Resource Competition and Selective Grazing of Plankton in a Multispecies Pelagic Food Web Model*     

Roel Riegman  Bouwe R. Kuipers 《Marine Ecology》1994,15(2):153-164

Abstract. An idealized mathematical, multispecies model of the pelagic foodweb was used to explore the impact of resource competition, selective grazing, and species composition within each trophic level on the structure and functioning of a foodweb at a variety of light irradiance and nutrient levels. This concept is based on the general assumption that poorly edible species are poor competitors. The concept demonstrates that with increasing eutrophication, specialists in competition will be replaced by generalists and subsequently by poorly edible primary producers. Controlling factors not only determine the growth rate of primary producers but also select predators according to their ability to utilize the energy fixation of the system. Plankton community structure seems to adapt in such a way that the energy fixation rate of the system is optimized at a given set of nutrients and light. Red tides, dense algal blooms in shallow lakes and the varying results of biomanipulation in shallow lakes, can be explained by the concept of selective grazing. If this assumption is correct, the same ecosystem concept may be applicable for terrestrial ecosystems.  相似文献   

The Wild Life of Pesticides: urban agriculture,institutional responsibility,and the future of biodiversity in Sydney's Hawkesbury-Nepean River     

Roel Plant  Jeremy Walker  Scott Rayburg  Jacqueline Gothe  Teresa Leung 《The Australian geographer》2012,43(1):75-91

Agricultural chemicals are a notoriously intractable source of environmental pollution. Offering enhanced agricultural productivity, they simultaneously risk degrading the ecological basis upon which agriculture depends. This paper considers chemicalisation as a cause of the erosion of aquatic biodiversity and ecosystem resilience, focusing on the Hawkesbury-Nepean River and the small-scale horticulturalists who supply the city's fresh vegetable markets, working under the pressure of urbanisation, retail monopolies, indifferent land-use planning, and often without access to information about pesticide use in the languages they understand. Arguing that standard practices of ‘risk management’ are unable to adequately control chemical contamination, the paper presents findings from interviews with actors within the ‘assemblage’ of institutions with responsibility for agriculture, water quality, and environmental protection, in order to assess the effectiveness of pesticide governance in the Greater Sydney Basin. It appears that pesticide pollution is far from being tamed: it is rarely measured nor monitored, neither is it a priority of any particular agency. Arguing that public health, the long-term viability of local farming and the ecological well-being of the Hawkesbury-Nepean River are mutually consistent goals, we conclude that these vital elements of the common-weal are currently subject to a system of ‘organised irresponsibility’. The paper concludes by proposing several ways forward.  相似文献   

A unified approach to ray bending, ray perturbation and paraxial ray theories     

Roel Snieder  Carl Spencer 《Geophysical Journal International》1993,115(2):456-470

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Resolving a low-velocity zone with surface-wave data   总被引：2，自引：0，他引：2  

Hendrik Jan van Heijst  Roel Snieder  Robert Nowack 《Geophysical Journal International》1994,118(2):333-343

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The Theory of Coda Wave Interferometry   总被引：7，自引：0，他引：7  

Roel Snieder 《Pure and Applied Geophysics》2006,163(2-3):455-473

Coda waves are sensitive to changes in the subsurface because the strong scattering that generates these waves causes them to repeatedly sample a limited region of space. Coda wave interferometry is a technique that exploits this sensitivity to estimate slight changes in the medium from a comparison of the coda waves before and after the perturbation. For spatially localized changes in the velocity, or for changes in the source location, the travel-time perturbation may be different for different scattering paths. The coda waves that arrive within a certain time window are therefore subject to a distribution of travel-time perturbations. Here I present the general theory of coda wave interferometry, and show how the time-shifted correlation coefficient can be used to estimate the mean and variance of the distribution of travel-time perturbations. I show how this general theory can be used to estimate changes in the wave velocity, in the location of scatterer positions, and in the source location.  相似文献   

Waveform inversions and the significance of surface-wave mode coupling     

Henk Marquering  Roel Snieder  Guust Nolet 《Geophysical Journal International》1996,124(1):258-278

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Error in shear-wave polarization and time splitting     

Gwénola Michaud  Roel Snieder 《Geophysical Prospecting》2004,52(2):123-132

Shear‐wave polarization and time delay are attributes commonly used for fracture detection and characterization. In time‐lapse analysis these parameters can be used as indicators of changes in the fracture orientation and density. Indeed, changes in fracture characteristics provide key information for increased reservoir characterization and exploitation. However, relative to the data uncertainty, is the comparison of these parameters over time statistically meaningful? We present the uncertainty in shear‐wave polarization and time delay as a function of acquisition uncertainties, such as receiver and source misorientation, miscoupling and band‐limited random noise. This study is applied to a time‐lapse borehole seismic survey, recorded in Vacuum Field, New Mexico. From the estimated uncertainties for each survey, the uncertainty in the difference between the two surveys is 31° for the shear‐wave polarization angle and 4 ms for the shear‐wave time delay. Any changes in these parameters greater than these error estimates can be interpreted with confidence. This analysis can be applied to any time‐lapse measurement to provide an interval of confidence in the interpretation of shear‐wave polarization angles and time splitting.  相似文献   

Surface wave tomography of North America and the Caribbean using global and regional broad-band networks: phase velocity maps and limitations of ray theory   总被引：1，自引：0，他引：1  

Stéphanie Godey  Roel Snieder  Antonio Villaseñor  Harley M. Benz 《Geophysical Journal International》2003,152(3):620-632

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Uncertainty analysis for the integration of seismic and controlled source electro‐magnetic data     

Myoung Jae Kwon  Roel Snieder 《Geophysical Prospecting》2011,59(4):609-626

We study the appraisal problem for the joint inversion of seismic and controlled source electro‐magnetic (CSEM) data and utilize rock‐physics models to integrate these two disparate data sets. The appraisal problem is solved by adopting a Bayesian model and we incorporate four representative sources of uncertainty. These are uncertainties in 1) seismic wave velocity, 2) electric conductivity, 3) seismic data and 4) CSEM data. The uncertainties in porosity and water saturation are quantified by a posterior random sampling in the model space of porosity and water saturation in a marine one‐dimensional structure. We study the relative contributions from the four individual sources of uncertainty by performing several statistical experiments. The uncertainties in the seismic wave velocity and electric conductivity play a more significant role on the variation of posterior uncertainty than do the seismic and CSEM data noise. The numerical simulations also show that the uncertainty in porosity is most affected by the uncertainty in the seismic wave velocity and that the uncertainty in water saturation is most influenced by the uncertainty in electric conductivity. The framework of the uncertainty analysis presented in this study can be utilized to effectively reduce the uncertainty of the porosity and water saturation derived from the integration of seismic and CSEM data.  相似文献   

An exploratory study on the use of enzyme activities as sediment tracers: biochemical fingerprints?   总被引：1，自引：0，他引：1  

Kazem NOSRATI  Gerard GOVERS  Hassan AHMADI  Forood SHARIFI  Mohammad Ali AMOOZEGAR  Roel MERCKX  Matthias VANMAERCKE 《国际泥沙研究》2011,26(2):136-151

Little information exists on the potential of soil enzyme activities,which are sensitive to soil properties and management,for the characterization of sediment sources at the catchment scale.The objective of this study is to explore and evaluate enzyme activity as tracer for sediment fingerprinting in the Hiv catchment(55 km~2),Iran.Therefore,four enzymes were measured from 42 different sampling sites,covering three sediment source areas(rangeland/surface erosion,orchard/surface erosion,and streambank erosion),as well as from 12 sediment samples from reservoir check dams (sediment sinks).The results indicate that,based upon backward mode discriminant analysis,βglucosidase and dehydrogenase,allowed more than 95%of the samples to be correctly assigned to their source areas.These enzymes were selected as input data for a mixing-model to determine the relative contribution of the sampled sediment sources.The mean contributions from rangeland, orchard and streambank sources in the study area were estimated as 11.3%,15.1%and 73.7%, respectively.Using geochemical tracers,the mean contribution from rangeland,orchard and streambank sources was estimated as 14.1%,9.5%and 74.8%,respectively.Combined biochemical and geochemical tracers,similar values were obtained(18.7%,10.7%and 70.7%,respectively).Our results indicate that soil enzyme activity allows for a good characterization of sediment sources,and can provide a complementary tool to currently existing sediment fingerprinting approaches.However, the method should be also tested in other regions.  相似文献