排序方式： 共有8条查询结果，搜索用时 128 毫秒
1
1.
The eelgrass Zostera marina is a key structural and functional species across the European coastline. The separate and interactive effects of eelgrass canopy removal and sediment addition on the sediment characteristics and the structure of benthic communities were studied in a factorial field experiment in the Northern Baltic Sea in July–August 2006. The removal of eelgrass canopy temporarily increased the sediment oxygen consumption, reduced the content of fine particles (<100 μm) and organic matter in the sediment, and increased the share of sand fraction (250–500 μm). Sediment addition increased the content of fine particles (<100 μm) and reduced the share of sand fraction (250–1000 μm). The effects were strongest in the presence of eelgrass canopy. Benthic invertebrates and macroalgae were affected by eelgrass canopy removal but not by sediment addition. The removal of eelgrass canopy significantly decreased benthic species richness and invertebrate and macroalgal densities. To conclude, our experiment demonstrates that Z. marina defines the patterns of benthic macroalgae and invertebrates but has moderate effects on sediment structure and metabolism in the Northern Baltic Sea. 相似文献
2.
Iceland： a window on North-Atlantic divergent plate tectonics and geologic processes 总被引：3，自引：0，他引：3
Freysteinn Sigmundsson Kristjan Scemundsson 《《幕》》2008,31(1):92-97
Opening of the North Atlantic about 60 million years ago was associated with massive basaltic volcanism, now found on both sides of the Atlantic Ocean. Divergence of the North American and Eurasian plates since then has formed the ocean floor in the North Atlantic, with the Mid-Atlantic Ridge marking the present day plate boundary. Iceland is the only large sub-aerial portion of the ridge. These unique conditions provide a ＂window＂ on divergent plate tectonics and related geologic processes, some of the main features of which are described and explained in this paper： The geological record in Iceland reaches back into late Tertiary and is particularly notable for the interplay of rift-volcanism and glaciation during the last three million years of rapid climate change. 相似文献
3.
Agust Gudmundsson Niels Oskarsson Karl Gronvold Kristjan Saemundsson Oddur Sigurdsson Ragnar Stefansson Sigurdur R Gislason Pall Einarsson Bryndis Brandsdottir Gudrun Larsen Haukur Johannesson Thorvaldur Thordarson 《Bulletin of Volcanology》1992,54(3):238-246
The eruption that started in the Hekla volcano in South Iceland on 17 January 1991, and came to an end on 11 March, produced mainly andesitic lava. This lava covers 23 km2 and has an estimated volume of 0.15 km3. This is the third eruption in only 20 years, whereas the average repose period since 1104 is 55 years. Earthquakes, as well as a strain pulse recorded by borehole strainmeters, occurred less than half an hour before the start of the eruption. The initial plinian phase was very short-lived, producing a total of only 0.02 km3 of tephra. The eruption cloud attained 11.5 km in height in only 10 min, but it became detached from the volcano a few hours later. Several fissures were active during the first day of the eruption, including a part of the summit fissure. By the second day, however, the activity was already essentially limited to that segment of the principal fissure where the main crater subsequently formed. The average effusion rate during the first two days of the eruption was about 800 m3 s–1. After this peak, the effusion rate declined rapidly to 10–20 m3 s–1, then more slowly to 1 m3 s–1, and remained at 1–12 m3 s–1 until the end of the eruption. Site observations near the main crater suggest that the intensity of the volcanic tremor varied directly with the force of the eruption. A notable rise in the fluorine concentration of riverwater in the vicinity of the eruptive fissures occurred on the 5th day of the eruption, but it levelled off on the 6th day and then remained essentially constant. The volume and initial silica content of the lava and tephra, the explosivity and effusion rate during the earliest stage of the eruption, as well as the magnitude attained by the associated earthquakes, support earlier suggestions that these parameters are positively related to the length of the preceeding repose period. The chemical difference between the eruptive material of Hekla itself and the lavas erupted in its vicinity can be explained in terms of a density-stratified magma reservoir located at the bottom of the crust. We propose that the shape of this reservoir, its location at the west margin of a propagating rift, and its association with a crustal weakness, all contribute to the high eruption frequency of Hekla. 相似文献
4.
Chris Zevenbergen Sebastiaan van Herk Jeroen Rijke Pavel Kabat Pieter Bloemen Richard Ashley Andrew Speers Berry Gersonius William Veerbeek 《Natural Hazards》2013,65(3):1217-1225
There is a growing international recognition that flood risk management in optima forma should be a programmed and flexible process of continuously improving management practices by active learning about the outcome of earlier and ongoing interventions and drivers of change. In the Netherlands, such a long-term, adaptive flood risk management strategy is now being implemented. This so-called second Delta Programme aims to identify and exploit opportunities and capitalize on short-term benefits and opportunistic synergies that arise from change and will require adaptive policymaking. It also requires the financial and institutional means to operate in a long-lasting way, which at the very least, means engaging stakeholders, gathering and disseminating results and adaptation of future plans. Transferring the Dutch approach to other countries is a major challenge that calls for fundamental changes in institutional arrangements at various levels and thus requires customized programmes for strategic institutional change. Recent examples of transfer will provide important lessons of how institutional change can successfully occur and will contribute insights for other countries that attempting to reform their flood risk management strategies. Continuous monitoring and evaluation and sharing international experiences will become crucial for the effective delivery and wider uptake of these new strategies around the globe. 相似文献
5.
Jonne Kotta Kristjan Herkül Ilmar Kotta Helen Orav-Kotta & Velda Lauringson 《Marine Ecology》2009,30(S1):56-64
The importance of suspension-feeding mussels is particularly apparent in benthic communities; however, the role of this feeding strategy on the development of macroalgal and associated invertebrate communities is in general poorly known. The effect of suspension-feeding mussels Mytilus trossulus on benthic communities was studied in an in situ factorial field experiment in the Northern Baltic Sea over one ice-free season. The experiment was performed under different regimes of wave exposure (low and moderate) and on different sedimentary habitats (soft bottom with high organic content, soft bottom with low organic content, and hard bottom). In general the presence of mussels was associated with increased biomass of filamentous algae, herbivores and deposit feeders and decreased biomass of charophytes. The effect of M. trossulus interacted with the effect of exposure and substrate. Stronger responses were observed in moderately exposed than in sheltered areas. The presence of M. trossulus affected charophytes and deposit feeders on sand with low content of organic matter and filamentous algae on pebbles but not on other substrate types. The magnitude of the effects varied between months. The results suggest that (i) even in dynamic coastal systems the biodeposits and excretions of mussels are at least partly assimilated locally and are not flushed away to the open sea, (ii) the accumulation of faecal material induced elevated growth of deposit feeders, (iii) mussels enhanced the growth of ephemeral macroalgae and reduced the growth of perennial macroalgae, and (iv) together with increasing benthic primary production, mussels indirectly increase the production of herbivores. 相似文献
6.
Comparison of benthic and pelagic suspension feeding in shallow water habitats of the Northeastern Baltic Sea 总被引：1，自引：0，他引：1
Velda Lauringson Jonne Kotta Helen Orav-Kotta Ilmar Kotta Kristjan Herkül & Arno Põllumäe 《Marine Ecology》2009,30(S1):43-55
Combining field experiments with the biomass distribution data of dominant suspension feeders we compared the benthic and pelagic suspension feeding rates in shallow non-tidal brackish water coastal habitats. We found that pelagic grazing exceeded benthic grazing in almost all cases, on average from 14 to 4819 times depending on the site. Benthic grazing rates were related to site-specific environmental parameters and showed no relationship with water chlorophyl a (Chl a ) content, whereas pelagic grazing rates varied both spatially and temporally and were related to water Chl a content. Our results indicate that in several shallow coastal habitats, pelagic suspension feeding substantially exceeds benthic suspension feeding. This suggests that pelagic recycling is higher than the amounts of energy redirected from pelagic to benthic food webs by benthic suspension feeders. These results increase our knowledge of the energy flows in coastal ecosystems. 相似文献
7.
8.
This study reports K/Ar ages for basalts from four areas in central Iceland where erosion of structural highs has exposed stratigraphically older levels of the lava pile. The four areas are the Eyjafjördur regional anticline and the Tjörnes horst in north Iceland and the Borgarnes and Hreppar regional anticlines in south Iceland. Three of the areas have their older plateau basalts within the range of at least 8.5–9.5 m.y. old. Only the Hreppar area does not have any exposed rocks much older than about 2.5 m.y. The Tjörnes data confirm that the exposed Husavik faults have played a major role in the transform displacement of the Tjörnes Fracture Zone. The results are further evidence that the spreading axes through Iceland have had a history of shifting their location. Analysis of our results suggests that the regional anticlines of Iceland, a seeming structural anomaly in a spreading regime, have resulted from shifting spreading axes which transitionally coexist and create regional anticlines in between. 相似文献
1