7,159 research outputs found
The species composition of Antarctic phytoplankton interpreted in terms of Tilman's competition theory
An attempt was made, to test for the impact of resource competition on Antarctic marine phytoplankton. According to theory, species composition near competitive equilibrium should be determined by the ratios of limiting resources. Enrichment bioassays identified silicon and nitrogen as limiting nutrients for some of the most important phytoplankton species during early austral summer in the region near the Antarctic Peninsula. Together with the generally acknowledged limiting resource light, this gave three meaningful ratios of essential resources (Si:N, Si:light, N:light) and one ratio of substitutable resources (NO sub(3):NH sub(4)). Phytoplankton species assemblages were found to be well separated by the ratios of the essential resources and by mixing depth
Early Thermal Evolution of Planetesimals and its Impact on Processing and Dating of Meteoritic Material
Radioisotopic ages for meteorites and their components provide constraints on
the evolution of small bodies: timescales of accretion, thermal and aqueous
metamorphism, differentiation, cooling and impact metamorphism. Realising that
the decay heat of short-lived nuclides (e.g. 26Al, 60Fe), was the main heat
source driving differentiation and metamorphism, thermal modeling of small
bodies is of utmost importance to set individual meteorite age data into the
general context of the thermal evolution of their parent bodies, and to derive
general conclusions about the nature of planetary building blocks in the early
solar system. As a general result, modelling easily explains that iron
meteorites are older than chondrites, as early formed planetesimals experienced
a higher concentration of short-lived nuclides and more severe heating.
However, core formation processes may also extend to 10 Ma after formation of
Calcium-Aluminum-rich inclusions (CAIs). A general effect of the porous nature
of the starting material is that relatively small bodies (< few km) will also
differentiate if they form within 2 Ma after CAIs. A particular interesting
feature to be explored is the possibility that some chondrites may derive from
the outer undifferentiated layers of asteroids that are differentiated in their
interiors. This could explain the presence of remnant magnetization in some
chondrites due to a planetary magnetic field.Comment: 24 pages, 9 figures, Accepted for publication as a chapter in
Protostars and Planets VI, University of Arizona Press (2014), eds. H.
Beuther, R. Klessen, C. Dullemond, Th. Hennin
Signatures of gravitational fixed points at the LHC
We study quantum-gravitational signatures at the CERN Large Hadron Collider (LHC) in the context of theories with extra spatial dimensions and a low fundamental Planck scale in the TeV range. Implications of a gravitational fixed point at high energies are worked out using Wilson¿s renormalization group. We find that relevant cross sections involving virtual gravitons become finite. Based on gravitational lepton pair production we conclude that the LHC is sensitive to a fundamental Planck scale of up to 6 TeV
Ecological mechanisms associated with the positive diversity-productivity relationship in an N-limited grassland
Phytoplankton competition along a gradient of dilution rates
Natural phytoplankton from Lake Constance was used for chemostat competition experiments performed at a variety of dilution rates. In the first series at high Si:P ratios and under uniform phosphorus limitation for all species, Synedra acus outcompeted all other species at all dilution rates up to 1.6 d-1, only at the highest dilution rate (2.0 d-1) Achnanthes minutissima was successful. In the second series in the absence of any Si a green algal replacement series was found, with Mougeotia thylespora dominant at the lowest dilution rates, Scenedesmus acutus at the intermediate ones, and Chlorella minutissima at the highest ones. The outcome of interspecific competition was not in contradiction with the Monod kinetics of P-limited growth of the five species, but no satisfactorily precise prediction of competitive performance can be derived from the Monod kinetics because of insufficient precision in the estimate of k s
An experimental study of the effects of pH and nitrogen on grassland vegetation
The response of plant species to a full-factorial experimental manipulation of soil pH and soil N in a Minnesota old field is compared to patterns observed in a chronosequence of old fields. Agropyron repens reached its greatest relative abundance in low pH but high N treatments. Schizachyrium scoparium was less affected by N, but was most abundant at low pH. Poa pratensis had its greatest relative abundance in high N and high pH treatments. Total biomass and species richness were also influenced by the treatments. Thus, the common plant species were differentiated in their responses to pH and N manipulations. However, these pH and N responses did not explain the patterns observed in the old field chronosequence unless the differential colonization abilities of the species and the relationships between soil pH, soil N and successional age were considered
Interaction flow method for many-fermion systems
We propose an interaction flow scheme that sums up the perturbation expansion
of many-particle systems by successively increasing the interaction strength.
It combines the unbiasedness of renormalization group methods with the
simplicity of straight-forward perturbation theory. Applying the scheme to
fermions in one dimension and to the two-dimensional Hubbard model we find that
at one-loop level and low temperatures there is ample agreement with previous
one-loop renormalization group approaches. We furthermore present results for
the momentum-dependence of spin, charge and pairing interactions in the
two-dimensional Hubbard model.Comment: 14 pages, 14 figure
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