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241.
The gulf pipefish,Syngnathus scovelli, is the dominant syngnathid found in coastal regions of the Gulf of Mexico and is the only species in this region known to
occur in both freshwater and saltwater habitats. Relatively little is known about the population and reproductive cycles ofS. scovelli, particularly for those found in low salinity environments. The focus of the current study was to collect population structure
and reproductive data forS. scovelli from a low salinity habitat, Meaher Park, located at the mouth of Mobile Bay, Alabama. Sampling was conducted twice a month
from January 2003 to January 2004. Environmental parameters, as well as population and reproductive parameters, were collected.
Water temperature was the primary abiotic factor associated with both the appearance ofS. scovelli and their breeding cycle. Based on gonadosomatic (GSI) and brood pouch somatic indices (BPSI), females and males were reproductively
active throughout the summer. Peaks in male GSI and BPSI were consistent with the subsequent appearance of a large number
of juveniles in early fall. These event coincided with the higher temperatures seen during late summer and early fall. Sex
ratios (male : female 1 : 1.64) and operational sex ratios (1 : 4.09) indicated that this was a female biased population.
Although there was no difference in the average size of males and females in this population, the largest individuals collected
were female. The female-biased sex ratio supports previous genetic analyses suggesting thatS. scovelli exhibits true sex-role reversal and that operational sex ratios are a reliable indicator of the intensity of mating competition
and sex roles. 相似文献
242.
243.
We simulate a large-scale flooding in the province of South-Holland in the economic centre of the Netherlands. In traditional research, damage due to flooding is computed with a unit loss method coupling land use information to depth-damage functions. Normally only direct costs are incorporated as an estimate of damage to infrastructure, property and business disruption. We extend this damage concept with the indirect economic effects on the rest of the regional and national economy on basis of a bi-regional input output table.We broaden this damage estimation to the concept of vulnerability. Vulnerability is defined as a function of dependence, redundancy and susceptibility. Susceptibility is the probability and extent of flooding. Dependency is the degree to which an activity relates to other economic activities in the rest of the country. Input–output multipliers form representations of this dependency. Redundancy is the ability of an economic activity to respond to a disaster by deferring, using substitutes or relocating. We measure redundancy as the degree of centrality of an economic activity in a network. The more central an activity is, the less it encounters possibilities to transfer production and the more vulnerable it is for flooding. Vulnerability of economic activities is then visualized in a GIS. Kernel density estimation is applied to generalize point information on inundated firms to sectoral information in space. We apply spatial interpolation techniques for the whole of the province of South-Holland. Combining information of sectoral data on dependency and redundancy, we are able to create maps of economic hotspots. Our simulation of a flood in the centre of Holland reveals the vulnerability of a densely populated delta. 相似文献
244.
245.
Anne Duperret Said Taibi Rory N. Mortimore Martin Daigneault 《Engineering Geology》2005,78(3-4):321-343
The aim of this paper is to evaluate the role of groundwater and sea weathering on the strength of the chalk rocks exposed on the coastline of the English Channel in Normandy, NW France. We present a study of the rock strength variations of three representative chalk units (Lewes Chalk, Seaford Chalk and Newhaven Chalk) exposed at various locations on the coastal chalk cliffs. The combination of UCS tests and SEM observations have been used (1) on dry natural chalk samples, (2) on chalk samples at various moisture contents, (3) on dry chalk samples submitted to a 10-day cycle of alternating wetting and drying by distilled water and by sea water. Dry chalk samples show low UCS strength (3.46–4 MPa) indicative of very weak rocks. When chalk samples are submitted to progressive water wetting, they present a decrease of UCS strength and Young's modulus of 40% to 50%. This behaviour begins at low values of water content within the chalk, i.e., for a degree of water saturation ranging between 10% and 17%. When chalk samples are submitted to an artificial weathering cycle with distilled water, a decrease in strength is observed, whereas the Young's modulus increases. SEM observations indicate the occurrence of microcracks and particle aggregates in the sample. When chalk samples are submitted to an artificial weathering with sea water, the decrease of UCS strength and Young's modulus achieves a minimum. SEM observations indicate salt crystals within the chalk. On the coastal cliffs of NW France, weathering processes depend both on chalk lithology, which show a range of sensitivity to weathering and on the location of the chalk in the coastal area. Processes allied to the degree of weathering (e.g., salt crystallisation or fresh water disaggregation) differ in the chalk massif, on the cliff face and on the shore platform. 相似文献
246.
247.
Anne M. Gellatly 《Geochimica et cosmochimica acta》2005,69(15):3813-3829
Concentrations of oceanic and atmospheric oxygen have varied over geologic time as a function of sulfur and carbon cycling at or near the Earth’s surface. This balance is expressed in the sulfur isotope composition of seawater sulfate. Given the near absence of gypsum in pre-Phanerozoic sediments, trace amounts of carbonate-associated sulfate (CAS) within limestones or dolostones provide the best available constraints on the isotopic composition of sulfate in Precambrian seawater. Although absolute CAS concentrations, which range from those below detection to ∼120 ppm sulfate in this study, may be compromised by diagenesis, the sulfur isotope compositions can be buffered sufficiently to retain primary values.Stratigraphically controlled δ34S measurements for CAS from three mid-Proterozoic carbonate successions (∼1.2 Ga Mescal Limestone, Apache Group, Arizona, USA; ∼1.45-1.47 Ga Helena and Newland formations, Belt Supergroup, Montana, USA; and ∼1.65 Ga Paradise Creek Formation, McNamara Group, NW Queensland, Australia) show large isotopic variability (+9.1‰ to +18.9‰, −1.1‰ to +27.3‰, and +14.1‰ to +37.3‰, respectively) over stratigraphic intervals of ∼50 to 450 m. This rapid variability, ranging from scattered to highly systematic, and overall low CAS abundances can be linked to sulfate concentrations in the mid-Proterozoic ocean that were substantially lower than those of the Phanerozoic but higher than values inferred for the Archean. Results from the Belt Supergroup specifically corroborate previous arguments for seawater contributions to the basin. Limited sulfate availability that tracks the oxygenation history of the early atmosphere is also consistent with the possibility of extensive deep-ocean sulfate reduction, the scarcity of bedded gypsum, and the stratigraphic δ34S trends and 34S enrichments commonly observed for iron sulfides of mid-Proterozoic age. 相似文献
248.
Urban transformation of river landscapes in a global context 总被引:8,自引:1,他引:8
Over the past 50 years considerable progress has been made in understanding the impacts of urban development on river processes and forms. Such advances have occurred as urban population growth has accelerated around the world. Using a compilation of research results from more than 100 studies conducted in a range of areas (58 addressing morphological change), this paper describes how urbanization has transformed river landscapes across Earth’s surface, emphasizing the distribution of impacts in a global comparative context. Urban development induces an initial phase of sediment mobilization, characterized by increased sediment production (on the order of 2–10 times) and deposition within channels, followed by eventual decline that couples with erosion from increased runoff to enlarge channels. Data from humid and temperate environments around the world indicate that channels generally enlarge to 2–3 times and as much as 15 times the original size. Although research has emphasized temperate environments, recent studies of tropical areas indicate a tendency for channel reduction resulting from strong sediment erosion and deposition responses because of intense precipitation and highly weathered soils. Embryonic research in arid environments further suggests variable river responses to urbanization that are characterized by rapid morphological change over short distances. Regardless of location, the persistence of the sediment production phase varies from months to several years, whereas several decades are likely needed for enlarging channels to stabilize and potentially reach a new equilibrium. Urbanizing streams pose particular challenges for management given an inherent changing nature. Successful management requires a clear understanding of the temporal and spatial variations in adjustment processes. 相似文献
249.
250.
Key design and operational aspects for thermal diffusion cloud chamber (TDCC) applications are discussed in the context of a two-dimensional solution to the mass and energy balances describing diffusion through a stagnant background gas. The important issue of buoyancy-driven convective disturbances and their impact upon nucleation measurements made using a diffusion cloud chamber are discussed. A new derivation of the relation that predicts the upper limit of total pressure allowed for stable (the absence of buoyancy-driven convective disturbances) operation of the diffusion cloud chamber is presented. For the first time, this limit of stable operation can be predicted prior to making experimental measurements. Nucleation data obtained in our laboratory are examined in the context of this predicted limit of stable operation. New nucleation data are presented for 1-pentanol using helium as a background gas. Only data corresponding to stable operation in the cloud chamber is used in the analysis. The effect of background gas on nucleation we have reported previously is confirmed for 1-pentanol, as well as for all the other alcohols that have been investigated in our laboratory. 相似文献