全文获取类型
收费全文 | 71510篇 |
免费 | 818篇 |
国内免费 | 405篇 |
专业分类
测绘学 | 1902篇 |
大气科学 | 5431篇 |
地球物理 | 12863篇 |
地质学 | 27133篇 |
海洋学 | 5665篇 |
天文学 | 15665篇 |
综合类 | 254篇 |
自然地理 | 3820篇 |
出版年
2020年 | 349篇 |
2019年 | 347篇 |
2018年 | 3710篇 |
2017年 | 3541篇 |
2016年 | 2597篇 |
2015年 | 812篇 |
2014年 | 1203篇 |
2013年 | 2457篇 |
2012年 | 2186篇 |
2011年 | 4193篇 |
2010年 | 3955篇 |
2009年 | 4457篇 |
2008年 | 3709篇 |
2007年 | 4345篇 |
2006年 | 1734篇 |
2005年 | 2019篇 |
2004年 | 1856篇 |
2003年 | 1847篇 |
2002年 | 1596篇 |
2001年 | 1258篇 |
2000年 | 1192篇 |
1999年 | 1126篇 |
1998年 | 1063篇 |
1997年 | 1066篇 |
1996年 | 846篇 |
1995年 | 841篇 |
1994年 | 797篇 |
1993年 | 745篇 |
1992年 | 709篇 |
1991年 | 683篇 |
1990年 | 785篇 |
1989年 | 678篇 |
1988年 | 650篇 |
1987年 | 744篇 |
1986年 | 624篇 |
1985年 | 823篇 |
1984年 | 952篇 |
1983年 | 921篇 |
1982年 | 865篇 |
1981年 | 831篇 |
1980年 | 741篇 |
1979年 | 699篇 |
1978年 | 693篇 |
1977年 | 631篇 |
1976年 | 602篇 |
1975年 | 518篇 |
1974年 | 600篇 |
1973年 | 590篇 |
1972年 | 366篇 |
1971年 | 335篇 |
排序方式: 共有10000条查询结果,搜索用时 312 毫秒
941.
942.
943.
944.
Previous studies have shown that metal partitioned to a subcellular compartment containing trophically available metal (TAM) is readily available to predators and may be enhanced by increased binding of metal to heat-stable proteins (HSP - e.g., metallothioneins). The aim of the current investigation was to determine the influence of TAM on the trophic transfer of Cd along an experimental, three-level food chain: Artemia franciscana (brine shrimp)-->Palaemonetes pugio (grass shrimp)-->Fundulus heteroclitus (mummichog). P. pugio were fed for 7 days on A. franciscana exposed to Cd in solution (including (109)Cd as radiotracer) and subjected to subcellular fractionation or fed to F. heteroclitus. An HSP-driven increase in the percentage of Cd associated with TAM (TAM-Cd%) in A. franciscana exposed to 1 muM Cd resulted in a bioenhancement (i.e., a greater than linear increase with respect to A. franciscana exposure) of Cd trophic transfer to P. pugio. Increased dietary Cd exposure did not affect TAM-Cd% in P. pugio nor trophic transfer to F. heteroclitus. 相似文献
945.
Geracitano LA Bocchetti R Monserrat JM Regoli F Bianchini A 《Marine environmental research》2004,58(1):1-17
Acute (4 days) and chronic (14 days) effects of copper were evaluated on the antioxidant defenses of Laeonereis acuta (Polychaeta) collected in unpolluted (UP) and polluted (P) sites. In the acute assay (125 and 250 micro g Cu/l) superoxide dismutase (SOD) and glutathione S-transferase (GST) activities did not change, whereas catalase (CAT) increased in worms from both the sites. Lipid peroxidation was higher in copper exposed worms from the P site. In the chronic assay (62.5 micro g Cu/l) polychaetes from the P site showed enhanced activities of SOD, GST and CAT and higher contents of metallothionein-like proteins and sulfhydrils compared to worms from UP. Differences in responses between polychaetes from UP and P sites suggest that organisms from the polluted site, P, are more susceptible to oxidative stress conditions. 相似文献
946.
947.
Abstract. The red band-fish, Cepola rubescens L., lives in burrows in sublittoral muddy sediments. The authors first presented information on the burrows of this species in the 1970s. The present paper presents new information on burrow structure, describes the method of excavation, and comments on the bioturbatory significance of the species. The work derives from field and laboratory studies. A burrow typically consists of a vertical shaft which opens into an expanded terminal chamber. In some cases a side shaft may be added. The paper includes a size analysis of 130 burrows measured in the field and detailed morphological information from a selection of burrows which were cast with polyester resin. The fish burrows are frequently intersected by the burrows of other species and interspecific associations may develop. Burrow size reflects the size of the occupant and may approach 1 m in depth. The biogenic movement of water and particles to this depth is often overlooked in bioturbation studies and is discussed. Burrow distribution is aggregate, which has implications for the bioturbatory impact of the species. Burrows are constructed by mouth excavation and this is described in detail. Fish transport fine material within their mouths and coarse material is grasped in the jaws. Large spoil heaps occur at burrow openings. One obvious effect of this bioturbatory activity at the field site was the redistribution of coarse material (shell gravel) from depth to the sediment surface. 相似文献
948.
R.H.M. Eertman C.L.F.M.G. Groenink B. Sandee H. Hummel A.C. Smaal 《Marine environmental research》1995,39(1-4)
The polycyclic aromatic hydrocarbons fluoranthene and benzo(a)pyrene significantly reduced the feeding rate of mussels. For both compounds the tissue concentration resulting in a 50% reduction of the clearance rate (TEC50) was calculated. At high tissue concentrations both aromatic compounds reduced the tolerance of mussels to aerial exposure, whereas at low tissue concentrations an improved response was noticed. The activities of the antioxidant enzymes Superoxide dismutase (SOD) and catalase were elevated only at low tissue concentrations of fluoranthene and benzo(a)pyrene. At the highest measured tissue concentrations the activity of both enzymes was reduced, possibly due to a narcotic effect. The reproductive success rate of mussels appeared to be affected negatively by the investigated hydrocarbons. The results of a pilot experiment indicate that mussels can be used also for the testing of contaminated sediments. 相似文献
949.
The nonlinear energy transfer through the wave spectrum is studied on the basis of the previously obtained explicit equation for matrix elements of a four-wave kinetic integral. The equation describes the evolution of a system of gravity waves at the surface of a sea of finite depth with a uniform distribution of broken ice over the sea surface. Particular attention is paid to the analytical part of the algorithm of the calculation of the kinetic integral. This part differs from the standard algorithm by a set of prominent features of the dispersion relation for wave oscillations in the ice-covered water. The kinetic integral for the system under consideration is calculated, and the results are compared with the results obtained for the ice-free water. 相似文献
950.
Until recently, the ideas about the age of the Black Sea deep-water basin have been based on land geological observations in the coastal areas at the interaction periphery, underwater observations from manned submersibles, and on the data of seismic reflection and refraction studies and drilling. Formerly, the scarcity of the information led to a wide scattering of the age determinations: from the Jurassic to the Eocene. Recently, with the appearance of reliable geological and geophysical data, the range of the age estimates has been considerably reduced during the last few years, although there is no commonly accepted opinion on this issue. Therefore, the first attempt to determine the age of the Western Black Sea basin using an analysis of the anomalous magnetic field is of certain interest. The following results were obtained: the basin probably opened between 71.338 and 71.587 My B.P. (subchron C32n.1r). During the interval 68.737–71.071 My B.P. (subchron C31r), extinction of the spreading axes took place. Thus, the total duration of the Campanian-Maestrichtian phase of the opening was about 3 My (interval from 71.587 to 68.737 My B.P.). This result does not agree with the geological and geophysical data available to date. To solve this problem, collection of new geological data and further studies of the structure of the anomalous magnetic field are required. 相似文献