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Abstract. Populations of Botryllus schlosseri PALLAS 1772. a cosmopolitan colonial ascidian. were examined on the undersurface of stones for the first time in three adjoining localities along the Israeli Mediterranean coast. A two-year study of 1589 stones which were inhabited by 1345 colonies, revealed that colony coverage was < 1.5 % of the total available substrate area; values were highest in spring, lowest in winter. The two years differed significantly in the number of colonies · m 2 , and these numbers were correlated with mean seawater temperatures. Most of the stones contained 1–5 Botryllus colonies year-round; no significant correlation was found between number and stone size. The brown morph was dominant 80 % at all three localities. The distribution of other colour morphs differed between localities. Reproductive colonies were either hermaphroditic or contained male gonads only. Peak reproduction was in the spring, but did not correlate with seawater temperatures. While colony size ranged between I and 1I55zooids, sexually mature colonies consisted of 171–273 zooids on the average, compared with 37–90 zooids for sterile colonies. The three populations differed significantly in several ecological and life history characteristics. This further confirmed past studies indicating that Botryllus populations are characteristically divided into local subpopulations exhibiting microgeographic differences in life history patterns. The results are compared with the accumulated data on other world-wide Botryllus populations residing in other habitats. 相似文献
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The coupled mechanics of fluid-filled granular media controls the physics of many Earth systems, for example saturated soils, fault gouge, and landslide shear zones. It is well established that when the pore fluid pressure rises, the shear resistance of fluid-filled granular systems decreases, and, as a result, catastrophic events such as soil liquefaction, earthquakes, and accelerating landslides may be triggered. Alternatively, when the pore pressure drops, the shear resistance of these geosystems increases. Despite the great importance of the coupled mechanics of grain–fluid systems, the basic physics that controls this coupling is far from understood. Fundamental questions that must be addressed include: what are the processes that control pore fluid pressurization and depressurization in response to deformation of the granular skeleton? and how do variations of pore pressure affect the mechanical strength of the grains skeleton? To answer these questions, a formulation for the pore fluid pressure and flow has been developed from mass and momentum conservation, and is coupled with a granular dynamics algorithm that solves the grain dynamics, to form a fully coupled model. The pore fluid formulation reveals that the evolution of pore pressure obeys viscoelastic rheology in response to pore space variations. Under undrained conditions elastic-like behavior dominates and leads to a linear relationship between pore pressure and overall volumetric strain. Viscous-like behavior dominates under well-drained conditions and leads to a linear relationship between pore pressure and volumetric strain rate. Numerical simulations reveal the possibility of liquefaction under drained and initially over-compacted conditions, which were often believed to be resistant to liquefaction. Under such conditions liquefaction occurs during short compactive phases that punctuate the overall dilative trend. In addition, the previously recognized generation of elevated pore pressure under undrained compactive conditions is observed. Simulations also show that during liquefaction events stress chains are detached, the external load becomes completely supported by the pressurized pore fluid, and shear resistance vanishes. 相似文献
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Motion of a large rock mass down a slope can either take the form of a catastrophic landslide, or can exhibit self-stabilization, where the mass arrests on the slope, after moving only a short distance. In order to study the parameters that control the stability of the sliding process, a thermo-poro-elastic model is investigated both numerically and analytically. This model assumes that the physics controlling sliding stability is dominated by coupling between frictional heating, thermal pressurization and sliding velocity: A temperature increase due to frictional heating causes thermal pressurization within a fluid-saturated shear zone. The pressure rise leads to reduction of frictional resistance, which in turns leads to higher sliding velocities. Results demonstrate that the permeability of the sliding mass is an important factor in controlling the sliding stability: Low permeabilities lead to catastrophic landslides, by allowing high pore pressure to develop and friction to be reduced. In contrast, high permeabilities lead to rapid arrest by promoting pore pressure diffusion. A pore pressure – velocity phase plane is described, divided by a separatrix distinguishing between catastrophic and arrested sliding. In this phase plane minute changes in permeability dictate a bifurcation in the dynamics of landslides. A sensitivity study reveals that various geometrical and mechanical parameters can control the sliding process dynamics in a similar manner. It is hypothesized that a third sliding regime observed in nature, creep sliding, may be generated by a sequence of arrested events, where the number of arrested events/unit time dictates the apparent creep velocity. 相似文献
4.
Shark interactions in pelagic longline fisheries 总被引：1，自引：0，他引：1
Eric Gilman Shelley Clarke Nigel Brothers Joanna Alfaro-Shigueto John Mandelman Jeff Mangel Samantha Petersen Susanna Piovano Nicola Thomson Paul Dalzell Miguel Donoso Meidad Goren Tim Werner 《Marine Policy》2008
Substantial ecological, economic and social problems result from shark interactions in pelagic longline fisheries. Improved understanding of industry attitudes and practices towards shark interactions assists with managing these problems. Information on fisher knowledge and new strategies for shark avoidance may benefit sharks and fishers. A study of 12 pelagic longline fisheries from eight countries shows that incentives to avoid sharks vary along a continuum, based on whether sharks represent an economic disadvantage or advantage. Shark avoidance practices are limited, including avoiding certain areas, moving when shark interaction rates are high, using fish instead of squid for bait and deeper setting. Some conventionally employed fishing gear and methods used to target non-shark species contribute to shark avoidance. Shark repellents hold promise; more research and development is needed. Development of specifically designed equipment to discard sharks could improve shark post release survival prospects, reduce gear loss and improve crew safety. With expanding exploitation of sharks for fins and meat, improved data collection, monitoring and precautionary shark management measures are needed to ensure that shark fishing mortality levels are sustainable. 相似文献
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P. Breen L.A. Robinson S.I. Rogers A.M. Knights G. Piet T. Churilova P. Margonski N. Papadopoulou E. Akoglu A. Eriksson Z. Finenko V. Fleming-Lehtinen B. Galil F. Goodsir M. Goren O. Kryvenko J.M. Leppanen V. Markantonatou S. Moncheva T. Oguz L. Paltriguera K. Stefanova F. Timofte F. Thomsen 《Marine Policy》2012
7.
A characteristic hallmark of the Pre‐Pottery Neolithic B (PPNB) in the southern Levant was the extensive use of lime plaster for architectural and other purposes. Yet no obvious kilns have been identified in archaeological contexts. Here we present details of an experimental pit‐kiln modeling lime‐plaster production based on observed burnt stone accumulations in pits at the PPNB site of Kfar HaHoresh in the lower Galilee. The experimental kiln was loaded in layers with ∼500 kg of limestone (pebbles and stones) and ∼1000 kg of fuel (branches and dung). Fired for 24 hours, and reaching a maximum 870°C, the kiln yielded almost 250 kg of quicklime (calcium oxide, CaO). Micromorphological samples, general observations, and scaled plan view drawings made immediately following and nine years after ignition demonstrate that the original shape of the kiln and residual quicklime within and around it rapidly dissipated through bioturbation, trampling by animals, erosion, rain, and exposure to the elements. This could account for the seeming absence of kilns within sites, although they were probably located close to where lime‐plaster was applied, given the unstable nature and toxic effects of handling quicklime. Calculations of the manpower and fuel involved indicate that PPNB lime‐plaster production may have been less labor intensive and less detrimental to the environment than previously portrayed. © 2008 Wiley Periodicals, Inc. 相似文献
8.
Fish Biodiversity in the Vermetid Reef Of Shiqmona (Israel) 总被引：1，自引：0，他引：1
Abstract. A study of the structure of the fish community associated with a vermetid reef at Shiqmona, Israel, revealed the highest fish biodiversity (36 species) of any habitat along the Mediterranean coast of Israel. Despite the prevalence of fish of Erythrean origin in the Levantine littoral, the benthic fish community at the site is entirely autochthonous. The families richest in members were the Blenniidae and the Gobiidae. Eighteen species are benthic, sixteen supra-benthic and two pelagic. 相似文献
9.
Analogue modelling of the initiation of subduction 总被引：3，自引：0，他引：3
10.
Ruth Shahack‐Gross Avner Ayalon Paul Goldberg Yuval Goren Boaz Ofek Rivka Rabinovich Erella Hovers 《Geoarchaeology》2008,23(1):43-62
Hearths are important archaeological features, serving to infer past practices related to hominin subsistence and social behaviors. The identification of hearths is not always straightforward due to post‐depositional processes. In karstic cave environments in particular it is not always easy to distinguish, in the field or in the laboratory, between geogenic and anthropogenic cemented features because both are composed of calcite. Here we present a novel study in which bulk oxygen and carbon isotopic analyses were used in order to unequivocally identify anthropogenic calcite derived from hearths in the Neanderthal site of Amud Cave (Israel). We demonstrate that the isotopic composition of wood ash is distinctively different from that of geogenic calcite. The linear relationship observed between carbon and oxygen isotopic compositions of the various sediment types in Amud Cave is interpreted as a mixing line between wood ash and geogenic calcite, thus enhancing our understanding of the origins of sedimentological variability at the site and showing that not all of the cemented sediments in Amud Cave should be regarded as hearths. Our study suggests that the relative abundance of validated hearths in the various areas of the cave may be associated with Neanderthal use of space in Amud Cave. © 2008 Wiley Periodicals, Inc. 相似文献