to a state with multiple vortices. The Bogoliubov excitation
spectrum of the soliton state contains complex frequencies, which disappear for
sufficiently small numbers of atoms or large transverse confinement. The
relationship between these complex modes and the snake instability is
investigated numerically by propagation in real time.Comment: 11 pages, 8 embedded figures (two in color
Entangled coherent states under dissipation
We study the evolution of entangled coherent states of the two quantized
electromagnetic fields under dissipation. Characteristic time scales for the
decay of the negativity are found in the case of large values of the phase
space distance among the states of each mode. We also study how the
entanglement emerges among the reservoirs.Comment: 13 pages and 4 figures, published versio
Search for Spontaneous Nucleation of Magnetic Flux During Rapid Cooling of YBCO films Through Tc
We describe an experimental search for spontaneous formation of flux lines
during a rapid quench of thin YBaCuO films through Tc. This effect is expected
according to the Kibble-Zurek mechanism of a creation of topological defects of
the order parameter during a symmetry breaking phase transition. Spontaneously
formed vortices were previously observed in superfluid 3He, while a similar
experiment in superfluid 4He gave negative results. Using a high Tc SQUID, we
measured both the magnetic flux in the sample during a quench with a
sensitivity of 20 phi-0/cm^2, and the field noise which one would expect from
flux lines pinned in the film. The sensitivity was sufficient to detect
spontaneous flux at a level corresponding to 10^(-3) of the prediction. Within
our resolution, we saw no evidence for this effect.Comment: Manuscript and 4 figure
- …