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111.
The WN5 star HD 50896 is a well known variable star but the cause of the variations is uncertain. As part of an extensive observing campaign on HD 50896, 68 spectra were obtained with the AAT and UCL echelle spectrograph over three nights in January 1991. The variations seen in the emission line profiles of He II 4686, He II 4860, N V 4945 and N V 4603, 4620, sampling different regions of the wind, are described. Gross night-to-night changes are seen in the tops of all the emission lines, with the pattern of variability being different for each line, as a consequence of the different line formation regions. The edges of the emission line profiles are seen to shift but not in a systematic way, as expected for binary motion. In terms of hourly variations which monitor the propagation of instabilities in the wind, distinct blobs are seen moving outwards through the emission line profiles of the He II lines but are absent in NV 4945. This appears to indicate that, at least for the epoch of these observations, the instabilities develop farther out in the wind than the N V formation region and may well rule out photospheric-induced wind instabilities.  相似文献   
112.
Hydrogen isotopic compositions of individual lipids from Methylococcus capsulatus, an aerobic, methane-oxidizing bacterium, were analyzed by hydrogen isotope-ratio-monitoring gas chromatography-mass spectrometry (GC-MS). The purposes of the study were to measure isotopic fractionation factors between methane, water, and lipids and to examine the biochemical processes that determine the hydrogen isotopic composition of lipids. M. capsulatus was grown in six replicate cultures in which the δD values of methane and water were varied independently. Measurement of concomitant changes in δD values of lipids allowed estimation of the proportion of hydrogen derived from each source and the isotopic fractionation associated with the utilization of each source.All lipids examined, including fatty acids, sterols, and hopanols, derived 31.4 ± 1.7% of their hydrogen from methane. This was apparently true whether the cultures were harvested during exponential or stationary phase. Examination of the relevant biochemical pathways indicates that no hydrogen is transferred directly (with C-H bonds intact) from methane to lipids. Accordingly, we hypothesize that all methane H is oxidized to H2O, which then serves as the H source for all biosynthesis, and that a balance between diffusion of oxygen and water across cell membranes controls the concentration of methane-derived H2O at 31%. Values for αl/w, the isotopic fractionation between lipids and water, were 0.95 for fatty acids and 0.85 for isoprenoid lipids. These fractionations are significantly smaller than those measured in higher plants and algae. Values for αl/m, the isotopic fractionation between lipids and methane, were 0.94 for fatty acids and 0.79 for isoprenoid lipids. Based on these results, we predict that methanotrophs living in seawater and consuming methane with typical δD values will produce fatty acids with δD between −50 and −170‰, and sterols and hopanols with δD between −150 and −270‰.  相似文献   
113.
Impacts of Climate Change on the Global Forest Sector   总被引:1,自引:0,他引:1  
The path and magnitude of future anthropogenic emissions of carbon dioxide will likely influence changes in climate that may impact the global forest sector. These responses in the global forest sector may have implications for international efforts to stabilize the atmospheric concentration of carbon dioxide. This study takes a step toward including the role of global forest sector in integrated assessments of the global carbon cycle by linking global models of climate dynamics, ecosystem processes and forest economics to assess the potential responses of the global forest sector to different levels of greenhouse gas emissions. We utilize three climate scenarios and two economic scenarios to represent a range of greenhouse gas emissions and economic behavior. At the end of the analysis period (2040), the potential responses in regional forest growing stock simulated by the global ecosystem model range from decreases and increases for the low emissions climate scenario to increases in all regions for the high emissions climate scenario. The changes in vegetation are used to adjust timber supply in the softwood and hardwood sectors of the economic model. In general, the global changes in welfare are positive, but small across all scenarios. At the regional level, the changes in welfare can be large and either negative or positive. Markets and trade in forest products play important roles in whether a region realizes any gains associated with climate change. In general, regions with the lowest wood fiber production cost are able to expand harvests. Trade in forest products leads to lower prices elsewhere. The low-cost regions expand market shares and force higher-cost regions to decrease their harvests. Trade produces different economic gains and losses across the globe even though, globally, economic welfare increases. The results of this study indicate that assumptions within alternative climate scenarios and about trade in forest products are important factors that strongly influence the effects of climate change on the global forest sector.  相似文献   
114.
Coastal ecosystems such as eelgrass beds and salt marshes have always been valued for their high productivity and rich bounty of fish and shellfish. High plant productivity, complex physical structure, and suitable environmental characteristics combine to create areas of high production of important recreational and commercial species. If we are to successfully manage and restore these ecosystems, it is important to understand the mechanisms by which support of nekton may be affected by nutrient enrichment. A review of the literature suggests that there are some similarities and differences in the effects of nutrient enrichment on the support of nekton by seagrass and salt marsh ecosystems. Nutrient enrichment may compromise the ability of these habitats to support fish and invertebrates before the habitat itself is gone. In both ecosystems, alteration of characteristics within the ecosystem (for example, stem density in seagrass and food webs in marshes) affect the support of nekton, even though the basic ecosystem is still clearly extant. Because of differences in natural ecosystem characteristics, loss of ecosystem function does not occur through the same mechanisms. In seagrass systems, physical structure is usually lost first, followed by alteration of food webs and finally changes in dissolved oxygen. In salt marsh systems, loss of dissolved oxygen may occur early in the process, followed by food web alterations and eventually changes in the physical structure may occur. For both seagrass and salt marsh ecosystems, the mechanisms suggested to operate at the ecosystem-level are often based on relatively small-scale plot experiments that have been conducted in only a few locations. A better understanding of how these ecosystems function across broad geographic regions will be needed to ensure functioning coastal ecosystems.  相似文献   
115.
As we enter the new millennium, geographers have a momentous opportunity to reflect upon the historical development of our discipline and the academic culture within which it thrives, with the aim of setting out an antiracist agenda. We advocate a fundamental refashioning of the discipline, not simply an extension of its research agenda; for racism, like gender, is not just another item in the lexicon of geographical subjects. The agenda includes, but is not limited to: clarifying relations between racism and law; racism and immigration policy; racism and poverty; and mobilizing racialized groups around policy issues. These items need to be addressed both through scholarship and through activism, as centering geographical practices in the streets rather than in the academy impels not only more effective social change, but also new theoretical understanding of geographies of engagement. Our agenda for antiracist geography also involves three aspects of institutional change: to build up on and extend traditional geographical scholarship; to change the basis of the discipline by extending the principles of antiracism throughout our institutional practices, particularly in the classroom; and to change the face of the discipline by increasing the participation and contributions of geographers of color.  相似文献   
116.
Land use changes from natural ecosystems to industrial agriculture have impacted water quality and wildlife populations in the Mississippi River basin. Government programs providing technical assistance and monetary incentives have not resulted in adequate adoption rates of conservation practices. While there has been a plethora of research examining the factors associated with conservation adoption, significantly less is understood about the relative importance of these factors. Using the Analytic Hierarchy Process (AHP) with agricultural producers in three Midwestern watersheds, we assess the relative importance of environmental and production decision criteria when making decisions to adopt conservation practices. Although AHP provides insight into how decisions may be made at the watershed scale, this methodology also provides insight into how individuals make conservation decisions and may also provide a method for delivering tailored conservation advice and communications.  相似文献   
117.
118.
Geographic citizen science has much potential to assist in wildlife research and conservation, but the quality of observation data is a key concern. We examined the effects of sampling design on the quality of spatial data collected for a koala citizen science project in Australia. Data were collected from three samples—volunteers (n = 454), an Internet panel (n = 103), and landowners (n = 35)to assess spatial data quality, a dimension of citizen science projects rarely considered. The locational accuracy of koala observations among the samples was similar when benchmarked against authoritative data (i.e., an expert‐derived koala distribution model), but there were differences in the quantity of data generated. Fewer koala location data were generated per participant by the Internet panel sample than the volunteer or landowner samples. Spatial preferences for land uses affecting koala conservation were also mapped, with landowners more likely to map locations for residential and tourism development and volunteers less likely. These spatial preferences have the potential to influence the social acceptability of future koala conservation proposals. With careful sampling design, both citizen observations and land use preferences can be included within the same project to augment scientific assessments and identify conservation opportunities and constraints.  相似文献   
119.
The Cassini-Huygens spacecraft flew by Jupiter on December 30, 2000. The instruments aboard the spacecraft started making scientific observations three months earlier. Joint, collaborative observations were carried out with the teams of other spacecraft, notably Galileo, and with Earth-based observers. An operational overview of the flyby is presented and attention drawn to contributions of the eleven papers of this series which follow. Prime achievements of this campaign have been to better define the present state of fundamental elements of the jovian system, confirming many previously tentative conclusions. Particularly noteworthy is that the interactions between the solar wind and the jovian magnetosphere have been explored far deeper than before, along with the link to the morphology and dynamics of the jovian aurora.  相似文献   
120.
We report preliminary results of an observing run at the 1--m telescope at CTIO during which we followed RR Pic over several nights in February 2005. The resulting light curves show no sign of an eclipse. Apart from the 3.48 h period, which is usually interpreted as the orbital one, we find an additional period at P=3.78 h, which we interpret as the superhump period of the system.We furthermore present high–resolution Doppler maps in Hα, Hβ and He II, which we derived from observations at the NTT, La Silla in February 2004. They show strong variations in the emission distribution from one day to the next. While Hα and Hβ emission clearly show the accretion disc with some additional isolated sources, the He II emission is confined to an elongated region at low velocities.  相似文献   
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