全文获取类型
收费全文 | 857篇 |
免费 | 122篇 |
国内免费 | 265篇 |
专业分类
测绘学 | 34篇 |
大气科学 | 147篇 |
地球物理 | 95篇 |
地质学 | 359篇 |
海洋学 | 263篇 |
天文学 | 239篇 |
综合类 | 62篇 |
自然地理 | 45篇 |
出版年
2024年 | 14篇 |
2023年 | 14篇 |
2022年 | 29篇 |
2021年 | 37篇 |
2020年 | 28篇 |
2019年 | 38篇 |
2018年 | 24篇 |
2017年 | 22篇 |
2016年 | 25篇 |
2015年 | 29篇 |
2014年 | 42篇 |
2013年 | 48篇 |
2012年 | 49篇 |
2011年 | 42篇 |
2010年 | 51篇 |
2009年 | 55篇 |
2008年 | 61篇 |
2007年 | 57篇 |
2006年 | 81篇 |
2005年 | 88篇 |
2004年 | 59篇 |
2003年 | 54篇 |
2002年 | 48篇 |
2001年 | 47篇 |
2000年 | 44篇 |
1999年 | 38篇 |
1998年 | 23篇 |
1997年 | 10篇 |
1996年 | 16篇 |
1995年 | 16篇 |
1994年 | 5篇 |
1993年 | 16篇 |
1992年 | 11篇 |
1991年 | 1篇 |
1990年 | 4篇 |
1989年 | 6篇 |
1988年 | 2篇 |
1987年 | 2篇 |
1985年 | 2篇 |
1984年 | 4篇 |
1981年 | 1篇 |
1978年 | 1篇 |
排序方式: 共有1244条查询结果,搜索用时 15 毫秒
61.
R Henriques WM Potts CV Santos WHH Sauer PW Shaw 《African Journal of Marine Science》2018,40(1):13-24
The West Coast dusky kob Argyrosomus coronus is a commercially exploited fish with a distribution confined to the Angola–Benguela Frontal Zone (ABFZ) of the southeastern Atlantic Ocean. A previous study revealed that during a recent period of local warming the species extended its distribution into Namibian waters, where it hybridised with the resident and congeneric Argyrosomus inodorus. Environmental changes are a major threat to marine biodiversity and when combined with overfishing have the potential to accelerate the decline of species. However, little is known regarding the evolutionary history and population structure of A. coronus across the ABFZ. We investigated genetic diversity, population structure and historical demographic changes using mtDNA control region sequences and genotypes at six nuclear microsatellite loci, from 180 individuals. A single, genetically homogeneous population was indicated across the distributional range of A. coronus (φST = 0.041, FST = 0.000, D = 0.000; p > 0.05). These findings imply that the oceanographic features within the ABFZ do not appear to significantly influence population connectivity in A. coronus, which simplifies management of the species. However, reconstruction of the demographic history points to a close link between the evolutionary history of A. coronus and the environmental characteristics of the ABFZ. This outcome suggests the species’ vulnerability to the rapid environmental changes being observed across this region, and highlights a pressing need for transboundary management to mitigate the impacts of climate change in this global hotspot of seawater temperature changes. 相似文献
62.
本文综述了国内外关于介核生物(主要包括甲藻和裸藻)分子生物学方面的研究进展。介绍了介核生物的分子分类进展及其特殊的分类地位,甲藻毒素和裸藻在环境检测上的应用,同时简介了介核藻类活性物质的开发利用前景。 相似文献
63.
Clibanarius , a genus of hermit crabs, is one of the most diverse genera within Diogenidae. However, studies on the phylogeny within Clibanarius is very limited. We reconstruct the molecular phylogeny of the genus Clibanarius based on two mitochondrial (16S rRNA and COI) and two nuclear protein-coding (NaK and PEPCK) genes using a multigene phylogenetic approach. Eleven selected Clibanarius species from the Indo-West Pacifi c are analyzed and the Bayesian and maximum likelihood analyses produced identical results in topology. Results suggest that C . rutilus and C . infraspinatus are more closely related to each other than to C . snelliusi , and C . striolatus shows a close relationship to C . longitarsus . The close association of C . merguiensis with C . englaucus and C . humilis is strongly supported by the divergence of C . virescens from them. Our phylogenetic results contradict the morphological classifi cation scheme proposed for Clibanarius and indicate that the relative length of dactyls and propodi is not phylogenetically signifi cant in Clibanarius . In addition, we speculate that the morphological characteristic of the median cleft of the telson might be phylogenetically important for Clibanarius . 相似文献
64.
东海原甲藻的分子鉴定 总被引:4,自引:2,他引:4
采用nrDNAITS序列及18S序列两种分子标记,对东海原甲藻和美国国家海洋藻种保藏中心(CCMP)的具齿原甲藻进行分子鉴定,结果表明,两者之间序列非常相似,两者的nrDNAITS序列碱基差异值仅为0.002,nrDNA18S序列的碱基差异值为0.000,根据分子数据,东海原甲藻和美国国家海洋藻种保藏中心的具齿原甲藻应为同一个种.从基因库获取原甲藻的另外3个种nrD-NAITS序列和8个种的18S序列,用计算机软件进行分析和构建分子系统树,结果显示18S序列用于原甲藻的分子鉴定过于保守,而nrDNAITS更适合于该种属界定的分子标准. 相似文献
65.
66.
傅承启 《中国科学院上海天文台年刊》2002,(1)
根据银心区的CO谱线观测 ,对 30 0pc气体分子环提出了一种开普勒模型 :气体分子分布在一个膨胀或收缩的椭圆环上 ,环上的气体分子按开普勒定律绕银心转动。在这样一种模型下 ,气体分子的辐射可以产生类似CO谱线观测的图形。简单讨论了各种模型参数对图形的可能影响 相似文献
67.
68.
R. G.Smith † R. D.Blum † D. E.Quinn K.Sellgren D. C. B.Whittet † 《Monthly notices of the Royal Astronomical Society》2002,330(4):837-843
To better understand the conditions under which ice mantles form on grains in molecular clouds, three globules in the Southern Coalsack have been searched for the presence of H2 O ice. Given the total lack of star formation in the Coalsack, it is an ideal site for studying unprocessed icy molecular mantles. In our sample of eight field stars lying behind the Coalsack we detect strong H2 O ice absorption in the lines of sight to two stars and possible weak absorption in four others. We estimate H2 O ice column densities or upper limits for these lines of sight. Compared to dark clouds such as Taurus, the Coalsack H2 O ice column densities are lower than expected given the quiescent nature of the Coalsack region. It is possible that the chemical evolution of the Coalsack may simply be at too early a stage for significant ice mantles to appear on the grains, except perhaps in the densest parts of some of the globules. Alternatively, the presence or absence of ice absorption may be related to the distribution of dust along each line of sight, specifically, the relative contributions of dense globules and a more extended diffuse component. For example, our observations are consistent with an ice threshold extinction similar to that observed in the Taurus dark cloud if extinction amounting to A V ∼5 towards Globules 2 and 3 arises in the extended component. Globule 1 appears to have no extended component. 相似文献
69.
70.