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Lower Boundary (lower + boundary)
Terms modified by Lower Boundary Selected AbstractsThe responses of photosynthesis and oxygen consumption to short-term changes in temperature and irradiance in a cyanobacterial mat (Ebro Delta, Spain)ENVIRONMENTAL MICROBIOLOGY, Issue 4 2000Eric Epping We have evaluated the effects of short-term changes in incident irradiance and temperature on oxygenic photosynthesis and oxygen consumption in a hypersaline cyanobacterial mat from the Ebro Delta, Spain, in which Microcoleus chthonoplastes was the dominant phototrophic organism. The mat was incubated in the laboratory at 15, 20, 25 and 30°C at incident irradiances ranging from 0 to 1000 µmol photons m,2 s,1. Oxygen microsensors were used to measure steady-state oxygen profiles and the rates of gross photosynthesis, which allowed the calculation of areal gross photosynthesis, areal net oxygen production, and oxygen consumption in the aphotic layer of the mat. The lowest surface irradiance that resulted in detectable rates of gross photosynthesis increased with increasing temperature from 50 µmol photons m,2 s,1 at 15°C to 500 µmol photons m,2 s,1 at 30°C. These threshold irradiances were also apparent from the areal rates of net oxygen production and point to the shift of M. chthonoplastes from anoxygenic to oxygenic photosynthesis and stimulation of sulphide production and oxidation rates at elevated temperatures. The rate of net oxygen production per unit area of mat at maximum irradiance, J0, did not change with temperature, whereas, JZphot, the flux of oxygen across the lower boundary of the euphotic zone increased linearly with temperature. The rate of oxygen consumption per volume of aphotic mat increased with temperature. This increase occurred in darkness, but was strongly enhanced at high irradiances, probably as a consequence of increased rates of photosynthate exudation, stimulating respiratory processes in the mat. The compensation irradiance (Ec) marking the change of the mat from a heterotrophic to an autotrophic community, increased exponentially in this range of temperatures. [source] Simulation of water flow and solute transport in free-drainage lysimeters and field soils with heterogeneous structuresEUROPEAN JOURNAL OF SOIL SCIENCE, Issue 2 2004H. M. Abdou Summary Lysimeters are valuable for studying the fate and transport of chemicals in soil. Large-scale field lysimeters are used to assess pesticide behaviour and radionuclide transport, and are assumed to represent natural field conditions better than laboratory columns. Field lysimeters are usually characterized by a free-draining lower boundary. As a result, the hydraulic gradient is disrupted, and leachate cannot be collected until the bottom of the lysimeter becomes saturated. We compared heterogeneously structured, free-drainage lysimeters and field soils with respect to water flow and solute transport. Numerical simulations were carried out in a two-dimensional heterogeneous sandy soil under unsaturated water flow conditions with the CHAIN_2D code. Three different soil structures (isotropic, horizontal, and vertical) were generated, and Miller,Miller similitude was used to scale the hydraulic properties of the soil. The results showed that ponding occurs at the bottom of the lysimeter for the three soil structures and that it occurred faster and was more pronounced with the vertical structure (preferential flow effect). Breakthrough curves of a conservative solute (bromide) showed that solutes are moving faster in the field than in the lysimeters. Fewer differences between lysimeters and field soils were found with the horizontal soil structure than with the isotropic and vertical structures. [source] Consequences of climatic change for water temperature and brown trout populations in Alpine rivers and streamsGLOBAL CHANGE BIOLOGY, Issue 1 2006RENATA E. HARI Abstract Twenty-five years of extensive water temperature data show regionally coherent warming to have occurred in Alpine rivers and streams at all altitudes, reflecting changes in regional air temperature. Much of this warming occurred abruptly in 1987/1988. For brown trout populations, the warming resulted in an upward shift in thermal habitat that was accelerated by an increase in the incidence of temperature-dependent Proliferative Kidney Disease at the habitat's lower boundary. Because physical barriers restrict longitudinal migration in mountain regions, an upward habitat shift in effect implies habitat reduction, suggesting the likelihood of an overall population decrease. Extensive brown trout catch data documenting an altitudinally dependent decline indicate that such a climate-related population decrease has in fact occurred. Our analysis employs a quantitatively defined reference optimum temperature range for brown trout, based on the sinusoidal regression of seasonally varying field data. [source] General Steady-State Shape Factor for a Partially Penetrating WellGROUND WATER, Issue 1 2010Vitaly A. Zlotnik We present the closed form of a general steady-state shape factor for a partially penetrating well in a uniform anisotropic aquifer. Our simple analytical expression for the shape factor has a uniform representation for full range of parameters and meets or exceeds the accuracy of previous results obtained through semiempirical methods (e.g., Bouwer and Rice [1976] equations). This general shape factor pertains to the flow of fluids (water or air) in subsurface formations when the upper formation boundary has constant potential and the lower boundary is impermeable. The results of our investigation are directly applicable to analyses of (1) slug tests with falling or rising head and (2) injection/extraction tests with constant head, essential techniques for the characterization of hydraulic conductivity of aquifers, streambeds, or lakebeds as well as the design of aquifer and soil remediation systems. [source] Interaction of metamorphism, deformation and exhumation in large convergent orogensJOURNAL OF METAMORPHIC GEOLOGY, Issue 1 2002R. A. Jamieson Abstract Coupled thermal-mechanical models are used to investigate interactions between metamorphism, deformation and exhumation in large convergent orogens, and the implications of coupling and feedback between these processes for observed structural and metamorphic styles. The models involve subduction of suborogenic mantle lithosphere, large amounts of convergence (, 450 km) at 1 cm yr,1, and a slope-dependent erosion rate. The model crust is layered with respect to thermal and rheological properties , the upper crust (0,20 km) follows a wet quartzite flow law, with heat production of 2.0 ,W m,3, and the lower crust (20,35 km) follows a modified dry diabase flow law, with heat production of 0.75 ,W m,3. After 45 Myr, the model orogens develop crustal thicknesses of the order of 60 km, with lower crustal temperatures in excess of 700 °C. In some models, an additional increment of weakening is introduced so that the effective viscosity decreases to 1019 Pa.s at 700 °C in the upper crust and 900 °C in the lower crust. In these models, a narrow zone of outward channel flow develops at the base of the weak upper crustal layer where T,600 °C. The channel flow zone is characterised by a reversal in velocity direction on the pro-side of the system, and is driven by a depth-dependent pressure gradient that is facilitated by the development of a temperature-dependent low viscosity horizon in the mid-crust. Different exhumation styles produce contrasting effects on models with channel flow zones. Post-convergent crustal extension leads to thinning in the orogenic core and a corresponding zone of shortening and thrust-related exhumation on the flanks. Velocities in the pro-side channel flow zone are enhanced but the channel itself is not exhumed. In contrast, exhumation resulting from erosion that is focused on the pro-side flank of the plateau leads to ,ductile extrusion' of the channel flow zone. The exhumed channel displays apparent normal-sense offset at its upper boundary, reverse-sense offset at its lower boundary, and an ,inverted' metamorphic sequence across the zone. The different styles of exhumation produce contrasting peak grade profiles across the model surfaces. However, P,T,t paths in both cases are loops where Pmax precedes Tmax, typical of regional metamorphism; individual paths are not diagnostic of either the thickening or the exhumation mechanism. Possible natural examples of the channel flow zones produced in these models include the Main Central Thrust zone of the Himalayas and the Muskoka domain of the western Grenville orogen. [source] Variability in temperature and geometry of the Norwegian Current over the past 600,yr; stable isotope and grain size evidence from the Norwegian marginJOURNAL OF QUATERNARY SCIENCE, Issue 7 2003Ida Malene Berstad Abstract Core P1-003MC was retrieved from 851,m water depth on the southern Norwegian continental margin, close to the boundary between the Norwegian Current (NC) and the underlying cold Norwegian Sea Deep Water. The core chronology was established by using 210Pb measurements and 14C dates, suggesting a sampling resolution of between 2 and 9,yr. Sea-surface temperature (SST) variations in the NC are reconstructed from stable oxygen isotope measurements in two planktonic Foraminifera species, Neogloboquadrina pachyderma (d.) and Globigerina bulloides. The high temporal resolution of the SST proxy records allows direct comparison with instrumental ocean temperature measurements from Ocean Weather Ship (OWS) Mike in the Norwegian Sea and an air temperature record from the coastal island Ona, western Norway. The comparison of the instrumental and the proxy SST data suggests that N. pachyderma (d.) calcify during summer, whereas G. bulloides calcify during spring. The ,18O records of both species suggest that the past 70,yr have been the warmest throughout the past 600,yr. The spring and summer proxy temperature data suggest differences in the duration of the cold period of the Little Ice Age. The spring temperature was 1,3°C colder throughout most of the period between ca. AD 1400 and 1700, and the summer temperature was 1,2°C colder throughout most of the period between ca. AD 1400 and 1920. Fluctuations in the depth of the lower boundary of the NC have been investigated by examining grain size data and benthic foraminiferal assemblages. The data show that the transition depth of the lower boundary of the NC was deeper between ca. AD 1400 and 1650 than after ca. AD 1750 until present. Copyright © 2003 John Wiley & Sons, Ltd. [source] | |||||||||||||