首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   152篇
  免费   3篇
  国内免费   1篇
测绘学   1篇
大气科学   13篇
地球物理   21篇
地质学   60篇
海洋学   30篇
天文学   11篇
自然地理   20篇
  2022年   1篇
  2021年   2篇
  2020年   5篇
  2019年   4篇
  2018年   3篇
  2017年   3篇
  2016年   8篇
  2015年   2篇
  2014年   5篇
  2013年   6篇
  2012年   8篇
  2011年   9篇
  2010年   6篇
  2009年   7篇
  2008年   5篇
  2007年   6篇
  2006年   4篇
  2005年   17篇
  2004年   8篇
  2003年   7篇
  2002年   6篇
  2000年   2篇
  1999年   1篇
  1998年   2篇
  1996年   1篇
  1995年   2篇
  1994年   2篇
  1992年   3篇
  1991年   2篇
  1989年   2篇
  1988年   1篇
  1986年   2篇
  1985年   1篇
  1981年   1篇
  1980年   1篇
  1979年   2篇
  1976年   1篇
  1975年   1篇
  1972年   1篇
  1931年   2篇
  1928年   3篇
  1927年   1篇
排序方式: 共有156条查询结果,搜索用时 15 毫秒
151.
The endangered Western population of the Steller sea lion declined for three decades for uncertain reasons. We present baseline data of metal concentrations in pups as a first step towards investigating the potential threat of developmental exposures to contaminants. Seven metals were investigated: arsenic, cadmium, silver, aluminum, mercury, lead and vanadium. Vanadium was detected in only a single blubber sample. Mercury appears to be the most toxicologically significant metal with concentrations in the liver well above the current action level for mercury in fish. The concentrations of aluminum, arsenic, silver, cadmium and lead were present in one-fourth to two-thirds of all samples and were at either comparable or below concentrations previously reported. Neither gender nor region had a significant effect on metal burdens. Future work should consider metal concentrations in juveniles and adults and toxicological studies need to be performed to begin to assess the toxicity of these metals.  相似文献   
152.
Synthesis of NaTiSi2O6 at high pressure and temperature (65 kbar and 1550° C) results in transparent, light-green crystals when quenched to ambient conditions. These crystals have a pyroxene structure and belong to the series NaRSi2O6 where R is (in order of increasing ionic radius) Al, Cr, Fe, Ti, Sc, and In. X-ray diffraction patterns and a diffuse reflectance spectrum help to characterize this material which is one of the few oxides known containing Ti3+.Contribution No. 1715.  相似文献   
153.
Ecosystem-based management of marine fisheries requires the use of simulation modelling to investigate the system-level impact of candidate fisheries management strategies. However, testing of fundamental assumptions such as system structure or process formulations is rarely done. In this study, we compare the output of three different ecosystem models (Atlantis, Ecopath with Ecosim, and OSMOSE) applied to the same ecosystem (the southern Benguela), to explore which ecosystem effects of fishing are most sensitive to model uncertainty. We subjected the models to two contrasting fishing pressure scenarios, applying high fishing pressure to either small pelagic fish or to adult hake. We compared the resulting model behaviour at a system level, and also at the level of model groups. We analysed the outputs in terms of various commonly used ecosystem indicators, and found some similarities in the overall behaviour of the models, despite major differences in model formulation and assumptions. Direction of change in system-level indicators was consistent for all models under the hake pressure scenario, although discrepancies emerged under the small-pelagic-fish scenario. Studying biomass response of individual model groups was key to understanding more integrated system-level metrics. All three models are based on existing knowledge of the system, and the convergence of model results increases confidence in the robustness of the model outputs. Points of divergence in the model results suggest important areas of future study. The use of feeding guilds to provide indicators for fish species at an aggregated level was explored, and proved to be an interesting alternative to aggregation by trophic level.  相似文献   
154.
Alkaline phosphatase (AP) was detected using ELF-97® in silicon-starved Pseudo-nitzschia multiseries cells; thus, we tested two, alternative hypotheses: Pseudo-nitzschia multiseries has a high phosphate demand, showing signs of phosphate deficiency even when concentrations of orthophosphate are high, or silicate deficiency can stimulate the AP enzyme in this species. We also studied the effect of silicon deficiency on AP in three other common marine diatoms: Thalassiosira pseudonana, Nitzschia pusilla, and Nitschia closterium. Each of the species tested showed a different pattern of AP regulation. AP levels, however, increased in the four diatoms as a result of silicon deficiency, suggesting that AP may be involved in a variety of intracellular processes related to silicon deficiency. Additionally, the results of this study indicate that AP could be stimulated by stressors other than phosphate deficiency, such as silicon deficiency; therefore, it should be used cautiously as an indicator of phosphate limitation.  相似文献   
155.
Recent research on food access has increasingly focused on how individuals’ daily mobility, much of it based on activity spaces created from GPS data. In this paper, we expand this research through an analysis of a large transit survey (n = 21,298 households) from Minneapolis/St. Paul, Minnesota. We do this using relational approach focused on the topological connections found in household travel patterns rather than measures of exposure based on geographic distance. Our exploratory data analysis analyzes both grocery shopping and eating out across the metropolitan area, focusing on the position of utilized food sources relative to home and work locations, utilized modes of transit, and other daily activities often combined with food shopping. Households often used food sources located outside their residential neighborhoods, usually moving toward the central city to do so. Eating out occurred farther from home than grocery shopping, though in many cases close to work. Automobile use was most common for grocery shopping trips, but less so in the lowest income households and in the central city. Our findings show that a relational approach can identify distinctive patterns in everyday food provisioning by emphasizing the connections between food shopping and other everyday household activities.  相似文献   
156.
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号