全文获取类型
收费全文 | 46678篇 |
免费 | 20604篇 |
国内免费 | 46170篇 |
专业分类
测绘学 | 5331篇 |
大气科学 | 22448篇 |
地球物理 | 14633篇 |
地质学 | 42393篇 |
海洋学 | 18483篇 |
天文学 | 901篇 |
综合类 | 5216篇 |
自然地理 | 4047篇 |
出版年
2024年 | 363篇 |
2023年 | 873篇 |
2022年 | 1453篇 |
2021年 | 2041篇 |
2020年 | 2864篇 |
2019年 | 6080篇 |
2018年 | 6167篇 |
2017年 | 5861篇 |
2016年 | 5932篇 |
2015年 | 5561篇 |
2014年 | 5010篇 |
2013年 | 5728篇 |
2012年 | 5646篇 |
2011年 | 5441篇 |
2010年 | 5446篇 |
2009年 | 4666篇 |
2008年 | 4086篇 |
2007年 | 4000篇 |
2006年 | 3525篇 |
2005年 | 3227篇 |
2004年 | 3309篇 |
2003年 | 2793篇 |
2002年 | 2606篇 |
2001年 | 2388篇 |
2000年 | 1965篇 |
1999年 | 1752篇 |
1998年 | 1721篇 |
1997年 | 1772篇 |
1996年 | 1396篇 |
1995年 | 1339篇 |
1994年 | 1190篇 |
1993年 | 1156篇 |
1992年 | 991篇 |
1991年 | 741篇 |
1990年 | 716篇 |
1989年 | 582篇 |
1988年 | 503篇 |
1987年 | 398篇 |
1986年 | 315篇 |
1985年 | 255篇 |
1984年 | 276篇 |
1983年 | 168篇 |
1982年 | 208篇 |
1981年 | 147篇 |
1980年 | 109篇 |
1979年 | 113篇 |
1978年 | 46篇 |
1977年 | 46篇 |
1971年 | 53篇 |
1970年 | 44篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
101.
Competition within the marine microalgae over the polar dark period in the Greenland Sea of high Arctic 总被引:2,自引:0,他引:2
Abstract-With the onset of winter, polar marine microalgae would have faced total darkness for aperiod of up to 6 months. A natural autumn community of Arctic sea ice microalgae was collected fordark survival experiments from the Greenland Sea during the ARKTIS-XI/2 Expedition of RV Po-larstern in October 1995. After a dark period of 161 days, species dominance in the algal assemblagehave changed from initially pennate diatoms to small phytoflagellates (<20μm). Over the entire darkperiod, the mean algal growth rate was-0.01 d~(-1). Nearly all diatom species had negative growthrates, while phytoflagellate abundance increased. Resting spore formation during the dark period was ob-served in less than 4.5% of all cells and only for dinoflagellates and the diatom Chaetoceros spp. We as-sume that facultative heterotrophy and energy storage are the main processes enabling survival during thedark Arctic winter. After an increase in light intensity, microalgal cells reacted with fast growth withindays. Phytoffa 相似文献
102.
103.
Photoinduced toxicity single and binary mixtures of four polycyclic aromatic hydrocarbons to the marine diatom Skeletonema costatum 总被引:1,自引:0,他引:1
Diatom Skeletonema costatum Cleve is one of the main predominant phytoplankton species in the Changjiang Estuary in China. In order to provide some basic information for future assessment of the potential risk on phytoplankton communities in this estuary caused by polycyclic aromatic hydrocarbons(PAHs), this alga was selected as a representative to investigate the photoinduced toxicity of PAHs, in single and mixture. Four PAHs including three-ring phenanthrene and anthracene, four-ring fluoranthene and pyrene were tested in the laboratory. The single toxicity of each PAH on this microalga was compared with and without the simulated solar UV radiation. The results showed that this microalga was sensitive to PAH's photoinduced toxicity. Ratios of the 72 h median effect concentration obtained for fluorescent and UV light tests were about 8.4 for phenanthrene, 13.0 for anthracene, 6.5 for fluoranthene, and 5.7 for pyrene, indicating that UV light enhanced the PAH toxicity to this alga significantly. Under the fluorescent radiation (lacking UV), the dose-response curves based on chemical concentrations revealed that the order of toxic strength was fluoranthene greater than pyrene greater than anthracene greater than phenanthrene; while under the UV radiation (476 μW/cm2 for UVA, 6.5 μW/cm2 for UVB) it became fluoranthene approximately equaling anthracene greater than pyrene greater than phenanthrene, indicating that the UV light also changed its relative toxicity to this alga. The photoinduced toxicity of PAHs to the marine diatom S. costatum might be a synergistic effect of photosensitization reactions (e.g., generation of single-state oxygen) and photomodification (photooxidation and/or photolysis).The combined effects of six binary mixtures on the marine diatom S. costatum were investigated using the additive-index method. Four binary-mixtures (phenanthrene plus anthracene; phenanthrene plus pyrene; anthracene plus fluoranthene; anthracene plus pyrene) were found to be synergistic joint action mode, while two binary-mixtures (phenanthrene plus fluoranthene; fluoranthene plus pyrene) displayed antagonist, revealing a complex pattern of possible interactions. The combined action mode of PAHs might be related to various factors such as the mixture compounds, mixture ratios and test conditions, etc. 相似文献
104.
罗氏沼虾育苗用水中Mg2+与Ca2+含量及Mg2+/Ca2+对出苗率的影响 总被引:13,自引:0,他引:13
1991年3-6月和1992年3-6月在南汇县东海水产养殖公司对罗氏沼虾育苗用水调配原则与不同基础水的调配方法进行试验研究。结果表明,育苗用水中Mg^2+与Ca^2+含量及Mg^2+/Ca^2+必须达到一定的范围,这是调配的基本原则。以鱼塘水作基础水时,需添加6种化学药品,其中Mg^2+与Ca^2+含量可适范围分别为300.0-440.0mg/L与170.0-244.0mg/L,Mg^2+/Ca^ 相似文献
105.
Thesecondorderspectrumoftwo-dimensionalrandomwaves¥DingPingxing;SunFuandYuZhouwen(ReceivedJanuary15,1993;acceptedMay31,1993)A... 相似文献
106.
Full-rangenonlinearanalysisoffatiguebehaviorsofreinforcedconcretestructuresbyfiniteelementmethod¥SongYupu;ZhaoShunbo;WangRuim... 相似文献
107.
A method combining boundary and finite element approaches as well as modal analysis to solve the acoustical scattering from elastic obstacle in infinite fluid is described. An elastic cylinder with finite length immersed in water is chosen as an example for numerical demonstration. The computed frequency response and direction pattern of the backscattering target strength of the cylinder are conformable to those measured. 相似文献
108.
饱和潜水技术可大大延长水下作业时间、提高潜水作业效率,是开发海底资源、水下施工、援潜救生、海洋考察和科学研究不可缺少的一种手段.近20年来,取得了较大的进展,人体氦氧饱和潜水模拟实验的深度已达650米,现场饱和潜水为460米,巡潜已达501米. 相似文献
109.
110.