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One of the most serious problems caused by eutrophication of shallow lakes is the disappearance of submerged macrophytes and the switch to a turbid, phytoplankton-dominated state. The reduction of external nutrient loads often does not result in a change back to the macrophyte-dominated state because stabilising mechanisms that cause resilience may delay a response. Additional internal lake restoration measures may therefore be needed to decrease the concentration of total phosphorus and increase water clarity. The re-establishment of submerged macrophytes required for a long-term stability of clear water conditions, however, may still fail, or mass developments of tall-growing species may cause nuisance for recreational use. Both cases are often not taken into account when restoration measures are planned in Germany, and existing schemes to reduce eutrophication consider the topic inadequately. Here we develop a step-by-step guideline to assess the chances of submerged macrophyte re-establishment in shallow lakes. We reviewed and rated the existing literature and case studies with special regard on (1) the impact of different internal lake restoration methods on the development of submerged macrophytes, (2) methods for the assessment of natural re-establishment, (3) requirements and methods for artificial support of submerged macrophyte development and (4) management options of macrophyte species diversity and abundance in Germany. This guideline is intended to help lake managers aiming to restore shallow lakes in Germany to critically asses and predict the potential development of submerged vegetation, taking into account the complex factors and interrelations that determine their occurrence, abundance and diversity.  相似文献   
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Vulnerability, adaptation and resilience are concepts that are finding increasing currency in several fields of research as well as in various policy and practitioner communities engaged in global environmental change science, climate change, sustainability science, disaster risk-reduction and famine interventions. As scientists and practitioners increasingly work together in this arena a number of questions are emerging: What is credible, salient and legitimate knowledge, how is this knowledge generated and how is it used in decision making? Drawing on important science in this field, and including a case study from southern Africa, we suggest an alternative mode of interaction to the usual one-way interaction between science and practice often used. In this alternative approach, different experts, risk-bearers, and local communities are involved and knowledge and practice is contested, co-produced and reflected upon. Despite some successes in the use and negotiation of such knowledge for ‘real’ world issues, a number of problems persist that require further investigation including the difficulties of developing consensus on the methodologies used by a range of stakeholders usually across a wide region (as the case study of southern Africa shows, particularly in determining and identifying vulnerable groups, sectors, and systems); slow delivery of products that could enhance resilience to change that reflects not only a lack of data, and need for scientific credibility, but also the time-consuming process of coming to a negotiated understanding in science–practice interactions and, finally, the need to clarify the role of ‘external’ agencies, stakeholders, and scientists at the outset of the dialogue process and subsequent interactions. Such factors, we argue, all hinder the use of vulnerability and resilience ‘knowledge’ that is being generated and will require much more detailed investigation by both producers and users of such knowledge.  相似文献   
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Tom Beer 《Natural Hazards》2007,42(3):469-480
The United Nations has declared 2008 to be the International Year of Planet Earth. It is being organised under the auspices of the International Union of Geological Sciences and UNESCO. Planning for the International Year of Planet Earth has consisted of establishing 10 major science themes including Hazards. The Hazards Theme is centred around the following key questions: (1) How have humans altered the geosphere, the biosphere and the landscape, thereby creating long-term changes detrimental to life and the environment and triggering certain hazards, while increasing societal vulnerability to geophysical (geological and hydrometeorological) hazards? (2) What technologies and methodologies are required to assess the vulnerability of people and places to hazards and how might these be used at a variety of spatial scales? (3) How do geophysical hazards compare relative to each other regarding current capabilities for monitoring, prediction and mitigation and what can be done in the short-term to improve these capabilities (4) What barriers exist to the utilisation of risk and vulnerability information by governments (and other entities) for risk and vulnerability reduction policies and planning (including mitigation) from each of the geophysical hazards? Following the 26 December 2004 Indian Ocean Tsunami and the UN World Conference on Disaster Reduction held in Kobe, Japan in January 2005, the International Council for Science (ICSU) decided to establish a major research programme and initiative on Natural and Human Induced Environmental Hazards and Disasters that will co-operate with the Hazards theme of the International Year and continue through to 2011.  相似文献   
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Geochemical Evidence for Slab Melting in the Trans-Mexican Volcanic Belt   总被引:3,自引:0,他引:3  
Geochemical studies of Plio-Quaternary volcanic rocks from theValle de Bravo–Zitácuaro volcanic field (VBZ) incentral Mexico indicate that slab melting plays a key role inthe petrogenesis of the Trans-Mexican Volcanic Belt. Rocks fromthe VBZ are typical arc-related high-Mg andesites, but two differentrock suites with distinct trace element patterns and isotopiccompositions erupted concurrently in the area, with a traceelement character that is also distinct from that of other Mexicanvolcanoes. The geochemical differences between the VBZ suitescannot be explained by simple crystal fractionation and/or crustalassimilation of a common primitive magma, but can be reconciledby the participation of different proportions of melts derivedfrom the subducted basalt and sediments interacting with themantle wedge. Sr/Y and Sr/Pb ratios of the VBZ rocks correlateinversely with Pb and Sr isotopic compositions, indicating thatthe Sr and Pb budgets are strongly controlled by melt additionsfrom the subducted slab. In contrast, an inverse correlationbetween Pb(Th)/Nd and 143Nd/144Nd ratios, which extend to lowerisotopic values than those for Pacific mid-ocean ridge basalts,indicates the participation of an enriched mantle wedge thatis similar to the source of Mexican intraplate basalts. In addition,a systematic decrease in middle and heavy rare earth concentrationsand Nb/Ta ratios with increasing SiO2 contents in the VBZ rocksis best explained if these elements are mobilized to some extentin the subduction flux, and suggests that slab partial fusionoccurred under garnet amphibolite-facies conditions. KEY WORDS: arcs; mantle; Mexico; sediment melting; slab melting  相似文献   
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Abstract— This study provides a complete data set of all five noble gases for bulk samples and mineral separates from three Martian shergottites: Shergotty (bulk, pyroxene, maskelynite), Zagami (bulk, pyroxene, maskelynite), and Elephant Moraine (EET) A79001, lithology A (bulk, pyroxene). We also give a compilation of all noble gas and nitrogen studies performed on these meteorites. Our mean values for cosmic‐ray exposure ages from 3He, 21Ne, and 38Ar are 2.48 Myr for Shergotty, 2.73 Myr for Zagami, and 0.65 Myr for EETA79001 lith. A. Serious loss of radiogenic 4He due to shock is observed. Cosmogenic neon results for bulk samples from 13 Martian meteorites (new data and literature data) are used in addition to the mineral separates of this study in a new approach to explore evidence of solar cosmic‐ray effects. While a contribution of this low‐energy irradiation is strongly indicated for all of the shergottites, spallation Ne in Chassigny, Allan Hills (ALH) 84001, and the nakhlites is fully explained by galactic cosmic‐ray spallation. Implanted Martian atmospheric gases are present in all mineral separates and the thermal release indicates a near‐surface siting. We derive an estimate for the 40Ar/36Ar ratio of the Martian interior component by subtracting from measured Ar in the (K‐poor) pyroxenes the (small) radiogenic component as well as the implanted atmospheric component as indicated from 129Xe, * excesses. Unless compromised by the presence of additional components, a high ratio of ~2000 is indicated for Martian interior argon, similar to that in the Martian atmosphere. Since much lower ratios have been inferred for Chassigny and ALH 84001, the result may indicate spatial and/or temporal variations of 40Ar/36Ar in the Martian mantle.  相似文献   
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