全文获取类型
收费全文 | 67558篇 |
免费 | 1217篇 |
国内免费 | 701篇 |
专业分类
测绘学 | 1975篇 |
大气科学 | 5389篇 |
地球物理 | 13933篇 |
地质学 | 23118篇 |
海洋学 | 5653篇 |
天文学 | 15437篇 |
综合类 | 181篇 |
自然地理 | 3790篇 |
出版年
2021年 | 455篇 |
2020年 | 511篇 |
2019年 | 618篇 |
2018年 | 1400篇 |
2017年 | 1303篇 |
2016年 | 1764篇 |
2015年 | 1014篇 |
2014年 | 1676篇 |
2013年 | 3202篇 |
2012年 | 1817篇 |
2011年 | 2444篇 |
2010年 | 2202篇 |
2009年 | 3002篇 |
2008年 | 2636篇 |
2007年 | 2536篇 |
2006年 | 2504篇 |
2005年 | 1951篇 |
2004年 | 1983篇 |
2003年 | 1872篇 |
2002年 | 1909篇 |
2001年 | 1683篇 |
2000年 | 1663篇 |
1999年 | 1440篇 |
1998年 | 1386篇 |
1997年 | 1406篇 |
1996年 | 1184篇 |
1995年 | 1125篇 |
1994年 | 1037篇 |
1993年 | 903篇 |
1992年 | 836篇 |
1991年 | 848篇 |
1990年 | 837篇 |
1989年 | 823篇 |
1988年 | 763篇 |
1987年 | 908篇 |
1986年 | 847篇 |
1985年 | 934篇 |
1984年 | 1143篇 |
1983年 | 1063篇 |
1982年 | 1010篇 |
1981年 | 945篇 |
1980年 | 824篇 |
1979年 | 818篇 |
1978年 | 847篇 |
1977年 | 736篇 |
1976年 | 666篇 |
1975年 | 666篇 |
1974年 | 695篇 |
1973年 | 713篇 |
1972年 | 475篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
551.
Analysis of microbiological and chitinase activities relative to the white shrimpPenaeus setiferus, and its chitinoclastic endosymbionts, have demonstrated the relevance of total chitinolytic processes in penaeid biology. Microorganisms may serve as a direct source of nutrients for the animal as well as in the elaboration of extracellularin situ chitinase enzymes. The enzyme produced by the predominant gut bacteria,Beneckea neptuna, is a moderately actively inducible chitinase while the shrimp has an indigenous constitutive chitinase and chitobiase system. Factors of temperature, pH, ion inhibition and reducing sugar ratios have been compared for the bacterial and animal enzymes. This dual enzyme system suggests that metabolic chitin transformation may play a vital role in crustacean metabolism. 相似文献
552.
553.
Formation of pockmarks by pore-water escape 总被引:1,自引:1,他引:1
P. K. Harrington 《Geo-Marine Letters》1985,5(3):193-197
During the course of a North Sea rig site investigation, a number of seabed depressions were observed on side-scan sonar records, some of which may be identified as pockmarks. Others are described as pits. A pockmark evolutionary series is proposed on the basis of all these features with a suggested mechanism which would favor release of pore water, but does not exclude gas escape. Acoustic voids are considered to be a related phenomenon. 相似文献
554.
Digital underwater acoustic voice communications 总被引:2,自引:0,他引:2
This paper describes the design of an underwater acoustic diver communication system controlled by a digital signal processor. The speech signal transmission rate is compressed by using linear predictive coding (LPC) and the extracted parameters are transmitted through the water to a synchronized receiver by employing digital pulse position modulation (DPPM). The pulse position in each time frame is estimated by an energy detection and decision algorithm which enables the received LPC parameters to be recovered and used to synthesize the speech signal 相似文献
555.
556.
Nicholas H. Vrolijk Nancy M. Targett Bruce R. Woodin John J. Stegeman 《Marine environmental research》1995,39(1-4)
The relationship between cytochrome P450 and feeding on terpenoid-rich gorgonian corals was investigated in a species of tropical butterflyfish and compared with two other sympatric congeners that do not feed on gorgonians. Fish were collected from non-polluted waters in Belize and the levels of two cytochrome P450 isozymes (CYP2B and CYP3A) were immunoquantitated in addition to quantification of total P450. Chaetodon capistratus regularly feeds on gorgonian corals and has higher levels of total hepatic microsomal cytochrome P450 than C. ocellatus or C. striatus. The content of hepatic P450 (0.588–0.794 nmol mg−1) in C. capistratus is among the highest ever reported in teleosts from non-polluted waters and is significantly greater than detected in C. ocellatus or C. striatus. Chaetodon capistratus also had a larger hepatic index (g liver per g fish) and more microsomal protein (mg protein per g liver), factors that translate into 3.3- to 8-fold more total P450 per g fish. Sexual differences in total P450 were observed between male and female C. capistratus, but not among the other species. The contents of proteins detected by immunoassay with polyclonal anti-scup P450B (CYP2B) and anti-human P4503A (CYP3A) were 2- to 10-fold and 2- to 20-fold greater, respectively, in C. capistratus than in the congeneric species. CYP2 and CYP3 gene families in mammals are thought to have evolved partially in response to dietary allelochemicals. These results suggest that these P450 isozymes may also be important in marine teleosts that feed on terpenoid-rich prey. 相似文献
557.
Observations from a surface mooring, in a weak-flow regime over the southeastern Bering Sea shelf, were used to derive surface heat fluxes for the period May–July 1996. Changes in heat content of the water column also were determined from subsurface temperature measurements. Agreement of net surface heat flux and change in heat content was within 2%. This result provides additional evidence that heat advection and diffusion are small in this region. 相似文献
558.
The basic relations of the classical dyamic method have been complemented with the condition allowing for the non-monotonicity
of the vertical kinematic structure. The elements of the multi-layered system of geostrophic currents have been found to correlate
over depth with seawater density and hydrostatic pressure distribution. An example of the analysis of the vertical structure
of currents, involving specific hydrological data, is given.
Translated by Vladimir A. Puchkin. 相似文献
559.
560.
The potential of heterotrophs to degrade aromatic hydrocarbons derived from oil in water and sediments in the northern North Sea around the Brent, Beryl, Forties and Murchison oilfields and at a number of stations, from the Forties field to the Firth of Forth, was estimated using 1-14C naphthalene and 7, 10-14C benzo(a)pyrene. The degradation of uniformly labelled 14C mixed 1-amino acids was used as a measure of total heterotrophic activity. Results showed that microorganisms have the potential to degrade the smaller aromatic hydrocarbon molecules rapidly in the water column and in surface sediments. Close to the Beryl platform, where diesel-washed drill cuttings have been continuously dumped, the rate of input of hydrocarbons to the sediment has exceeded the rate of degradation. Mineralization of benzo(a)pyrene, estimated in the sediments only, was minimal. Close to production platforms the biodegradation rate of mixed 1-amino acids showed no increase comparable with that found for naphthalene, suggesting that existing microbial populations had not increased but adapted to degrade oil in water and sediments. 相似文献