161 research outputs found
Changes in the location of biodiversity–ecosystem function hot spots across the seafloor landscape with increasing sediment nutrient loading
Declining biodiversity and loss of ecosystem function threatens the ability of habitats to contribute ecosystem services. However, the form of the
relationship between biodiversity and ecosystem function (BEF) and how relationships change with environmental change is poorly understood. This limits our ability to predict the consequences of biodiversity loss on ecosystem function, particularly in real-world marine ecosystems that are species rich, and where multiple ecosystem functions are represented by multiple indicators. We investigated spatial variation in BEF relationships across a 300 000 m2 intertidal sandflat by nesting experimental manipulations of sediment pore water nitrogen concentration into sites with contrasting macrobenthic community composition. Our results highlight the significance of many different elements of biodiversity associated with environmental characteristics, community structure, functional diversity, ecological traits or particular species (ecosystem engineers) to important functions of coastal marine sediments (benthic oxygen consumption, ammonium pore water concentrations and flux across the sediment–water interface). Using the BEF relationships developed from our experiment, we demonstrate patchiness across a landscape in functional performance and the potential for changes in the location of functional hot and cold spots with increasing nutrient loading that have important implications for mapping and predicating change in functionality and the concomitant delivery of ecosystem services
Generally Covariant Actions for Multiple D-branes
We develop a formalism that allows us to write actions for multiple D-branes
with manifest general covariance. While the matrix coordinates of the D-branes
have a complicated transformation law under coordinate transformations, we find
that these may be promoted to (redundant) matrix fields on the transverse space
with a simple covariant transformation law. Using these fields, we define a
covariant distribution function (a matrix generalization of the delta function
which describes the location of a single brane). The final actions take the
form of an integral over the curved space of a scalar single-trace action built
from the covariant matrix fields, tensors involving the metric, and the
covariant distribution function. For diagonal matrices, the integral localizes
to the positions of the individual branes, giving N copies of the single-brane
action.Comment: 34 pages, LaTeX. v2: comments and refs adde
In situ soft sediment nutrient enrichment: A unified approach to eutrophication field experiments
Adding fertiliser to sediments is an established way of studying the effects of eutrophication but a lack of consistent
methodology, reporting on enrichment levels, or guidance on application rates precludes rigorous synthesis
and meta-analysis. We developed a simple enrichment technique then applied it to 28 sites across an intertidal
sandflat. Fertiliser application rates of 150 and 600 g N m−2 resulted in pore water ammonium concentrations
respectively 1–110 and 4–580 × ambient, with greater elevations observed in deeper (5–7 cm) than surface
(0–2 cm) sediments. These enrichment levels were similar to eutrophic estuaries and were maintained for at
least seven weeks. The high between-site variability could be partially explained by the sedimentary environment
and macrofaunal community (42%), but only at the high application rate. We suggest future enrichment
studies should be conducted in situ across large environmental gradients to incorporate real world complexity
and increase generality of conclusions
D-brane Black Holes: Large-N Limit and the Effective String Description
We address the derivation of the effective conformal field theory description
of the 5-dimensional black hole, modelled by a collection of D1- and D5-
branes, from the corresponding low energy U(Q_1)xU(Q_5) gauge theory. Finite
horizon size at weak coupling requires both Q_1 and Q_5 to be large. We derive
the result in the moduli space approximation (say for Q_1>Q_5) and appeal to
supersymmetry to argue its validity beyond weak coupling. As a result of a
combination of quenched Z_{Q_1} Wilson lines and a residual Weyl symmetry, the
low-lying excitations of the U(Q_1)xU(Q_5) gauge theory are described by an
effective N=4 superconformal field theory with c=6 in 1+1 dimensions, where the
space is a circle of radius RQ_1Q_5. We also discuss the appearance of a
marginal perturbation of the effective conformal field theory for large but
finite values of Q_5.Comment: 15 pages, LaTeX, some comments added to make the arguments in
sections 4 and 5 more precise, to appear in Phys. Lett.
Macrofaunal Functional Diversity Provides Resilience to Nutrient Enrichment in Coastal Sediments
The degradation of ecosystems is often associated
with losses of large organisms and the concomitant
losses of the ecological functions they mediate.
Conversely, the resilience of ecosystems to stress is
strongly influenced by faunal communities and
their impacts on processes. Denitrification in
coastal sediments is a process that may provide
ecosystem resilience to eutrophication by removing
excess bioavailable nitrogen. Here, we conducted a
large-scale field experiment to test the effect of
macrofaunal community composition on denitrification
in response to two levels of nutrient
enrichment at 28 sites across a biologically
heterogeneous sandflat. After 7 weeks of enrichment,
we measured denitrification enzyme activity
(DEA) along with benthic macrofaunal community
composition and environmental variables. We
normalised treatment site specific DEA values by
those in ambient sediments (DEACN) to reveal the
underlying response across the heterogeneous
landscape. Nutrient enrichment caused reductions
in DEACN as well as functional changes in the
community; these were both more pronounced
under the highest level of nutrient loading (on
average DEACN was reduced by 34%). The degree
of suppression of DEACN following moderate
nitrogen loading was mitigated by a key bioturbating
species, but following high nitrogen loading
(which reduced the key species density) the
abundance and diversity of other nutrient processing
species were the most important factors
alleviating negative effects. This study provides a
prime example of the context-dependent role of
biodiversity in maintaining ecosystem functioning,
underlining that different elements of biodiversity
can become important as stress levels increase. Our
results emphasise that management and conservation
strategies require a real-world understanding
of the community attributes that facilitate nutrient
processing and maintain resilience in coastal
ecosystems
Use of dispersion modelling for Environmental Impact Assessment of biological air pollution from composting: Progress, problems and prospects
© 2017 The Authors With the increase in composting as a sustainable waste management option, biological air pollution (bioaerosols) from composting facilities have become a cause of increasing concern due to their potential health impacts. Estimating community exposure to bioaerosols is problematic due to limitations in current monitoring methods. Atmospheric dispersion modelling can be used to estimate exposure concentrations, however several issues arise from the lack of appropriate bioaerosol data to use as inputs into models, and the complexity of the emission sources at composting facilities. This paper analyses current progress in using dispersion models for bioaerosols, examines the remaining problems and provides recommendations for future prospects in this area. A key finding is the urgent need for guidance for model users to ensure consistent bioaerosol modelling practices
The outer halos of elliptical galaxies
Recent progress is summarized on the determination of the density
distributions of stars and dark matter, stellar kinematics, and stellar
population properties, in the extended, low surface brightness halo regions of
elliptical galaxies. With integral field absorption spectroscopy and with
planetary nebulae as tracers, velocity dispersion and rotation profiles have
been followed to ~4 and ~5-8 effective radii, respectively, and in M87 to the
outer edge at ~150 kpc. The results are generally consistent with the known
dichotomy of elliptical galaxy types, but some galaxies show more complex
rotation profiles in their halos and there is a higher incidence of
misalignments, indicating triaxiality. Dynamical models have shown a range of
slopes for the total mass profiles, and that the inner dark matter densities in
ellipticals are higher than in spiral galaxies, indicating earlier assembly
redshifts. Analysis of the hot X-ray emitting gas in X-ray bright ellipticals
and comparison with dynamical mass determinations indicates that non-thermal
components to the pressure may be important in the inner ~10 kpc, and that the
properties of these systems are closely related to their group environments.
First results on the outer halo stellar population properties do not yet give a
clear picture. In the halo of one bright galaxy, lower [alpha/Fe] abundances
indicate longer star formation histories pointing towards late accretion of the
halo. This is consistent with independent evidence for on-going accretion, and
suggests a connection to the observed size evolution of elliptical galaxies
with redshift.Comment: 8 pages. Invited review to appear in the proceedings of "Galaxies and
their Masks" eds. Block, D.L., Freeman, K.C. & Puerari, I., 2010, Springer
(New York
Closed string tachyons, flips and conifolds
Following the analysis of tachyons and orbifold flips described in
hep-th/0412337, we study nonsupersymmetric analogs of the supersymmetric
conifold singularity and show using their toric geometry description that they
are nonsupersymmetric orbifolds of the latter. Using linear sigma models, we
see that these are unstable to localized closed string tachyon condensation and
exhibit flip transitions between their two small resolutions (involving
2-cycles), in the process mediating mild dynamical topology change. Our
analysis shows that the structure of these nonsupersymmetric conifolds as
quotients of the supersymmetric conifold obstructs the 3-cycle deformation of
such singularities, suggesting that these nonsupersymmetric conifolds decay by
evolving towards their stable small resolutions.Comment: Latex, 22 pgs, 2 figs. v4: matches JHEP version, 29 pgs, 3 figures,
more elaborate Introduction, various clarifications adde
Recommended from our members
Closed Strings and Moduli in AdS3/CFT2
String theory on AdS3×S3×T4 has 20 moduli. We investigate how the perturbative closed string spectrum changes as we move around this moduli space in both the RR and NSNS flux backgrounds. We find that, at weak string coupling, only four of the moduli affect the energies. In the RR background the only effect of these moduli is to change the radius of curvature of the background. On the other hand, in the NSNS background, the moduli introduce worldsheet interactions which enable the use of integrability methods to solve the spectral problem. Our results show that the worldsheet theory is integrable across the 20 dimensional moduli space
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