全文获取类型
收费全文 | 4417篇 |
免费 | 355篇 |
国内免费 | 452篇 |
专业分类
测绘学 | 61篇 |
大气科学 | 176篇 |
地球物理 | 941篇 |
地质学 | 950篇 |
海洋学 | 813篇 |
天文学 | 1706篇 |
综合类 | 118篇 |
自然地理 | 459篇 |
出版年
2024年 | 8篇 |
2023年 | 28篇 |
2022年 | 80篇 |
2021年 | 92篇 |
2020年 | 96篇 |
2019年 | 110篇 |
2018年 | 93篇 |
2017年 | 95篇 |
2016年 | 104篇 |
2015年 | 106篇 |
2014年 | 126篇 |
2013年 | 169篇 |
2012年 | 93篇 |
2011年 | 164篇 |
2010年 | 150篇 |
2009年 | 338篇 |
2008年 | 341篇 |
2007年 | 359篇 |
2006年 | 371篇 |
2005年 | 284篇 |
2004年 | 251篇 |
2003年 | 261篇 |
2002年 | 207篇 |
2001年 | 173篇 |
2000年 | 231篇 |
1999年 | 209篇 |
1998年 | 188篇 |
1997年 | 87篇 |
1996年 | 70篇 |
1995年 | 76篇 |
1994年 | 53篇 |
1993年 | 38篇 |
1992年 | 27篇 |
1991年 | 12篇 |
1990年 | 23篇 |
1989年 | 13篇 |
1988年 | 6篇 |
1987年 | 11篇 |
1986年 | 5篇 |
1985年 | 20篇 |
1984年 | 11篇 |
1983年 | 11篇 |
1982年 | 15篇 |
1981年 | 15篇 |
1980年 | 2篇 |
1979年 | 1篇 |
1978年 | 1篇 |
排序方式: 共有5224条查询结果,搜索用时 31 毫秒
81.
82.
Responses of a Mediterranean soft bottom community to short-term (1993-1996) hydrological changes in the Rhone river 总被引:1,自引:0,他引:1
The polychaete fauna of muddy bottoms off the Rhone delta (NW Mediterranean) was seasonally sampled at two sites at 70 m depth, from 1993 to 1996. During this period, five severe flood events occurred. A clustering analysis (distance coefficient of Whittaker) and three way fixed factor ANOVAs (site x season x year) showed strong year-to-year changes in species density and community structure, changes that masked seasonal variations. Total density increased by a factor of 3 and density of most species significantly increased following the first flooding event. Changes in the community structure were due to the successive dominance of a few species. The opportunistic species, such as Cossura sp., Mediomastus sp. and Polycirrus sp., exhibited peaks in density 1-3 months after the flood. These peaks were followed by a drastic decline. For species with a long life span, such as Sternaspis scutata, a slower but continuous increase in density was observed which was maintained for several months. The density fluctuations of these species exhibited a good correlation with river flow with time lags of 1-2 years. The successional dynamics observed are explained according to the geographical origin of the floods and the biology and feeding ecology of species. 相似文献
83.
A tether management system of a winch module in a marine environment is proposed. For the purpose of this study the subsea body is considered as a mass attached at the free end of a long tether which is wrapped around a circular drum controlled by an external torque. The winch is considered mounted on a heaving platform. The effect of the heaving platform on the motions of the drum, tether and attached mass are studied with respect to the longitudinal vibrations of the tether in one dimension. The hydrodynamic effects are considered on the deployed portion of the tether and the tethered mass, both of which are assumed submerged in otherwise still water. The resulting nonlinear system of equations of motion is developed and solved numerically for an example of a marine tether system. The effect of the tether extensibility on the operation threshold of the system is identified. Comparison with an inextensible tether case is provided. The numerical results and their analysis for the retrieval/deployment of the system are presented. 相似文献
84.
1994年6,7月,调查了广州、深圳、珠海及珠江口海区的空气微生物粒子沉降量状况。发现空气微生物粒子总量、真菌粒子量、真菌粒子数/总菌粒子数百分比在三市中不一致。这三种数据一般是陆上的高于海上。随着距陆的增加,都市影响的减弱和海面的开阔,空气微生物含量呈减少之势,显示出海洋新鲜空气对陆上污染空气的调节、净化作用。文中还分析了空气微生物有关参数与气温、风力或相对湿度间的相关关系。 相似文献
85.
86.
87.
The Princeton Ocean Model with realistic bottom topography has been used to investigate the summer temperature decrease in
the past 25 years in Fukuoka Bay. The vertical mixing of the model is expressed by a scheme that effectively includes the
influences of interannual variations of tidal currents and wind. The results show that the historical temperature decrease
in summer has been caused by tidal currents and wind weakening in the past 25 years in Fukuoka Bay. The weakening of tidal
currents and wind gives rise to weakening of the vertical mixing, and to enhancement of the estuarine circulation in the bay.
The enhancement of the estuarine circulation activates the inflow of open-ocean water toward Fukuoka Bay. Coastal water in
summer has therefore tended to be colder and more saline in the past 25 years. This interannual variation in coastal waters
is called “open-oceanization” in this study. On the basis of the numerical model, it is anticipated that the temperature will
decrease by 0.2°C in the next 25 years in Fukuoka Bay if the tide and wind weaken persistently as in the present bay. 相似文献
88.
89.
S. C. Gonalves J. C. Marques M. A. Pardal M. F. Bouslama M. El Gtari F. Charfi-Cheikhrouha 《Estuarine, Coastal and Shelf Science》2003,58(4):901-916
The biology, population dynamics, and production of Talorchestia brito were studied at two sandy beaches located on the Atlantic (Portugal) and on the Mediterranean (Tunisia) coasts, respectively. The seasonal variation in abundance and the overall densities were similar in both populations. Reproduction occurred from February to September in the Atlantic, and from March to early November in the Mediterranean. The sex ratio was male biased in the Atlantic, and female biased in the Mediterranean. Based on data from the Atlantic population, both abundance and the proportion of reproductive females were positively correlated with temperature, while the proportion of juveniles in the population was positively correlated with temperature and sediment moisture. On average, individuals from the Atlantic were larger than the ones from the Mediterranean. Life span was estimated at six to nine months in the Atlantic, and five to eight months in the Mediterranean. Talorchestia brito was shown to be a semiannual species, with iteroparous females producing two broods per year, and exhibited a bivoltine life cycle. The minimum age required for males' and females' sexual differentiation and for female sexual maturation was shorter in the Mediterranean. Growth production (P) was estimated at 0.19 g m−2 y−1 ash free dry weight (AFDW; 4.3 kJ m−2 y−1) in the Atlantic population, and 0.217 g m−2 y−1 AFDW (4.9 kJ m−2 y−1) in the Mediterranean one. Elimination production (E) was estimated at 0.35 g m−2 y−1 AFDW (7.9 kJ m−2 y−1) in the Atlantic, and 0.28 g m−2 y−1 AFDW (6.3 kJ m−2 y−1) in the Mediterranean. The average annual biomass (
) (standing stock) was estimated at 0.032 g m−2 in the Atlantic beach, and 0.029 g m−2 in the Mediterranean one, resulting, respectively, in
ratios of 5.9 and 7.5 and
ratios of 10.8 and 9.6. Like other talitrids, T. brito exhibited geographic variation in morphometrical characteristics, sex ratio, growth rates, life span, and reproduction period, with the Atlantic population presenting a slower life history. 相似文献
90.