21,891 research outputs found
Flow alteration-ecology relationships in Ozark Highland streams: Consequences for fish, crayfish and macroinvertebrate assemblages
We examined flowalteration-ecology relationships in benthic macroinvertebrate, fish, and crayfish assemblages in Ozark Highland streams, USA, over two years with contrasting environmental conditions, a drought year (2012) and a flood year (2013). We hypothesized that: 1) there would be temporal variation in flow alteration-ecology relationships between the two years, 2) flow alteration-ecology relationshipswould be stronger during the drought year vs the flood year, and 3) fish assemblages would show the strongest relationships with flow alteration. We used a quantitative richest-targeted habitat (RTH) method and a qualitative multihabitat (QMH) method to collect macroinvertebrates at 16 USGS gaged sites during both years. We used backpack electrofishing to sample fish and crayfish at 17 sites in 2012 and 11 sites in 2013.Weused redundancy analysis to relate biological response metrics, including richness, diversity, density, and community-based metrics, to flow alteration.We found temporal variation in flow alteration-ecology relationships for all taxa, and that relationships differed greatly between assemblages. We found relationships were stronger for macroinvertebrates during the drought year but not for other assemblages, and that fish assemblage relationships were not stronger than the invertebrate taxa. Magnitude of average flow, frequency of high flow, magnitude of high flow, and duration of high flow were the most important categories of flow alteration metrics across taxa. Alteration of high and average flows was more important than alteration of low flows. Of 32 important flow alteration metrics across years and assemblages, 19 were significantly altered relative to expected values. Ecological responses differed substantially between drought and flood years, and this is likely to be exacerbated with predicted climate change scenarios. Differences in flow alteration-ecology relationships among taxonomic groups and temporal variation in relationships illustrate that a complex suite of variables should be considered for effective conservation of stream communities related to flow alteration
Could There Be A Hole In Type Ia Supernovae?
In the favored progenitor scenario, Type Ia supernovae arise from a white
dwarf accreting material from a non-degenerate companion star. Soon after the
white dwarf explodes, the ejected supernova material engulfs the companion
star; two-dimensional hydrodynamical simulations by Marietta et. al. show that,
in the interaction, the companion star carves out a conical hole of opening
angle 30-40 degrees in the supernova ejecta. In this paper we use
multi-dimensional Monte Carlo radiative transfer calculations to explore the
observable consequences of an ejecta-hole asymmetry. We calculate the variation
of the spectrum, luminosity, and polarization with viewing angle for the
aspherical supernova near maximum light. We find that the supernova looks
normal from almost all viewing angles except when one looks almost directly
down the hole. In the latter case, one sees into the deeper, hotter layers of
ejecta. The supernova is relatively brighter and has a peculiar spectrum
characterized by more highly ionized species, weaker absorption features, and
lower absorption velocities. The spectrum viewed down the hole is comparable to
the class of SN 1991T-like supernovae. We consider how the ejecta-hole
asymmetry may explain the current spectropolarimetric observations of SNe Ia,
and suggest a few observational signatures of the geometry. Finally, we discuss
the variety currently seen in observed SNe Ia and how an ejecta-hole asymmetry
may fit in as one of several possible sources of diversity.Comment: 11 pages, 9 figures, submitted to Ap
Tightness for the interface of the one-dimensional contact process
We consider a symmetric, finite-range contact process with two types of
infection; both have the same (supercritical) infection rate and heal at rate
1, but sites infected by Infection 1 are immune to Infection 2. We take the
initial configuration where sites in have Infection 1 and sites
in have Infection 2, then consider the process defined as
the size of the interface area between the two infections at time . We show
that the distribution of is tight, thus proving a conjecture posed by
Cox and Durrett in [Bernoulli 1 (1995) 343--370].Comment: Published in at http://dx.doi.org/10.3150/09-BEJ236 the Bernoulli
(http://isi.cbs.nl/bernoulli/) by the International Statistical
Institute/Bernoulli Society (http://isi.cbs.nl/BS/bshome.htm
Pion Photoproduction Amplitude Relations in the 1/N_c Expansion
We derive expressions for pion photoproduction amplitudes in the 1/N_c
expansion of QCD, and obtain linear relations directly from this expansion that
relate electromagnetic multipole amplitudes at all energies. The leading-order
relations in 1/N_c compare favorably with available data, while the
next-to-leading order relations seem to provide only a small improvement.
However, when resonance parameters are compared directly, the agreement at
O(1/N_c) or O(1/N_c^2) is impressive.Comment: 19 pages, ReVTeX, 50 eps files combine into 5 compound figure
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