排序方式： 共有5条查询结果，搜索用时 15 毫秒
1
1.
Water-level fluctuations may be used to promote the expansion of emergent vegetation along lakeshores. We present the case of the lake Volkerak-Zoommeer in the Netherlands, a fresh-water lake created in 1987 after the enclosure of an estuary. Using an experimental area in which the water level could be manipulated, it was shown that partial summer drawdown of the shoreline created suitable conditions for germination and growth of tall emergent species (in particular Phragmites australis). Plant survival and growth depended on subsequent water-level fluctuations and grazing by waterbirds. Based on the experiment and empirical data, a model was developed to predict the effects of the water-level regime on potential reed bed development. The model was applied for four hydrological scenarios that have been considered for the water-level management of the lake. 相似文献
2.
临界转换的早期预警信号 总被引：1，自引：0，他引：1
Marten Scheffer George Sugihara Jordi Bascompte Victor Brovkin Vasilis Dakos Hermann Held Max Rietkerk 李迎春 闫伟 蒋长胜 《国际地震动态》2009,(9):1-14
从生态系统到金融市场和气候在内的很多复杂动力系统,都会有临界点,在这样的点上系统可能会发生突变,从而演变到一个对立的动力模式上。在这样的临界点到达之前对其进行预测极为困难,但是现在,不同科学领域的研究工作表明,普遍性的早期预警信号有迹可循。对一系列不同类型的系统来说,这些信号会指示是否正在趋近一个临界阈值。 相似文献
3.
Although striking regime shifts have been observed in marine systems, mechanistic explanations for these phenomena remain scarce. Here, we review studies of regime shifts in lakes and compare them to what has been found in marine systems to explore the extent to which lakes can be viewed as microcosms for studying essential ecosystem feedbacks in marine systems. We conclude that despite obvious differences, which have been discussed by several authors, lakes are in fact quite similar to oceans in many aspects. In particular, similar mechanisms may be involved in causing alternative attractors (and thus a tendency to show occasional regime shifts) in both systems. A striking similarity exists between the mechanisms that may have caused massive marine anoxic events in the distant past, and the shift of many deep lakes to a mode of anoxic phosphorus recycling. Biotic interactions are relatively poorly understood in the oceans. However, based on what we know from lakes, we expect competitive interactions and diffuse multi-species interactions to be a common cause of alternative attractors in marine communities. In addition, overexploitation traps are an obvious cause of multiple attractors to be expected in the oceans. These should not be limited to direct effects of human overexploitation. Shifts between overexploited and under-exploited states may happen at various trophic levels. Finally, we conclude from observed patterns and theoretical results that implications of alternative attractors will be quite different in benthic and open ocean systems. Benthic regime shifts might happen easily but can be relatively local. By contrast, open ocean shifts might not arise so easily but tend to be impressive in magnitude and scale when they happen. 相似文献
4.
Recent sediments in the Vågen, Bergen harbour, are subjected to injection of untreated sewage from around 15,000 person equivalent. Here the distribution of trace elements is mapped in surface and subsurface sediments. Selected trace-element concentrations decrease from the inner towards the outer Vågen. These concentration gradients do not correlate with minerogenic grain size. Linear correlation, combined with enriched sediment trace elements, are used to infer sources for these elements and possible reasons for their enrichment. (1) Frequent use of dental amalgam might be related to the enrichments of and correlation between Ag, Cu, Hg and Sn in surficial sediments pre-1994. (2) Enrichment and correlation of Cu/Zn may be related to the commercial and private use of anti-fouling boat-paint. Further, tributyltin is suggested as a source of Sn enrichment. (3) Increased traffic in the city of Bergen with gasoline combustion and car-wheel wear-off release Pb and Zn. (4) Hg and Pb might have been atmospherically supplied due to the combustion of coal in a nearby gas plant pre-1985. (5) No correlation is observed between enriched Vågen trace metals and bedrock element composition of the surrounding mountains, indicating a minor supply from erosion. 相似文献
5.
Properly accounting for the mechanical anisotropy of shales can be critical for successful drilling of high inclination wells, because shales are known to be weak along bedding planes. To optimize the drilling parameters in such cases, a sufficiently representative, anisotropic rock mechanical model is therefore required. This paper presents such a model developed to better match results from a dedicated, extensive set of uniaxial and triaxial compression tests performed on plugs of Mancos outcrop shale with different orientations relative to the bedding plane. Post failure inspection of the plugs shows that the failure planes are to some extent affected by the orientation of the applied stress relative to the bedding planes, indicating that the bedding planes may represent weak planes which tend to fail before intrinsic failure occurs, whenever the orientation of these planes is suitable. The simple “plane of weakness” model is commonly used to predict strength as function of orientation for such a rock. A comparison of this model to the experimental data shows, however, that the weak planes seem to have an impact on strength even outside the range of orientations where the model predicts such impact. An extension of this model allowing the weak planes to be heterogeneous in terms of patchy weakness was therefore developed. In this model, local shear sliding may occur prior to macroscopic failure, leading to enhanced local stresses and corresponding reduction in strength. The model is found to give better match with strength data at intermediate orientations. The model is also able to partly predict the qualitatively different variation of Young’s modulus with orientation for this data set. 相似文献
1