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The Basin of Mexico is a closed basin of lacustrine character, with an average elevation of 2200 m above sea level. The watershed covers a vast extension in five states. Mexico City and its metropolitan area are located within this basin. The aquifer system is the main source of water supply for more than 20 million people. Water consumption is about 60 m3/s. The aquifer supplies about 43 m3/s from around 1000 wells at 70–200 m depth. Pumping policies have generated subsidence and degradation of the ground water quality in the Basin of Mexico The lacustrian clay layers play an important role in the local hydrogeology, protecting the aquifer from pollution, but the transition and piedmont areas are highly vulnerable to surface pollutants.  相似文献   
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The main objectives of fisheries management are generally similar throughout the world. These are often stated in policy documents such as the Common Fisheries Policy and the Magnuson–Stevens Fishery Conservation and Management Act. However, at the local level often the key objectives of management are more detailed, characterised by both the overriding management structure and the status and type of fishery concerned. In this paper, we consider case study fisheries from the UK, France, Spain and Denmark to compare some of the various types of fisheries and fisheries management systems that exist in the European Union. From this, we define the key objectives for each management system.  相似文献   
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In order to understand the structure of fish assemblages in the modified Lima basin (Northern Portugal), two distinct datasets concerning the presence and abundance of fish species were subjected to multivariate analysis. On the River Lima two types of flow modification are present within kilometres of one another: (a) a reduced and constant flow due to hypolimnetic release; and (b) an intense and irregular flow. A comparison of their influence on fish assemblages revealed a gradient of assemblage types from tributaries to main river sites. The latter were characterised by a strong dominance of cyprinids, particularly Iberian barbel (Barbus bocagei). The former harboured two kinds of fish assemblages: those closer to the river mouth were dominated by the cyprinids Iberian chub (Squalius carolitertii) and Iberian nase (Chondrostoma polylepis), which were also frequently present in the main river; while in those further upstream the predominant species was the brown trout (Salmo trutta). Although explanatory variables such as distance from source, altitude, substrate coarseness and width were the primary correlates of fish assemblage composition, dam construction and flow regulation also had a significant effect upon assemblage structure, particularly by: i) reducing the importance of migratory species; ii) constraining the presence of trout in the regulated segments; and iii) simplifying the community, especially in the case of the constant and reduced flow regime.  相似文献   
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Knowledge of the distribution of species life stages at multiple spatial scales is fundamental to both a proper assessment of species management and conservation programmes and the ability to predict the consequences of human disturbances for river systems. The habitat requirements of three native cyprinid species—the Iberian barbel Barbus bocagei Steindachner, the Iberian straight-mouth nase Pseudochondrostoma polylepis (Steindachner), and the Northern straight-mouth nase Pseudochondrostoma duriense (Coelho)—were examined at 174 undisturbed or minimally disturbed sites in 8 river catchments across western Iberia, by modelling occurrence and counts of species life stages at two spatial scales—large (regional) and instream (local)—using hurdle models. All the life stages of the barbel showed a negative association with upstream high-gradient river reaches, whereas juvenile P. duriense favoured such areas. Stream width and openness were negatively related with the occurrence of juvenile and small adult barbel, but not with large adults. Juvenile nase, on the other hand, were found to be mainly confined to fast-flowing habitats with high instream cover and coarser substrata. Advanced life stages of the barbel were mainly associated with the “pure” regional and shared components, whereas the purely local attributes accounted for much of the model variation among nases, in particular juveniles, and juvenile barbel. The results of this study are useful for setting or refining management goals, and highlight the need to separately consider life stages when performing conservation-related studies of species distribution.  相似文献   
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The presence of the metamorphic high-pressure/low-temperature mineral association of lawsonite-sodic amphibole is reported at Cape Smith on Smith Island, located in the southwestern end of the South Shetland Archipelago. This association confirms the existence of a high-pressure/low-temperature metamorphic belt along the Pacific margin of the Antarctic Peninsula. This belt is considered to be the meridional prolongation of the metamorphic belt of similar metamorphic conditions known along portions of the Pacific margin of South America.  相似文献   
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Representative samples of crude oils from Cusiana, Cupiagua, Apiay, Castilla and Chichimene fields in the Eastern Llanos Basin of Colombia were analyzed to determine its compound-specific stable carbon isotope composition (CSIA) using gas chromatography–isotopic ratio–mass spectrometry (GC–IRMS). GC–IRMS analyses of n-alkanes allowed differentiating between Cretaceous and Cretaceous/Tertiary oil samples. Cretaceous sourced samples have δ13C-enriched values than Cretaceous/Tertiary sourced samples; the heavier isotope composition of these samples is due to their major terrigenous organic matter input. Their isotope distribution patterns suggest significant algal and/or bacterial contribution (marine origin). The analysis of the n-alkane fractions by GC–IRMS confirms that the organic matter has marine origin in those samples from Cusiana, Cupiagua and Apiay while Castilla and Chichimene have marine origin with terrestrial inputs. The results were confirmed by gas chromatography/FID and gas chromatography/mass spectrometry (GC/MS). Basic geochemical composition show that samples from Cupiagua/Cusiana fields and Apiay/Castilla/Chichimene fields in the Llanos basin, Colombia present different characteristics reflecting a specific for each depositional environment.  相似文献   
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Chemical and mineralogical characterization of waste materials present in an abandoned Pb, Zn–Ag mining site (SE, Spain) was carried out. In unaltered rocks, the mineralogy is characterized by plagioclase, pyroxene, magnetite, ilmenite, amphibole, biotite and quartz. Trace-element contents of these samples represent unaltered values. In mine-waste materials, pH ranged from acidic to slightly acidic and trace-element content was generally high, especially for Pb and Zn, although there were also substantial As concentrations. X-ray diffraction results suggested that these samples have a complex mineralogy, including alteration products. Surficial materials in the study area were affected by weathering processes, generating supergene assemblages, including Fe and Mn oxides and hydroxides, carbonates, hydrated sulfates and jarosite. Knowledge of the geochemical processes that took place in the past and which are still taking place provide an important tool for assessing associated environmental problems in this area.  相似文献   
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Natural Hazards - Empirical vulnerability models are fundamental tools to assess the impact of future earthquakes on urban settlements and communities. Generally, they consist of sets of fragility...  相似文献   
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