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Physical Gradient (physical + gradient)

Distribution by Scientific Domains

Life Sciences	75%

Selected Abstracts

Centering the projection reconstruction trajectory: Reducing gradient delay errors,

MAGNETIC RESONANCE IN MEDICINE, Issue 1 2003
Dana C. Peters
Abstract The projection reconstruction (PR) trajectory was investigated for the effect of gradient timing delays between the actual and requested start time of each physical gradient. Radial trajectories constructed with delayed gradients miss the center of k -space in an angularly dependent manner, causing effective echo times to vary with projection angle. The gradient timing delays were measured in phantoms, revealing delays on the x, y, and z gradients which differed by as much as 5 ,sec. Using this one-time calibration measurement, the trajectories were corrected for gradient delays by addition of compensatory gradient areas to the prephasers of the logical x and y readout gradients. Effective projection-to-projection echo time variability was reduced to less than 1 ,sec for all imaging orientations. Using corrected trajectories, artifacts were reduced in phantom images and in volunteer studies. This correction should potentiate greater clinical use of the PR trajectory. Magn Reson Med 50:1,6, 2003. Published 2003 Wiley-Liss, Inc. [source]

Connectivity and patch area in a coastal marine landscape: Disentangling their influence on local species richness and composition

AUSTRAL ECOLOGY, Issue 6 2009
ANA INÉS BORTHAGARAY
Abstract Landscape ecology emerged as a terrestrial discipline to evaluate the effect of spatial configuration of natural systems on ecological patterns. The advances in marine systems have been comparatively scarce perhaps as a consequence of a long-standing view about the greater dispersal potential of marine species and its effect on the spatial homogenization of marine landscapes. Herein we used an intertidal rocky system as a model to analyse the effect of landscape attributes on local species richness and composition. We evaluated the effect of patch area, landscape connectivity and salinity gradient on local species richness of macro-invertebrates, and the effect of geographic distance on species similarity. We sampled 19 rocky patches along the Uruguayan Atlantic coast one time during the spring of 2003. The relative contribution of the variables assessed on specific richness of sessile, mobile and total macrofauna was analysed with a stepwise multiple linear regression. For the mobile macrofaunal richness, we also incorporated the sessile macrofaunal richness as another independent variable. The effect of geographic distance on biological similarity was assessed by a Mantel test. We showed that landscape connectivity, as a descriptor of the average physical isolation of a biological community in the landscape, is an important factor explaining the community species richness for sessile macrofauna, what indirectly increases the mobile macrofaunal richness. The geographic distance between sites was negatively related to species similarity. We suggest that at the landscape scale, connectivity among sites can be important to understand the local structure of marine communities, particularly in rocky intertidal systems. Also the distance-decay of similarity in community composition provides a useful descriptor of how biological composition varies along a physical gradient. Our results contribute to reinforce the view that mesoscale connectivity (101,2 km) in coastal marine landscapes plays a more important role in local community structure than previously assumed. [source]

Lake morphometry predicts the degree of habitat coupling by a mobile predator

OIKOS, Issue 8 2009
Rebecca Dolson
Habitat coupling is an ecosystem process whereby semi-discontinuous habitats are connected through the movement of energy and nutrients by chemical, physical or biological processes. One oft-cited example is that of littoral,pelagic coupling in lakes. Theory has argued that such habitat coupling may be critical to food web dynamics, yet there have been few empirical studies that have quantified ecological factors that affect the degree of habitat coupling in ecosystems. Specifically, the degree to which habitat coupling occurs across important physical gradients has largely been ignored. To address this, we investigate the degree of littoral habitat coupling (i.e. the degree to which a top predator lake trout, Salvelinus namaycush, derives energy from the littoral zone) along a gradient of lake shape, where lake shape modifies the relative quantity of coupled epilimnetic benthic and pelagic habitats within each lake. Herein we demonstrate that littoral habitat coupling is intensified in simple circular lakes compared to their reticulate counterparts in seven Canadian Shield lakes. Although the more reticulate lakes had larger areas of epilimnetic benthic habitat, littoral food sources comprised 11% compared to 24% of lake trout diet in reticulate and circular lakes, respectively. This heightened interaction in circular lakes also appears to translate into increased omnivory in more circular lakes compared to reticulate lakes such that lake trout of circular lakes have a significantly lower trophic position than lake trout of reticulate lakes (F1,5=6.71 p=0.05). These results suggest that it is the accessibility of littoral production via thermal refugia, and not the amount of littoral production, that determines the degree to which lake trout couple littoral and pelagic habitats in lakes. [source]

Habitat Overlap and Facilitation in Tamarisk and Box Elder Stands: Implications for Tamarisk Control Using Native Plants

RESTORATION ECOLOGY, Issue 3 2010
John M. DeWine
Invasive plants are typically managed using top-down control techniques that focus on the removal of the target organism. Bottom-up control limits the resources available to the undesired species by manipulating disturbance, competition, and successional processes, and thus may prevent reinvasion. Tamarisk species (Tamarix sp.) have invaded riparian areas throughout western North America, resulting in expansive control efforts. A companion study has shown that a native competitor, Box elder (Acer negundo), is capable of outcompeting and killing established Tamarisk through light interception in canyons of Dinosaur National Monument (DNM), Colorado. The goal of this study was to determine the feasibility of using Box elder as a bottom-up control agent by (1) determining the distributional overlap of the two species in DNM; (2) determining if Tamarisk facilitates Box elder establishment; and (3) analyzing Box elder seedling survival across a range of physical gradients. The distribution of Tamarisk and Box elder overlapped considerably throughout the study area. Box elder seedlings were planted under Tamarisk canopies or areas with the canopy removed. Survival was significantly higher under Tamarisk canopies, indicating that Tamarisk facilitates Box elder seedling survival. Box elder seedling survival was tested across soil texture, litter depth, groundwater depth, and shade intensities indicative of conditions found in the canyons of DNM, and survival was high for all treatments. The manipulation of competitive and successional processes through the promotion of Box elder and other native tree establishment is suggested as a means of bottom-up Tamarisk control to complement traditional control techniques. [source]