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161.
Trudy Ann Cameron Jeffrey Englin 《Journal of Environmental Economics and Management》1997,33(3):296-313
Respondent experience (i.e., a respondent's information set) has long been suspected to influence contingent valuation estimates of environmental values. We assess the influence of experience by explicitly modeling the relationship between respondent experience and both fitted individual resource values and the conditional variance of these estimated values. Using three different joint specifications for experience and WTP—normal/censored-normal, Poisson/censored-normal, and zero-inflated Poisson/censored-normal—we find discrete jumps in resource values as experience increases from zero and that more-experienced respondents have smaller conditional variances. Simulation of arbitrary levels of experience allows standardization of the amount of information when developing welfare estimates. 相似文献
162.
163.
Watershed-Based Survey Designs 总被引:2,自引:0,他引:2
Detenbeck NE Cincotta D Denver JM Greenlee SK Olsen AR Pitchford AM 《Environmental monitoring and assessment》2005,103(1-3):59-81
Watershed-based sampling design and assessment tools help serve the multiple goals for water quality monitoring required under
the Clean Water Act, including assessment of regional conditions to meet Section 305(b), identification of impaired water
bodies or watersheds to meet Section 303(d), and development of empirical relationships between causes or sources of impairment
and biological responses. Creation of GIS databases for hydrography, hydrologically corrected digital elevation models, and
hydrologic derivatives such as watershed boundaries and upstream–downstream topology of subcatchments would provide a consistent
seamless nationwide framework for these designs. The elements of a watershed-based sample framework can be represented either
as a continuous infinite set defined by points along a linear stream network, or as a discrete set of watershed polygons.
Watershed-based designs can be developed with existing probabilistic survey methods, including the use of unequal probability
weighting, stratification, and two-stage frames for sampling. Case studies for monitoring of Atlantic Coastal Plain streams,
West Virginia wadeable streams, and coastal Oregon streams illustrate three different approaches for selecting sites for watershed-based
survey designs. 相似文献
164.
Ecological Monitoring and Assessment Network's Proposed Core Monitoring Variables: An Early Warning of Environmental Change 总被引:6,自引:0,他引:6
This article reports on the evaluation of existing ecologicalmonitoring variables from a variety of sources to select a suiteof core variables suitable for monitoring at the Ecological Monitoring and Assessment Network (EMAN) sites located acrossCanada. The purpose of EMAN is to promote the acquisition of relevant and consistent data that can be used to report on national trends and provide an early warning of ecosystem change. Existing monitoring variables were evaluated in twosteps. In the first step, three primary criteria were used to pre-screen preliminary variables. In the second step, a moredetailed evaluation considered twenty criteria based on dataquality, applicability, data collection methods, data analysisand interpretation, existing data and programs, and cost effectiveness to select a draft set of core monitoring variables(CMV). An ecological framework was developed to organize the CMVin a manner that permitted a gap analysis to confirm the CMV assessed a wide range of relevant environment components. Thesuite of CMV were then tested to determine their effectiveness in detecting ecosystem change caused by stressors with ecosystemresponses that have been well documented in the literature. Thisproject is part of a process lead by Environment Canada to select CMV to detect and track ecosystem change at EMAN sites. It is anticipated that the proposed CMV will undergo future discussion and development leading to the final selection of asuite of CMV for use at EMAN sites. 相似文献
165.
166.
The re-use of faecal pellets in the water column before sinking to the seafloor is known as an important pathway in marine
food webs. Especially planktonic copepods seems to be actively use their faecal pellets. Since benthic copepods (order Harpacticoida)
live in the vicinity of their pellets, it remains unclear how important these pellets are for their feeding ecology. In the
present study a laboratory experiment was conducted to analyse the importance of faecal pellets for the feeding ecology of
the harpacticoid Paramphiascella fulvofasciata and its grazing pressure on two diatom species (Seminavis robusta, Navicula phyllepta). By quantifying the amount and volume of the produced faecel pellets in different treatments, it was tested to what extent
the food source and the lack of faecal pellets would influence the production of faecal pellets. We found that the grazing
pressure of P. fulvofasciata depended on the diatom density since only a top-down effect could be found on the smaller Navicula cells during its initial exponential growth phase. The grazer had a negative effect on the diatom growth and controlled the
cell density to about 4,000 cells/cm2. In spite of the fact that the addition of faecal pellets did not show a significant positive effect on the assimilation
of diatoms, the removal of faecal pellets strongly promoted the pellet production. Especially when grazing on Navicula the harpacticoid P. fulvofasciata produced significantly more and smaller faecal pellets when the pellets were removed. This outcome illustrates the need for
faecal pellets of this harpacticoid copepod when grazing on the diatom Navicula. Apart from its selection for smaller diatom cells, it was suggested that the colonisation of heterotrophic bacteria enriched
these pellets. This study is the first to indicate that trophic upgrading occurs on faecal pellets and not only on the initial
autotrophic food sources per se. 相似文献
167.
168.
John V. Gartner Jr Kenneth J. Sulak Steve W. Ross Ann Marie Necaise 《Marine Biology》2008,153(5):825-841
Submersible observations during four missions over the North Carolina and Virginia continental slopes (184–900 m) documented
the occurrence of large aggregations of mesopelagic fishes and macronektonic invertebrates near or on the bottom. Aggregated
mesopelagics formed a layer up to tens of meters deep positioned from a few centimeters to 20 m, usually <10 m, above the
substrate. Aggregations were numerically dominated by microvores, notably the myctophid fish Ceratoscopelus maderensis and the penaeid shrimp Sergestes arcticus. Consistently present but in relatively lower numbers, were mesopelagic predators, including the paralepidids Notolepis rissoi and Lestidium atlanticum, the eel Nemichthys scolopaceus, the stomiid fishes Chauliodus sloani and Stomias boa ferox, and squids Illex spp. Near-bottom aggregations do not appear to be an artifact due to attraction to the submersible. Based on submersible
observations in three areas in 4 years spanning a decade, near-bottom aggregations of midwater organisms appear to be a geographically
widespread and persistent phenomenon along the continental slope of the southeastern US Aggregations may exploit areas of
enhanced food resources at the bottom. 相似文献
169.
Ricardo Lavandier Natalia Quinete Rachel Ann Hauser-Davis Patrick Simões Dias Satie Taniguchi Rosalinda Montone Isabel Moreira 《Chemosphere》2013,90(9):2435-2443
Polychlorinated biphenyls (PCBs) and Polybrominated Diphenyls Ethers (PBDEs) are two highly lipophilic classes of persistent organic pollutants able to resist degradation and with the ability to bioaccumulate through the food chain. In Brazil, there are still few studies on PCBs and PBDEs in aquatic organisms. In this study, we determined the levels of PBDEs and PCBs in three different fish species from the Ilha Grande Bay, located in the southern state of Rio de Janeiro, Brazil. PBDEs levels were very low, with values below the limit of quantification. PCBs concentrations ranged from 2.29 to 27.60 ng g?1 ww in muscle and from 3.41 to 34.22 ng g?1 ww in liver of the three investigated fish species. Significant positive correlations were established between PCBs concentration and fish biometric variables in mullet (length and lipid content) and a statistically significant change between seasons in croaker was observed. 相似文献
170.