Distribution and abundance of overwintering blue crabs,Callinectes sapidus,in the lower Chesapeake Bay     

Linda C. Schaffner  Robert J. Diaz 《Estuaries and Coasts》1988,11(1):68-72

A study was conducted to define winter distribution patterns of blue crabs,Callinectes sapidus, in the lower Chesapeake Bay and to relate these patterns to environmental variation. During February 1986 a stratified random survey was conducted to examine the distribution of blue crabs with respect to three major habitat types: 1) high energy, wave- and tide-dominated, spits and shoals; 2) moderate energy, tide-dominated basins; and 3) variable energy, tide-dominated or quiescent channels (natural or cut). Each major habitat type was further stratified on the basis of location (to account for possible salinity effects), resulting in a total of 17 habitat-stratum combinations. Blue crabs exhibited significant differences in abundance among habitats. Crabs were most abundant in the basin habitat and least abundant in the shoal and spit habitat. A posteriori evaluations of abundance patterns in relation to sediment type and depth showed that crabs were significantly more abundant where sediments contained between 41 and 60% sand and at depths exceeding 9 m. The sampled population of blue crabs was dominated by mature females. There were no significant differences in crab sex ratios between habitats, but significant differences between two fixed sites sampled through the winter showed that there were proportionately more male crabs at the western site than there were at the eastern site. The observed patterns indicate that some differential habitat utilization occurs and that overwintering female crabs are found preferentially in areas characterized by moderate energy regimes and fine, but sandy sediments.  相似文献   

On the relationship between silica and carbonate diagenesis in deep-sea sediments     

Linda A. Hobert  Andreas Wetzel 《International Journal of Earth Sciences》1989,78(3):765-778

Silica diagenesis and carbonate diagenesis are interrelated. This is confirmed by observations of DSDP Sites 462, 463, 465, 466, and 577. Carbonate sediments containing chert (1) tend to be more indurated and display more advanced diagenetic alterations, regardless of sub-bottom depth; and (2) microfossil components are more strongly affected (overgrown and/or dissolved), while the amount of micritic particles and larger, euhedral calcite crystals is greater. In addition, mass physical properties, porosity in particular, vary more widely in sediment sections containing chert. Furthermore, in the studied similarly composed sediments recrystallization of biogenic opal is indicated by a significant reduction of the specific surface area, reaching a minimum value when quartz is formed.One possible mechanism involved is the production of «surplus« dissolved carbonate created by the replacement of carbonate material by silica during the process of chert formation and silicification. The «extra« carbonate is then available for precipitation as overgrowths and cement outside the chert nodules and silicified zones. Hence silica diagenesis, if it occurs early enough in the sediment, bears some influence on carbonate diagenesis. It is therefore suggested that silica diagenesis be added to the list of factors included in the «diagenetic potential« equation ofSchlanger &Douglas (1974).

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Zusammenfassung Die Diagenese von biogenem Silikat und Karbonat steht in engem Zusammenhang, wie Beobachtungen an Sedimenten der DSDP Sites 462, 463, 465, 466 und 577 zeigen. Karbonatische Sedimente, die biogenes SiO₂ enthalten, zeichnen sich aus durch (1) größere Verfestigung und stärkere diagenetische Veränderungen — unabhängig von der Tiefe im Sediment, (2) mehr Lösung und Überwuchs an Mikrofossil-Komponenten, (3) höheren Anteil mikritischer Partikel wie auch größerer idiomorpher Kalzitkristalle, (4) stärkere Variation der sedimentphysikalischen Eigenschaften, speziell der Porosität und damit zusammenhängender Parameter.Die Umkristallisation des biogenen SiO₂ führt in ähnlich Zusammengesetzen Sedimenten zu einer drastischen Abnahme der spezifischen Oberfläche. Minimalwerte werden erreicht, wenn sich Quarz bildet.Diagenetisch wichtig ist die Produktion von zusätzlichem Karbonat durch die Silizifizierung von Karbonatschalen. Dieses »Überschuß«-Karbonat wird dann als Überwuchs, Zement oder außen an den »Hornstein«-Aggregaten gefällt. Demzufolge beeinflußt die Diagenese von biogenem SiO₂ auch die Karbonatdiagenese. Daher ist es sinnvoll, die Diagenese von biogenem SiO₂ mit zu den Faktoren zu rechnen, die das »diagenetische Potential« — wie esSchlanger &Douglas (1974) definierten — ausmachen.

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Résumé L'étude de sédiments provenant des sites DSDP 462, 463, 465, 466 et 577 montre qu'il existe une relation entre la diagenèse de la silice et celle du carbonate biogéniques. Les sédiments carbonatés qui renferment de la silice biogémque présentent: 1) une induration plus marquée et des modifications diagénétiques plus poussées — et ce indépendamment de la profondeur sous la surface du fond; 2) une dissolution et/ou un accroissement plus développés des micro-fossiles; 3) une plus grande teneur en particules micritiques et une plus grande taille des calcites idiomorphes; 4) un éventail plus large de leurs propriétés physiques, particulièrement de la porosité et des paramètres qui en dépendent.La cristallisation de l'opale biogénique, dans des sédiments de compositions semblables, se traduit par une réduction drastique de la surface spécifique, qui atteint une valeur minimale lorsque du quartz est formé.Un rôle diagénétique important est joué par l'excès de carbonate dissous engendré par la silicification de coquilles carbonatées; cet excès de carbonate est dès lors disponible pour la précipitation des auréoles d'accroissement et du ciment hors des nodules de chert et des zones silicifiées. Il s'ensuit que la diagenèse du SiO₂ biogénique influence la diagenèse du carbonate. Il conviendrait dès lors d'ajouter la diagenèse de la silice à la liste des facteurs qui interviennent dans l'équation du «potentiel diagénétique» deSchlanger etDouglas (1974).

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462, 463, 465, 466 577, . , , : 1) , ; 2) ; 3) , 4) , . SiO₂ . . . »« , , « ». , .. «, 1974 Schlanger & Douglas.

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Optical observations of line profile variability in the WN5 star HD 50896     

Linda J. Smith  Allan J. Willis 《Astrophysics and Space Science》1994,221(1-2):189-196

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   

Hydrogen isotope fractionation in lipids of the methane-oxidizing bacterium Methylococcus capsulatus     

Alex L Sessions  Linda L JahnkeArndt Schimmelmann  John M Hayes 《Geochimica et cosmochimica acta》2002,66(22):3955-3969

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 α_l/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 α_l/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   

Impacts of Climate Change on the Global Forest Sector   总被引：1，自引：0，他引：1  

John Perez-Garcia  Linda A. Joyce  A. David Mcguire  Xiangming Xiao 《Climatic change》2002,54(4):439-461

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   

Origin and significance of 13-hydroxychlorophyll derivatives in sediments     

J Stuart Walker  Angela H Squier  Dominic A Hodgson  Brendan J Keely   《Organic Geochemistry》2002,33(12)

The authenticity of hydroxychlorophyll derivatives in sediments has been verified by subjecting pure chlorophyll preparations to the extraction method used for sediments. Model studies of chlorophyll autoxidation reveal hydroxychlorophyll as the major product formed in the presence of hydrogen peroxide. Its stability to further oxidation implies that hydroxychlorophyll derivatives are not precursors of aetioporphyrins and are more likely to produce cycloalkanoporphyrins via the processes of diagenesis. The occurrence of 13²-hydroxyphaeophytin a throughout a sediment core from Loch Ness confirms it to be a widespread chlorophyll oxidation product formed during early diagenesis. Profiles of hydroxychlorophyll derivatives in a sediment core from an Antarctic lake demonstrate their potential for use as markers of oxidation processes in palaeoenvironmental assessment.  相似文献   

Microphytobenthos contribution to nutrient-phytoplankton dynamics in a shallow coastal lagoon     

Ian T. Webster  Phillip W. Ford  Bruce Hodgson 《Estuaries and Coasts》2002,25(4):540-551

Nutrient fluxes and primary production were examined in Lake Illawarra (New South Wales, Australia), a shallow (Z_mean=1.9 m) coastal lagoon with a surface area of 35 km², by intensive measurement of dissolved nutrients and oxygen profiles over a 22-h period. Rates of primary production and nutrient uptake were calculated for the microphytobenthos, seagrass beds, macroalgae, and pelagic phytoplankton. Although gross nutrient release rates to the water column and sediment pore waters were potentially high, primary production by microphytobenthos rapidly sequesters the re-mineralized nutrients so that net releases, averaged over times longer than a day, were low. Production in the water column was closely coupled with the relatively low sediment net nutrient release rates and detrital decomposition in the water column. Dissolved inorganic nitrogen and silica concentrations in the water column are drawn down at the beginning of the day. The system did not appear to be light limited so photosynthesis occurs as fast as the nutrients become available to the phytoplankton and microphytobenthos. We conjecture that microphytobenthos are the dominant primary producers and, as has been shown previously, that the nutrient uptake occurs in phase with the various stages of the diatom growth.  相似文献   

Lessons learned: The effects of nutrient enrichment on the support of nekton by seagrass and salt marsh ecosystems     

Linda A. Deegan 《Estuaries and Coasts》2002,25(4):727-742

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   

Policies and Practices for an Antiracist Geography at the Millennium     

Linda Peake  Audrey Kobayashi 《The Professional geographer》2002,54(1):50-61

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   

A GIS-Based Model to Determine Site Suitability of Emergency Evacuation Shelters   总被引：6，自引：0，他引：6  

Bandana Kar  Michael E Hodgson 《Transactions in GIS》2008,12(2):227-248

In recent years, the increase in the number of hurricanes and other costal hazards in the US pose a tremendous threat to the residents of coastal states. According to the National Hurricane Center, Florida is the most vulnerable coastal state to hurricanes. Mitigation policies have been formulated to reduce mortality and provide emergency services by evacuating people from the hazard zone. Many of these evacuees, particularly the elderly or lower income populations, rely on evacuation shelters for temporary housing. Because of the cost and limited use, evacuation shelters are almost exclusively dual use shelters where the primary purpose of the facility is for some other public function (e.g. school, hospital, etc.). In 2000, the estimated shortage of public shelter spaces in Florida was about 1.5 million. The purpose of this study was to rank the existing and candidate shelters (schools, colleges, churches and community centers) available in the state based on their site suitability. The research questions examined in this study include: (1) How many candidate shelters are located in physically suitable areas (e.g. not in a flood prone area, not near hazardous facilities, etc.)?; (2) How many existing shelters are located in physically un suitable areas, but in socially suitable areas (situated in areas with demand)?; (3) How many alternative existing and/or candidate shelters with high/very high physical suitability are located near physically un suitable existing shelters and thus, may be better choices for a shelter?; and (4) How many existing shelters located in physically un suitable areas are not near alternative existing and/or candidate shelters? A Geographic Information System‐based suitability model integrating Weighted Linear Combination (WLC) with a Pass/Fail screening technique was implemented for the 17 counties of Southern Florida. It was found that 48% of the existing shelters are located in physically unsuitable areas. Out of all the candidate shelters, 57% are located in physically unsuitable areas. For 15 of the existing shelters in unsuitable locations, no alternative candidate or existing shelter with medium to high physical suitability exists within 10 miles (16.1 km).  相似文献