全文获取类型
收费全文 | 257994篇 |
免费 | 4330篇 |
国内免费 | 3371篇 |
专业分类
测绘学 | 6967篇 |
大气科学 | 19081篇 |
地球物理 | 54209篇 |
地质学 | 88977篇 |
海洋学 | 21341篇 |
天文学 | 56104篇 |
综合类 | 1011篇 |
自然地理 | 18005篇 |
出版年
2021年 | 2256篇 |
2020年 | 2602篇 |
2019年 | 2857篇 |
2018年 | 3297篇 |
2017年 | 2988篇 |
2016年 | 5601篇 |
2015年 | 4228篇 |
2014年 | 6931篇 |
2013年 | 14245篇 |
2012年 | 6440篇 |
2011年 | 7762篇 |
2010年 | 6679篇 |
2009年 | 9403篇 |
2008年 | 8320篇 |
2007年 | 7685篇 |
2006年 | 9703篇 |
2005年 | 7709篇 |
2004年 | 7655篇 |
2003年 | 7155篇 |
2002年 | 6774篇 |
2001年 | 6043篇 |
2000年 | 5975篇 |
1999年 | 5211篇 |
1998年 | 5232篇 |
1997年 | 5047篇 |
1996年 | 4694篇 |
1995年 | 4445篇 |
1994年 | 4120篇 |
1993年 | 3869篇 |
1992年 | 3661篇 |
1991年 | 3597篇 |
1990年 | 3771篇 |
1989年 | 3524篇 |
1988年 | 3307篇 |
1987年 | 3859篇 |
1986年 | 3407篇 |
1985年 | 4241篇 |
1984年 | 4748篇 |
1983年 | 4418篇 |
1982年 | 4332篇 |
1981年 | 3938篇 |
1980年 | 3647篇 |
1979年 | 3511篇 |
1978年 | 3483篇 |
1977年 | 3281篇 |
1976年 | 3046篇 |
1975年 | 2960篇 |
1974年 | 2922篇 |
1973年 | 3078篇 |
1972年 | 2029篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
971.
972.
The data on the species composition, trophic structure, and distribution of macrozoobenthos in Amur Bay obtained in 2001 are presented. Long-term changes in the benthos are analyzed. In 2001, the total benthos biomass significantly increased, although the parameters of the species richness declined as compared to 1986–1989. In 2001, as well as in the 1970s and 1980s, the benthos trophic structure was characterized by the prevalence of deposit feeders. The entire structure of the benthos is evaluated as an eutrophic one. Eutrophication of the bay is considered to be the most probable cause of the negative changes in the benthic communities of the bay in 2001, as well as 15 years ago. 相似文献
973.
Martina A. Doblin Stephen B. Baines Lynda S. Cutter Gregory A. Cutter 《Estuarine, Coastal and Shelf Science》2006,67(4):681-694
As part of a study of estuarine selenium cycling, we measured the concentration, chemical form (speciation), and distribution of particulate selenium under various river flow conditions in the North San Francisco Bay (from the Golden Gate to the Sacramento and San Joaquin Rivers). We also conducted laboratory studies on the accumulation of selenium by phytoplankton, the critical first step in the transformation of dissolved to particulate selenium. Total particulate selenium concentration in the North SF Bay was relatively constant between high and low flow periods, ranging spatially from 0.05 to 0.35 nmol l−1 and comprising between 5 and 12% of the total water column selenium inventory. Mean concentrations were generally highest in the Carquinez Strait–Suisun Bay region (salinity 0–17) and lowest in Central Bay. However, selenium content of suspended particles varied with river flow, with higher content during low flow (9.76 ± 4.17 nmol g−1; mean ± sd; n = 67) compared to high flow (7.10 ± 4.24 nmol g−1; n = 39). Speciation analyses showed that most particulate selenium is organic selenide (45 ± 27%), with a smaller proportion (typically <30%) of adsorbed selenite + selenate and a varying proportion (35 ± 28%) of elemental selenium. Based on the amount of elemental selenium in the seston (total suspended material), we calculate that resuspension of estuarine sediments could contribute 29–100% of particulate selenium in the water column. While selenium content of SF Bay seston (>0.4 μm) is relatively unenriched compared to phytoplankton (13.6–155 nmol g−1 dry weight) on a mass basis, when normalized to carbon or nitrogen, seston contains a similar selenium concentration to SF Bay sediments or phytoplankton cultures. SF Bay seston is thus comprised of selenium-rich phytoplankton and phyto-detritus, but also inorganic clay mineral particles that effectively “dilute” total particulate selenium. Selenium concentrations in algal cultures (11 species) exposed to 90 nmol l−1 selenite show relatively large differences in selenium accumulation, with the diatoms, chlorophytes and cryptophytes generally having lower selenium cell content (3.8 ± 2.7 × 10−9 nmol selenium cell−1) compared to the dinoflagellates (193 ± 73 × 10−9 nmol selenium cell−1). Because phytoplankton are such a rich (but variable) source of selenium, their dynamics could have a profound effect on the particulate selenium inventory in the North SF Bay. 相似文献
974.
975.
976.
977.
978.
979.
K.F. Lambrakos 《Ocean Engineering》1982,9(4):385-405
A methodology has been developed to calculate the dynamic probabilistic movement and resulting stresses for marine pipelines subjected to storm waves. A directional wave spectrum is used with a Fourier series expansion to simulate short-crested waves and calculate their loads on the pipeline. The pipeline displacements resulting from these loads are solutions to the time-dependent beam-column equation which also includes the soil resistance as external loading. The statistics of the displacements for individual waves are combined with the wave statistics for a given period of time, e.g. pipeline lifetime, to generate probabilistic estimates for net pipeline movement. On the basis of displacements for specified probability levels the pipeline configuration is obtained from which pipeline stresses can be estimated using structural considerations, e.g. pipeline stiffness, end restraints, etc. 相似文献
980.
In this paper the peculiarities of compiled oceanographic data sets, which consist primarily in the presence of some characteristic information as well as in the planned incomplete filling of the files with observations, are discussed. A number of special ways are suggested which essentially improve the packing density of computerized data without applying special archiving algorithms. The effectiveness of these methods has been tested by treating thein situ hydrological/chemical data file compiled in the Black Sea in 1988.Translated by Vladimir A. Puchkin. 相似文献