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1.
m amamu n¶rt;ma au ¶rt; nmuaa mu n ma a, ¶rt;a ¶rt;¶rt; maua mu n ¶rt; nmam ¶rt;um n¶rt; nnmmu n. u m umau n aa mau a, m m nmmu ma nu ¶rt;¶rt; n naa u umuu n. maa a¶rt;aa a u um ¶rt;uam. a u nu¶rt;m um ua u au, nu u n a auu mam, n¶rt;ma [5, 6]. m um nu num m amamu au ¶rt; nmuaa mu n, n¶rt;mau u¶rt; ¶rt;a nu a¶rt;a. 相似文献
2.
Summary The solution of the internal and external Dirichlet problem is given without assuming rotational symmetry for a body, limited by two intersecting spherical surfaces. The problem is treated in a toroidal system of co-ordinates, using the generalized Mehler integral transform. The results are compared with the solution, presented by Lebedev [1] for the case of rotational symmetry. 相似文献
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Rainer J. Klement 《Astronomy and Astrophysics Review》2010,18(4):567-594
The phase-space structure of our Galaxy holds the key to understand and reconstruct its formation. The ΛCDM model predicts
a richly structured phase-space distribution of dark matter and (halo) stars, consisting of streams of particles torn from
their progenitors during the process of hierarchical merging. While such streams quickly loose their spatial coherence in
the process of phase mixing, the individual stars keep their common origin imprinted into their kinematic and chemical properties,
allowing the recovery of the Galaxy’s individual “building blocks”. The field of Galactic Archeology has witnessed a dramatic
boost over the last decade, thanks to the increasing quality and size of available data sets. This is especially true for
the solar neighborhood, a volume of 1–2 kpc around the sun, where large scale surveys like SDSS/SEGUE continue to reveal the
full 6D phase-space information of thousands of halo stars. In this review, I summarize the discoveries of stellar halo streams
made so far and give a theoretical overview over the search strategies imployed. This article is intended as an introduction
to researchers new to the field, but also as a reference illustrating the achievements made so far. I conclude that disentangling
the individual fragments from which the Milky Way was built requires more precise data that will ultimately be delivered by
the Gaia mission. 相似文献
4.
Simone Daniela Langhans Urs Richard Janine Rueegg Urs Uehlinger Peter Edwards Michael Doering Klement Tockner 《Aquatic Sciences - Research Across Boundaries》2013,75(3):335-348
Quantifying spatial and temporal dynamics of organic matter (OM) is critical both for understanding ecosystem functioning and for predicting impacts of landscape change. To determine the influence of different habitats and coarse particulate OM (CPOM) types upon floodplain OM dynamics, we quantified aerial input, lateral surface transfer, and surface storage of CPOM over an annual cycle on the near-natural floodplain of the River Tagliamento in NE-Italy. Using these data, we modelled floodplain leaf dynamics, taking account of the spatial distribution and hydrologic connectivity of habitats, and using leaf storage as a response variable. Mean aerial CPOM input to the floodplain was similar from riparian forest and islands, but surface transfer was greater from islands, supporting the suggestion that these habitats act as “islands of fertility” along braided rivers. Leaves were the lateral conveyor of energy to more open parts of the floodplain, whereas CPOM was mainly stored as small wood in vegetated islands and riparian forest. Simulating the loss of habitat diversity (islands, ponds) decreased leaf storage on the whole floodplain, on exposed gravel and in large wood accumulations. In contrast, damming (loss of islands, ponds and floods plus floodplain overgrowth) greatly increased storage on exposed gravel. A random shuffle of habitats led to a storage increase on exposed gravel, while that in large wood accumulations and ponds declined. These results disentangle some of the complexities of CPOM dynamics in floodplain ecosystems, illustrate the value of models in understanding ecosystem functioning at a landscape level, and directly inform river management practice. 相似文献
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D. von Schiller V. Acuña D. Graeber E. Martí M. Ribot S. Sabater X. Timoner K. Tockner 《Aquatic Sciences - Research Across Boundaries》2011,73(4):485-497
Temporary streams are a dominant surface water type in the Mediterranean region. As a consequence of their hydrologic regime,
these ecosystems contract and fragment as they dry, and expand after rewetting. Global change leads to a rapid increase in
the extent of temporary streams, and more and more permanent streams are turning temporary. Consequently, there is an urgent
need to better understand the effects of flow intermittency on the biogeochemistry and ecology of stream ecosystems. Our aim
was to investigate how stream nutrient availability varied in relation to ecosystem contraction, fragmentation and expansion
due to hydrologic drying and rewetting. We quantified the temporal and spatial changes in dissolved nitrogen (N) and phosphorus
(P) concentrations along a reach of a temporary Mediterranean forest stream during an entire contraction–fragmentation–expansion
hydrologic cycle. We observed marked temporal changes in N and P concentrations, in the proportion of organic and inorganic
forms as well as in stoichiometric ratios, reflecting shifts in the relative importance of in-stream nutrient processing and
external nutrient sources. In addition, the spatial heterogeneity of N and P concentrations and their ratios increased substantially
with ecosystem fragmentation, reflecting the high relevance of in-stream processes when advective transport was lost. Overall,
changes were more pronounced for N than for P. This study emphasizes the significance of flow intermittency in regulating
stream nutrient availability and its implications for temporary stream management. Moreover, our results point to potential
biogeochemical responses of these ecosystems in more temperate regions under future water scarcity scenarios. 相似文献
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8.
A. M. Gurnell G. E. Petts N. Harris J. V. Ward K. Tockner P. J. Edwards J. Kollmann 《地球表面变化过程与地形》2000,25(3):255-275
After more than 300 years of widespread and intensive river management, few examples of complex, unmanaged river systems remain within Europe. An exception is the Fiume Tagliamento, Italy, which retains a riparian woodland margin and unconfined river channel system throughout almost the entire 170 km length of its river corridor. A research programme is underway focusing on a range of related aspects of the hydrology, fluvial geomorphology and ecology of the Tagliamento. This paper contributes to that programme by focusing on large wood retention. The paper adopts a simple force:resistance approach at the scale of the entire river corridor in order to identify reaches of the river with a high wood retention potential. Information on the character of the river corridor is derived from 1:10 000 scale topographic maps. A range of indices measured at 330 transects across the river corridor supports a classification of the geomorphological style of the river which reflects the presence and abundance of properties previously identified in the literature as large wood retention sites. This classification provides a qualitative representation of the ‘resistance’ of the corridor to wood movement and thus its overall wood‐retention potential. The map‐derived indices are also used to extrapolate estimates of the ten year return period flood to each of the 330 transects so that the downstream pattern of unit stream power can be quantified as an index representing ‘force’ in the analysis. Although input of wood is an important factor in many river systems, it is assumed not to be a limiting factor along the Tagliamento, where riparian woodland is abundant. Field observations of large wood storage illustrate that wood retention at eight sites along the river reflects the presence and abundance of the features incorporated in the classification of geomorphological style, including the complexity of the channel network, the availability of exposed gravel areas, and the presence of islands. In general at the time of survey in August 1998, open gravel areas were estimated to store approximately 1 t ha−1 of wood in single‐thread reaches and 6 t ha−1 in multiple‐thread reaches. Established islands were estimated to store an average of 80 t ha−1 of wood. Nevertheless, there was considerable variability between sites, and pioneer islands, which are not represented on maps or readily identified from air photographs because of their small size, were estimated to store an order of magnitude more wood than established islands. Furthermore, the wood storage from this sample of eight sites did not reflect variability in estimated unit stream power. A series of areas for further research are identified, which can be explored using field data, and which will throw more light on the processes of wood retention in this extremely dynamic fluvial environment. Copyright © 2000 John Wiley & Sons, Ltd. 相似文献
9.
Climate change is one of the main drivers of river warming worldwide. However, the response of river temperature to climate change differs with the hydrology and landscape properties, making it difficult to generalize the strength and the direction, of river temperature trends across large spatial scales and various river types. Additionally, there is a lack of long‐term and large‐scale trend studies in Europe as well as globally. In this study, we investigated the long‐term (25 years; 132 sites) and the short‐term (10 years; 475 sites) river temperature trends, patterns and underlying drivers within the period 1985–2010 in seven river basins of Germany. The majority of the sites underwent significant river warming during 1985–2010 (mean warming trend: 0.03 °C year?1, SE = 0.003), with a faster warming observed during individual decades (1985–1995 and 2000–2010) within this period. Seasonal analyses showed that, while rivers warmed in all seasons, the fastest warming had occurred during summer. Among all the considered hydro‐climatological variables, air temperature change, which is a response to climate forcing, was the main driver of river temperature change because it had the strongest correlation with river temperature, irrespective of the period. Hydrological variables, such as average flow and baseflow, had a considerable influence on river temperature variability rather than on the overall trend direction. However, decreasing flow probably assisted in a faster river temperature increase in summer and in rivers in NE basins (such as the Elbe basin). The North Atlantic Oscillation Index had a greater significant influence on the winter river temperature variability than on the overall variability. Landscape and basin variables, such as altitude, ecoregion and catchment area, induced spatially variable river temperature trends via affecting the thermal sensitivity of rivers, with the rivers in large catchments and in lowland areas being most sensitive. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
10.
Mary J. Harner Nora Opitz Keith Geluso Klement Tockner Matthias C. Rillig 《Aquatic Sciences - Research Across Boundaries》2011,73(1):35-42
Most vascular plants form symbioses with mycorrhizal fungi that associate with roots and provide nutrients to hosts in exchange
for carbohydrates, as well as serve a range of other functions. Mycorrhizal fungi have been studied extensively in upland
ecosystems, but we know less about their ecology at aquatic-terrestrial interfaces, especially their distributions at depth.
Our objectives were to determine whether abundances of arbuscular mycorrhizal fungi (AMF) change as floodplain islands develop
and to describe vertical distributions of AMF from the forest floor to the water table along the freely flowing Tagliamento
River in northeastern Italy. We sampled surface sediments (0–10 cm) from three stages of island development on the floodplain—fresh
deposits, pioneer islands, and established islands. We also sampled sediments vertically (0–150 cm) from the ground surface
to the water table on an established island. We characterized abundance of AMF propagules (colonized roots, spores, and hyphae)
within sediments. Roots available to host fungi were absent on fresh deposits; however, some viable spores and hyphae were
available at these sites. Pioneer and established islands each had similar hyphal lengths (~860 cm cm−3) and colonized root lengths (~3 cm cm−3). Abundance of spores increased from depositional (3.5 ± 0.9 (±SE) cm−3) to pioneer (17 ± 6.1 cm−3) to established (32 ± 6.3 cm−3) islands. On an established island, AMF propagules were present at all depths sampled, including at the water table, providing
first documentation of these symbionts to such depths in a riparian setting. Mycorrhizal fungi likely link aquatic and terrestrial
habitats by connecting plants, soil, and ground water and may influence nutrient transfers among these subsystems. 相似文献