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Same Layer (same + layer)
Selected AbstractsEnhanced exoenzyme activities in sediments in the presence of deposit-feeding Chironomus riparius larvaeFRESHWATER BIOLOGY, Issue 9 2007PETER STIEFArticle first published online: 10 JUN 200 Summary 1. The combined effects of deposit-feeding, bioturbation and bioirrigation by benthic macrofauna on the enzymatic hydrolysis of organic matter were studied in microcosms. Chironomus riparius larvae (Insecta, Diptera) served as model macrofauna and stinging nettle leaves (Urtica dioica) were used as a detrital food source. 2. In the upper 10 mm of the sediment (the habitat of C. riparius larvae), the activities of several exoenzymes, the contents of several fractions of particulate organic matter (POM), and the concentrations of dissolved oxidants (O2, NO) were measured on a small scale. Fluorescent particles (luminophores) were used to quantify the vertical redistribution of particles within the same layer. 3. In control sediment, the addition of detrital food enhanced exoenzyme activities in the 0,2 mm layer only. In the presence of C. riparius larvae, exoenzyme activities increased to 10 mm depth. Further, the content of POM in the 0,2 mm layer was lower in the presence than in the absence of larvae, suggesting ingestion and subduction of the added detritus. After prolonged incubation without further food addition, exoenzyme activities returned close to background values in both treatments, whereas the vertical distribution of POM remained unchanged. 4. The overall penetration depth of O2 and NO into the sediment was greater in the presence than the absence of C. riparius, the differences being more pronounced after prolonged incubation. Locally high O2 and NO concentrations due to bioirrigation by C. riparius were measured deep in the sediment. Net downward transport of particles was observed only in the presence of C. riparius larvae and only at the beginning of the incubation. 5. I conclude that deposit-feeding and bioturbation by macrofauna can quickly remove freshly deposited POM from the sediment surface and transfer it to less oxygenated sites (i.e. animal guts and deep sediment layers). Bioirrigation also increases the availability of oxidants deep in the sediment. The oscillation of oxidant supply to POM particles by ingestion,egestion, burial and re-burial, and the intermittent bioirrigation of subsurface sediment, is probably the cause of the increased rate of organic matter hydrolysis, the rate-limiting step in mineralization. [source] MLCC: A new hash-chained mechanism for multicast source authenticationINTERNATIONAL JOURNAL OF COMMUNICATION SYSTEMS, Issue 9 2009H. Eltaief Abstract Asymmetric cryptography has been widely used to generate a digital signature for message authentication. However, such a strategy cannot be used for packet authentication. Neither the source nor the receiver will be capable of handling the computational cost of asymmetric cryptography. For unicast communication, the solution adopted is based on symmetric cryptography. Solutions based on symmetric cryptography do not scale for multicast communication. Several solutions have been reported to authenticate multicast streams, with the possibility of packet losses. Proposed solutions are based on the concept of signature amortization, where a single signature is amortized on several packets. In this paper we present a new mechanism for multicast data source authentication based on signature amortization. Multi-layers connected chains divides the packet stream into a multi-layer structure, where each layer is a two-dimensional matrix. The hash of a packet is included into a forward chain of packets within the same layer as well as a downward chain of packets across multiple layers. The values of the key parameters that influence the mechanism efficiency as well as its performance are selected following a mathematical analysis. Comparisons of performance results with the well-known efficient multi-chained stream signature scheme as well as a recently reported scheme multiple connected chains model show that the proposed mechanism achieves a stronger resistance to packet losses with low overhead and high authentication probability. Copyright © 2008 John Wiley & Sons, Ltd. [source] The stratification theory for plant coexistence promoted by one-sided competitionJOURNAL OF ECOLOGY, Issue 3 2009Takashi Kohyama Summary 1It is an essential feature of plants that leaves at higher levels have better access to light than those at lower levels. Thus, larger plants generally enjoy greater success in competing for light than smaller ones. We analyse the effect of such size-asymmetry, or one-sided competition, on the successful coexistence of plant species, using an analytically tractable model for stratified populations, in which a plant in the same layer exhibits the same crowding effect as any other, irrespective of species. 2A two-layer population that is reproductive in upper layer and juvenile in lower layer has a uniquely stable (plant-size-weighted) equilibrium density, as long as its fecundity is sufficient to compensate for its mortality rate. We also calculate a unique threshold lower-layer density of this layered population when there is no upper-layer plant. This threshold lower-layer density is larger than the weighted equilibrium density with upper layer, except for the case of perfect two-sided competition. 3A two-layer species can stably coexist with a one-layer, understorey species as a result of one-sided, but not two-sided competition. The coexistence condition is that the equilibrium density of the one-layer species lies between the threshold lower-layer density and the equilibrium density of the two-layer species. For an understorey species to coexist successfully with a two-layer species, any advantage in demographic performance, most prominently in a sufficiently high fecundity per plant must offset the disadvantage of living in dark conditions. 4Results from a model of multi-layer populations suggest that several species differing in terms of maximum layer and fecundity can coexist under conditions of one-sided competition. We demonstrate an example of the stable coexistence of eight species. The inter-specific trade-offs predicted by the model correspond to patterns observed in a rain forest. 5Synthesis. We propose a stratification theory that explains the generation and maintenance of the successful coexistence of plant species. Under the condition of one-sided competition, a canopy population that takes advantage of escaping from understorey competition shows an ability to invade an understorey with a density higher than its own equilibrium density, and which offers opportunities for an understorey population with high fecundity and/or shade tolerance to coexist. The predicted coexistence of species that share maximum canopy height is most pronounced for trees of tropical rain forests. [source] Comparison of the effects of pressure on three layered hydrates: a partially successful attempt to predict a high-pressure phase transitionACTA CRYSTALLOGRAPHICA SECTION B, Issue 6 2009Russell D. L. Johnstone We report the effect of pressure on the crystal structures of betaine monohydrate (BTM), l -cysteic acid monohydrate (CAM) and S -4-sulfo- l -phenylalanine monohydrate (SPM). All three structures are composed of layers of zwitterionic molecules separated by layers of water molecules. In BTM the water molecules make donor interactions with the same layer of betaine molecules, and the structure remains in a compressed form of its ambient-pressure phase up to 7.8,GPa. CAM contains bi-layers of l -cysteic acid molecules separated by water molecules which form donor interactions to the bi-layers above and below. This phase is stable up to 6.8,GPa. SPM also contains layers of zwitterionic molecules with the waters acting as hydrogen-bond donors to the layers above and below. SPM undergoes a single-crystal to single-crystal phase transition above 1,GPa in which half the water molecules reorient so as to form one donor interaction with another water molecule within the same layer. In addition, half of the S -4-sulfo- l -phenylalanine molecules change their conformation. The high-pressure phase is stable up to 6.9,GPa, although modest rearrangements in hydrogen bonding and molecular conformation occur at 6.4,GPa. The three hydrates had been selected on the basis of their topological similarity (CAM and SPM) or dissimilarity (BTM) with serine hydrate, which undergoes a phase transition at 5,GPa in which the water molecules change orientation. The phase transition in SPM shows some common features with that in serine hydrate. The principal directions of compression in all three structures were found to correlate with directions of hydrogen bonds and distributions of interstitial voids. [source] The alkali hypophosphites KH2PO2, RbH2PO2 and CsH2PO2ACTA CRYSTALLOGRAPHICA SECTION C, Issue 5 2004Marina I. Naumova The structures of the hypophosphites KH2PO2 (potassium hypophosphite), RbH2PO2 (rubidium hypophosphite) and CsH2PO2 (caesium hypophosphite) have been determined by single-crystal X-ray diffraction. The structures consist of layers of alkali cations and hypophosphite anions, with the latter bridging four cations within the same layer. The Rb and Cs hypophosphites are isomorphous. 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