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Sharp Drop (sharp + drop)
Selected AbstractsLinking herbivore-induced defences to population dynamicsFRESHWATER BIOLOGY, Issue 3 2006IRENE VAN DER STAP Summary 1.,Theoretical studies have shown that inducible defences have the potential to affect population stability and persistence in bi- and tritrophic food chains. Experimental studies on such effects of prey defence strategies on the dynamics of predator,prey systems are still rare. We performed replicated population dynamics experiments using the herbivorous rotifer Brachionus calyciflorus and four strains of closely related algae that show different defence responses to this herbivore. 2.,We observed herbivore populations to fluctuate at a higher frequency when feeding on small undefended algae. During these fluctuations minimum rotifer densities remained sufficiently high to ensure population persistence in all the replicates. The initial growth of rotifer populations in this treatment coincided with a sharp drop in algal density. Such a suppression of algae by herbivores was not observed in the other treatments, where algae were larger due to induced or permanent defences. In these treatments we observed rotifer population densities to first rise and then decline. The herbivore went extinct in all replicates with large permanently defended algae. The frequency of herbivore extinctions was intermediate when algae had inducible defences. 3.,A variety of alternative mechanisms could explain differential herbivore persistence in the different defence treatments. Our analysis showed the density and fraction of highly edible algal particles to better explain herbivore persistence and extinctions than total algal density, the fraction of highly inedible food particles or the accumulation of herbivore waste products or autotoxins. 4.,We argue that the rotifers require a minimum fraction and density of edible food particles for maintenance and reproduction. We conjecture that induced defences in algae may thus favour larger zooplankton species such as Daphnia spp. that are less sensitive to shifts in their food size spectrum, relative to smaller zooplankton species, such as rotifers and in this way contributes to the structuring of planktonic communities. [source] A quarter century of declining suspended sediment fluxes in the Mississippi River and the effect of the 1993 flood,HYDROLOGICAL PROCESSES, Issue 1 2010Arthur J. Horowitz Abstract Annual fluxes, flow-weighted concentrations and linear least squares trendline calculations for a number of long-term Mississippi River Basin (MRB) sampling sites covering 1981 through 2007, whilst somewhat ,noisy', display long-term patterns of decline. Annual flow-weighted concentration plots display the same long-term patterns of decline, but are less noisy because they reduce/eliminate variations due to interannual discharge differences. The declines appear greatest in the middle MRB, but also are evident elsewhere. The pattern for the lower Ohio River differs and may reflect ongoing construction at the Olmsted lock and dam that began in 1993 and currently is ongoing. The ,Great Flood of 1993' appears to have superimposed a step function (a sharp drop) on the long-term rate of decline in suspended sediment concentrations (SSC), annual fluxes and flow-weighted concentrations in the middle MRB at St Louis and Thebes, Missouri and Vicksburg, Mississippi, and in the lower MRB at St Francisville, Louisiana. Evidence for a step function at other sites is less substantial, but may have occurred. The step function appears to have resulted from losses in available (erodible) sediment, rather than to a reduction in discharge; hence, the MRB appears to be supply limited rather than discharge limited. These evaluations support the need for daily discharge and SSC data collections in the MRB to better address questions regarding long-term trends in sediment-related issues. This is apparent when the results for the Mississippi River at Thebes and St Louis sites are compared with those from other MRB sites where intensive (daily) data collections are lacking. Published in 2009 by John Wiley & Sons, Ltd. [source] Peroxidase activity, chlorophylls and antioxidant profile of two leaf vegetables (Solanum nigrum L. and Amaranthus cruentus L.) under six pretreatment methods before cookingINTERNATIONAL JOURNAL OF FOOD SCIENCE & TECHNOLOGY, Issue 1 2008Odunayo Clement Adebooye Summary The study evaluated the effects of six pretreatment methods before cooking on the peroxidase activity, chlorophyll and antioxidant profile of Amaranthus cruentus L. and Solanum nigrum L. The six pretreatments methods used were chopped only (raw sample) (coded M1); chopped and dried at 50 °C for 5 h (coded M2); chopped and squeezed in water (at room temperature) (coded M3); chopped and soaked in warm water (approximately 60 °C), then cooled and squeezed (coded M4); chopped and soaked in salt-treated water (approximately 20 g NaCl per litre of water) for 15 min, then squeezed (coded M5) and chopped and soaked in boiling water (100 °C), then cooled and squeezed (coded M6). The main effect of vegetable type and the main effect of pretreatment methods have significant effects (P 0.05) on the parameters measured, while the interaction of vegetable type and pretreatment methods have no significant effect on the parameters measured. Statistical analyses (P 0.05) showed that chlorophyll a and b occur in ratio 3:1 in the two vegetables, irrespective of the pretreatment imposed. Peroxidase activity test showed that A. cruentus, irrespective of the pretreatment imposed showed, no peroxidase activity, while S. nigrum showed high peroxidase activity for all the pretreatments except for M6. Results showed that there was a significantly (P 0.05) higher content of carotenoids in A. cruentus when compared with S. nigrum, while the total phenolics, total flavonoids and total tannins contents were higher in S. nigrum when compared with A. cruentus, irrespective of the pretreatment method used. For the two vegetables, the percentage losses in total carotenoids, phenolics, flavonoids and total tannins at M6 when compared with M1 were 53.3,60.5%, 55.6,57.1%, 62.4,63.6% and 66.1,73.5%, respectively. There was a sharp drop in the carotenoids, phenolics, flavonoids and tannins contents of the two vegetables at M4 and M6, with both treatments having closely similar values for each parameter. [source] Primordial magnetic fields in the post-recombination era and early reionizationMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2005Shiv K. Sethi ABSTRACT We explore the ways in which primordial magnetic fields influence the thermal and ionization history of the post-recombination Universe. After recombination, the Universe becomes mostly neutral, resulting also in a sharp drop in the radiative viscosity. Primordial magnetic fields can then dissipate their energy into the intergalactic medium via ambipolar diffusion and, for small enough scales, by generating decaying magnetohydrodynamics turbulence. These processes can significantly modify the thermal and ionization history of the post-recombination Universe. We show that the dissipation effects of magnetic fields, which redshifts to a present value B0= 3 × 10,9 G smoothed on the magnetic Jeans scale and below, can give rise to Thomson scattering optical depths ,, 0.1, although not in the range of redshifts needed to explain the recent Wilkinson Microwave Anisotropy Probe (WMAP) polarization observations. We also study the possibility that primordial fields could induce the formation of subgalactic structures for z, 15. We show that early structure formation induced by nanoGauss magnetic fields is potentially capable of producing the early reionization implied by the WMAP data. Future cosmic microwave background observations will be very useful to probe the modified ionization histories produced by primordial magnetic field evolution and constrain their strength. [source] Thermal hysteresis of the phase-transition temperature of single-crystal GdB6PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 1 2006M. Reiffers Abstract The phase transition of a single-crystal sample of GdB6, oriented along the ,111, axis using the temperature dependence of electrical resistivity , (T ), susceptibility , (T ) and heat capacity C (T ) under an applied magnetic field was studied. , (T ) has shown 2 anomalies , a sharp drop at TN1 = 15.4 K and a small maximum at TN2 = 9.1 K with thermal hysteresis effect. , (T ) shows the anomalies at both transition temperatures. C (T ) shows similar thermal hysteresis effect at TN2. The small maximum at TN2 decreases its position to lower temperatures with increasing magnetic field. The peak at TN1 is practically unaffected by an applied magnetic field up to 9 T. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Thellungiella halophila, a salt-tolerant relative of Arabidopsis thaliana, possesses effective mechanisms to discriminate between potassium and sodiumPLANT CELL & ENVIRONMENT, Issue 1 2004V. VOLKOV ABSTRACT Thellungiella halophila is a salt-tolerant close relative of Arabidopsis thaliana. Significant mRNA similarity was confirmed by hybridization of T. halophila mRNA with the A. thaliana GeneChip ATH1. To establish a platform for future molecular comparison of the two species several physiological mechanisms, which may confer high salt tolerance to T. halophila, were investigated. Determination of ion content in shoots and roots of A. thaliana and T. halophila indicated different strategies of ion uptake and translocation from root to shoot in the two species. During salt stress T. halophila accumulated less sodium than A. thaliana. Tissue concentrations of sodium and potassium showed negative correlation in A. thaliana but not in T. halophila. Electrophysiological experiments proved high potassium/sodium selectivity of root plasma membrane channels in T. halophila. In particular, voltage-independent currents were more selective for potassium in T. halophila than in A. thaliana. Single cell sampling of T. halophila leaves during salt exposure revealed increased concentrations of sodium and decreased concentrations of potassium in epidermal cells suggesting that this cell type could function to ensure storage of sodium and exchange of potassium with the rest of leaf. Application of salt resulted in a sharp drop of transpiration in A. thaliana. By contrast, transpiration in T. halophila responded more slowly and was only slightly inhibited by salt treatment, thus maintaining high water uptake and ion transport. [source] Extension rheology of liquid-crystalline solution/layered silicate hybridsPOLYMER ENGINEERING & SCIENCE, Issue 4 2010Gleb B. Vasilyev The extension rheology of polymer/layered silicate composites based on liquid-crystalline (LC) solution of hydroxypropylcellulose (HPC) in oligomeric polyethyleneglycole (PEG) was studied. The HPC concentration was 60 wt%. Extension experiments have been carried out for materials in the different phase states. Compositions containing Na-montmorillonite (MMT) were prepared by a two-stage method. Final systems may be attributed to solutions of HPC in PEG, which intercalates into MMT galleries. The ordered domain structure of the LC matrix as well as hydrogen-bonded network between HPC and PEG molecules significantly reduce deformation at break and provide a strong nonlinear viscoelastic behavior at extension. Appearance of isotropic phase in solutions leads to a sharp drop of the elongation viscosity. The introduction of clay into LC solution only slightly affects the viscosity value but significantly suppresses the strain-hardening scale. In contrast, loading in biphasic state of HPC-PEG solution with even small amount (1 wt%) of MMT leads to the drastic viscosity increase that does not change in further growth of the filler concentration. Elastic properties of the systems under study demonstrate the similar behavior. This effect likely is caused by the interrelationship between deformability of the LC domain structure and the network strength formed by the clay particles. POLYM. ENG. SCI., 2010. © 2009 Society of Plastics Engineers [source] The lack of large compact symmetric objectsASTRONOMISCHE NACHRICHTEN, Issue 2-3 2009P. Augusto Abstract In recent years, ,baby' (<103 yr) and ,young' (103,105 yr) radio galaxies have been found and classified, although their numbers are still small (tens). Also, they have many different names, depending on the type of survey and scientific context in which they were found: compact steep spectrum sources (CSS), giga-Hertz peaked spectrum sources (GPS) and compact-medium symmetric objects (C-MSO). The latter have the radio galaxy structure more obvious and correspond to the ,babies' (CSOs; < 1 kpc) and ,young' (MSOs; 1,15 kpc) radio galaxies. The log-size distribution of CSOs shows a sharp drop at 0.3 kpc. This trend continues through flat-spectrum MSOs (over the full 1,15 kpc size range). In order to find out if this lack of large CSOs and flat-spectrum MSOs is due to poor sampling (lack of surveys that probe efficiently the 0.3,15 kpc size range) and/or has physical meaning (e.g. if the lobes of CSOs expand as they grow and age, they might become CSSs, ,disappearing' from the flat-spectrum MSO statistics), we have built a sample of 157 flat-spectrum radio sources with structure on ,0.3,15 kpc scales. We are using new, archived and published data to produce and inspect hundreds of multi-frequency multi-instrument maps and models. We have already found 13 new secure CSO/MSOs. We expect to uncover ,30,40 new CSOs and MSOs, most on the 0.3,15 kpc size range, when our project is complete (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] | |||||||||||||