1,996 research outputs found
Recommended from our members
Diffraction by a half-plane in a moving fluid
In the following work we solve the problem of the diffraction of a plane sound wave by an impedance half-plane in a moving fluid. Expressions for the total far field are derived for both the leading edge and trailing edge situations. In the trailing edge situation the problem has the added complication of a trailing vortex sheet or wake. Hence a Kutta-Joukowski edge condition is imposed to ensure that the fluid velocity is finite at the edge and to obtain a unique solution to the problem
On the roles of cell size and trophic strategy in North Atlantic diatom and dinoflagellate communities
We have examined the inter- and intra-group seasonal succession of 113 diatom and dinoflagellate taxa, as
surveyed by the Continuous Plankton Recorder (CPR) in the North Atlantic, by grouping taxa according to two
key functional traits: cell size (mg C cell21) and trophic strategy (photoautotrophy, mixotrophy, or heterotrophy).
Mixotrophic dinoflagellates follow photoautotrophic diatoms but precede their obligate heterotrophic
counterparts in the succession because of the relative advantages afforded by photosynthesizing when light and
nutrients are available in spring. The mean cell size of the sampled diatoms is smallest in the summer, likely
because of the higher specific nutrient affinity of smaller relative to larger cells. Contrastingly, we hypothesize that
mixotrophy diminishes the size selection based on nutrient limitation and accounts for the lack of a seasonal size
shift among surveyed dinoflagellates. Relatively small, heterotrophic dinoflagellates (mg C cell21 , 1023) peak
after other, larger dinoflagellates, in part because of the increased abundance of their small prey during nutrientdeplete
summer months. The largest surveyed diatoms (mg C cell21 . 1022) bloom later than others, and we
hypothesize that this may be because of their relatively slow maximum potential growth rates and high internal
nutrient storage, as well as to the slower predation of these larger cells. The new trait database and analysis
presented here helps translate the taxonomic information of the CPR survey into metrics that can be directly
compared with trait-based models
A unified hyperbolic formulation for viscous fluids and elastoplastic solids
We discuss a unified flow theory which in a single system of hyperbolic
partial differential equations (PDEs) can describe the two main branches of
continuum mechanics, fluid dynamics, and solid dynamics. The fundamental
difference from the classical continuum models, such as the Navier-Stokes for
example, is that the finite length scale of the continuum particles is not
ignored but kept in the model in order to semi-explicitly describe the essence
of any flows, that is the process of continuum particles rearrangements. To
allow the continuum particle rearrangements, we admit the deformability of
particle which is described by the distortion field. The ability of media to
flow is characterized by the strain dissipation time which is a characteristic
time necessary for a continuum particle to rearrange with one of its
neighboring particles. It is shown that the continuum particle length scale is
intimately connected with the dissipation time. The governing equations are
represented by a system of first order hyperbolic PDEs with source terms
modeling the dissipation due to particle rearrangements. Numerical examples
justifying the reliability of the proposed approach are demonstrated.Comment: 6 figure
Integration of temporal environmental variation by the marine plankton community
Theory and observations suggest that low frequency variation in marine plankton populations, or red noise, may arise through cumulative integration of white noise atmospheric forcing by the ocean and its ampliïŹcation within food webs. Here, we revisit evidence for the integration of stochastic atmospheric variations by comparing the power spectra of time series of atmospheric and oceanographic conditions to the population dynamics of 150 plankton taxa at Station L4 in the Western English Channel. The power spectra of oceanographic conditions (sea surface temperature, surface nitrate) are redder than those of atmospheric forcing (surface wind stress, net heat ïŹuxes) at Station L4. However, plankton populations have power spectral slopes across trophic levels and body sizes that are redder than atmospheric forcing but whiter than oceanographic conditions. While zooplankton have redder spectral slopes than phytoplankton, there is no signiïŹcant relationship between power spectral slope and body size or generation length. Using a predatorâprey model, we show that the whitening of plankton time series relative to oceanographic conditions arises from noisy plankton bloom dynamics in this strongly seasonal system. The model indicates that, for typical predatorâprey interactions, where the predator is on average 10 times longer than the prey, grazing leads to a modest reddening of phytoplankton variability by their larger and longer lived zooplankton consumers. Our ïŹndings suggest that, beyond extrinsic forcing by the environment, predatorâprey interactions play a role in inïŹuencing the power spectra of time series of plankton populations
Analytic Metaphysics versus Naturalized Metaphysics: The Relevance of Applied Ontology
The relevance of analytic metaphysics has come under criticism: Ladyman & Ross, for instance, have suggested do discontinue the field. French & McKenzie have argued in defense of analytic metaphysics that it develops tools that could turn out to be useful for philosophy of physics. In this article, we show first that this heuristic defense of metaphysics can be extended to the scientific field of applied ontology, which uses constructs from analytic metaphysics. Second, we elaborate on a parallel by French & McKenzie between mathematics and metaphysics to show that the whole field of analytic metaphysics, being useful not only for philosophy but also for science, should continue to exist as a largely autonomous field
Anthropogenic climate change impacts on copepod trait biogeography
Copepods are among the most abundant marine metazoans and form a key link between marine primary producers, higher trophic levels, and carbon sequestration pathways. Climate change is projected to change surface ocean temperature by up to 4°C in the North Atlantic with many associated changes including slowing of the overturning circulation, areas of regional freshening, and increased salinity and reductions in nutrients available in the euphotic zone over the next century. These changes will lead to a restructuring of phytoplankton and zooplankton communities with cascading effects throughout the food web. Here we employ observations of copepods, projected changes in ocean climate, and species distribution models to show how climate change may affect the distribution of copepod species in the North Atlantic.
On average species move northeast at a rate of 14.1 km decade. Species turnover in copepod communities will range from 5% to 75% with the highest turnover rates concentrated in regions of pronounced temperature increase and decrease. The changes in species range vary according to copepod traits with the largest effects found to occur in the cooling, freshening area in the subpolar North Atlantic south of Greenland and in an area of significant warming along the Scotian shelf. Large diapausing copepods (>2.5 mm) which are higher in lipids and a crucial food source for
whales, may have an advantage in the cooling waters due to their life-history strategy that facilitates their survival in the arctic environment. Carnivorous copepods show a basin wide increase in species richness and show significant habitat area increases when their distribution moves poleward while herbivores see significant habitat
area losses. The trait-specific effects highlight the complex consequences of climate change for the marine food web
Red Queen Dynamics with Non-Standard Fitness Interactions
Antagonistic coevolution between hosts and parasites can involve rapid fluctuations of genotype frequencies that are known as Red Queen dynamics. Under such dynamics, recombination in the hosts may be advantageous because genetic shuffling can quickly produce disproportionately fit offspring (the Red Queen hypothesis). Previous models investigating these dynamics have assumed rather simple models of genetic interactions between hosts and parasites. Here, we assess the robustness of earlier theoretical predictions about the Red Queen with respect to the underlying host-parasite interactions. To this end, we created large numbers of random interaction matrices, analysed the resulting dynamics through simulation, and ascertained whether recombination was favoured or disfavoured. We observed Red Queen dynamics in many of our simulations provided the interaction matrices exhibited sufficient âantagonicityâ. In agreement with previous studies, strong selection on either hosts or parasites favours selection for increased recombination. However, fast changes in the sign of linkage disequilibrium or epistasis were only infrequently observed and do not appear to be a necessary condition for the Red Queen hypothesis to work. Indeed, recombination was often favoured even though the linkage disequilibrium remained of constant sign throughout the simulations. We conclude that Red Queen-type dynamics involving persistent fluctuations in host and parasite genotype frequencies appear to not be an artefact of specific assumptions about host-parasite fitness interactions, but emerge readily with the general interactions studied here. Our results also indicate that although recombination is often favoured, some of the factors previously thought to be important in this process such as linkage disequilibrium fluctuations need to be reassessed when fitness interactions between hosts and parasites are complex
Background independent action for double field theory
Double field theory describes a massless subsector of closed string theory
with both momentum and winding excitations. The gauge algebra is governed by
the Courant bracket in certain subsectors of this double field theory. We
construct the associated nonlinear background-independent action that is
T-duality invariant and realizes the Courant gauge algebra. The action is the
sum of a standard action for gravity, antisymmetric tensor, and dilaton fields
written with ordinary derivatives, a similar action for dual fields with dual
derivatives, and a mixed term that is needed for gauge invariance.Comment: 45 pages, v2: minor corrections, refs. added, to appear in JHE
Cirsium species show disparity in patterns of genetic variation at their range-edge, despite similar patterns of reproduction and isolation
Genetic variation was assessed across the UK geographical range of Cirsium acaule and Cirsium heterophyllum. A decline in genetic diversity and increase in population divergence approaching the range edge of these species was predicted based on parallel declines in population density and seed production reported seperately. Patterns were compared with UK populations of the widespread Cirsium arvense.Populations were sampled along a latitudinal transect in the UK and genetic variation
assessed using microsatellite markers. Cirsium acaule
shows strong isolation by distance, a significant decline in diversity and an increase in divergence among range-edge populations. Geographical structure is also evident in
C. arvense, whereas no such patterns are seen in C.heterophyllum. There is a major disparity between patterns of genetic variation in C. acaule and
C. heterophyllum despite very similar patterns in seed production and population isolation in these species. This suggests it may be misleading to make assumptions
about the geographical structure of genetic variation within species based solely on the present-day reproduction and distribution of populations
Numerical continuation in nonlinear experiments using local Gaussian process regression
Control-based continuation (CBC) is a general and systematic method to probe the dynamics of nonlinear experiments. In this paper, CBC is combined with a novel continuation algorithm that is robust to experimental noise and enables the tracking of geometric features of the response surface such as folds. The method uses Gaussian process regression to create a local model of the response surface on which standard numerical continuation algorithms can be applied. The local model evolves as continuation explores the experimental parameter space, exploiting previously captured data to actively select the next data points to collect such that they maximise the potential information gain about the feature of interest. The method is demonstrated experimentally on a nonlinear structure featuring harmonically coupled modes. Fold points present in the response surface of the system are followed and reveal the presence of an isola, i.e. a branch of periodic responses detached from the main resonance peak
- âŠ