全文获取类型
收费全文 | 99696篇 |
免费 | 2078篇 |
国内免费 | 983篇 |
专业分类
测绘学 | 2531篇 |
大气科学 | 7778篇 |
地球物理 | 20550篇 |
地质学 | 35958篇 |
海洋学 | 8263篇 |
天文学 | 21561篇 |
综合类 | 300篇 |
自然地理 | 5816篇 |
出版年
2021年 | 899篇 |
2020年 | 1000篇 |
2019年 | 1049篇 |
2018年 | 2687篇 |
2017年 | 2551篇 |
2016年 | 3056篇 |
2015年 | 1891篇 |
2014年 | 2887篇 |
2013年 | 5157篇 |
2012年 | 3134篇 |
2011年 | 4112篇 |
2010年 | 3471篇 |
2009年 | 4544篇 |
2008年 | 4132篇 |
2007年 | 3766篇 |
2006年 | 3705篇 |
2005年 | 3359篇 |
2004年 | 3383篇 |
2003年 | 3063篇 |
2002年 | 2605篇 |
2001年 | 2350篇 |
2000年 | 2303篇 |
1999年 | 1871篇 |
1998年 | 1925篇 |
1997年 | 1897篇 |
1996年 | 1543篇 |
1995年 | 1557篇 |
1994年 | 1351篇 |
1993年 | 1220篇 |
1992年 | 1180篇 |
1991年 | 1107篇 |
1990年 | 1211篇 |
1989年 | 1045篇 |
1988年 | 969篇 |
1987年 | 1203篇 |
1986年 | 1037篇 |
1985年 | 1240篇 |
1984年 | 1395篇 |
1983年 | 1336篇 |
1982年 | 1269篇 |
1981年 | 1146篇 |
1980年 | 1053篇 |
1979年 | 997篇 |
1978年 | 994篇 |
1977年 | 905篇 |
1976年 | 849篇 |
1975年 | 811篇 |
1974年 | 863篇 |
1973年 | 824篇 |
1972年 | 536篇 |
排序方式: 共有10000条查询结果,搜索用时 78 毫秒
991.
Alkalinity, dissolved inorganic carbon (DIC), and 13C profiles from seep sites on the northwestern Gulf of Mexico upper slope show anomalously negative 13C values of up to –4.5 PDB, increased levels of DIC of up to 2.45 mmole/liter, and slight alkalinity rises of up to 2.54 meq/liter, relative to water column profiles from a seep-free site (0.63, 2.04 mmole/liter, and 2.39 meq/liter). The observed DIC enrichments coupled with the13C-depletions are attributed to the release of CO2 by microbial oxidation of crude oil in the seep environment, and its migration into the water column. The 13C composition of the migrating CO2 is estimated to be –26.0 on the basis of dissolved carbon inventory. Manifestation of DIC and 13C anomalies in the water column caused by hydrocarbon seepage holds promise to be useful for hydrocarbon reconnaissance surveys over large offshore tracts on account of the simplicity of sampling acquisition, and rapidity of analytical techniques in the laboratory. 相似文献
992.
Trends in biomass, density and diversity of North Sea macrofauna 总被引:6,自引:2,他引:6
993.
994.
B. T. G. Gowland A. D. McIntosh I. M. Davies C. F. Moffat L. Webster 《Marine environmental research》2002,54(3-5)
An aluminium smelter on the west coast of Scotland discharges an aqueous effluent containing polycyclic aromatic hydrocarbons (PAHs) at the head of Loch Leven. The loch also supports two mussel (Mytilus edulis) farms. Data are presented on burdens of PAHs in the soft tissues of mussels and the effect of these contaminants on glutathione S-transferase (GST) activity in mussel hepatopancreas. GST activity is shown to be correlated with total PAH burden and also with the concentrations of certain individual PAHs. These field data show that high molecular weight PAHs are closely correlated to GST activity, whereas low molecular weight PAHs are not. This suggests that 5- and 6-ring PAHs have a more pronounced role than 2- to 4-ring compounds in inducing GST activity in mussels from Loch Leven. It is proposed that it may be more appropriate to link GST activity with 5- and 6-ring compounds only, rather than with the total PAH burden. 相似文献
995.
S. Vanucci A. Dell'Anno A. Pusceddu M. Fabiano R. S. Lampitt R. Danovaro 《Progress in Oceanography》2001,50(1-4)
Downward fluxes of microbial assemblages associated with sinking particles sampled in sediment traps deployed at nominal depths of 1000 m (trap A), 3000 m (trap B) and 4700 m (trap C) were measured between October 1995 and August 1998 on the Porcupine Abyssal Plain (PAP, NE Atlantic). The goal of the study was to provide detailed information on the microbial contributions to the particulate organic carbon and DNA fluxes. Bacterial fluxes associated with settling particles in the PAP area were generally low and significantly lower than bacterial fluxes reported from the same area during 1989–90. Marked seasonal pulses in the microbial assemblages were observed in all years that were associated with particle flux maxima in April–June. No significant differences were found in microbial fluxes between 1000 and 4700 m depth, but both the bacterial biomass flux and the frequency of dividing bacteria increased with depth, suggesting that organic matter turnover and conversion into bacterial biomass increased in the deeper traps. The structure of microbial assemblages displayed clear changes with increasing depth; the ratios of bacteria to both flagellates and cyanobacteria increased up to 4-fold between 1000 and 4700 m, showing a marked increase in bacterial dominance in the deeper layers of the water column. A parallel increase of the bacterial contribution to particulate organic carbon (POC) and DNA fluxes was observed. Total microbial contribution to the POC flux in the PAP area was about 2%, whereas the contribution of cyanobacteria was negligible. Fluxes of microbial assemblages were significantly correlated with DNA fluxes and on average the bacteria accounted for 5% of DNA fluxes. Data reported here confirm that the “rain” of particulate bacterial DNA may represent an important source of nucleotides for deep-sea bacteria, but also suggests that a much larger pool of detrital DNA is potentially available to deep-sea micro-organisms. 相似文献
996.
997.
998.
H. Hummel C. Amiard-Triquet G. Bachelet M. Desprez J. Marchand B. Sylvand J.C. Amiard H. Rybarczyk R.H. Bogaards J. Sinke Y. De Wit L. De Wolf 《Journal of Sea Research》1996,35(4):315-321
Variation in the sensitivity to stress of Macoma balthica was measured in several French and Dutch estuaries. For adult and juvenile Macoma balthica exposed to copper under conditions of starvation, differences in mortality rate, condition, glycogen, burrowing rate and copper content were assessed. No significant differences were observed between adults and juveniles; the influence of treatment and origin was always evident. Animals from the most southern estuaries, Loire and Gironde, near to the species's southern limit of distribution, showed, in the field, the strongest deviations for the ecophysiological traits measured, and were in the experiments the most sensitive to stress. 相似文献
999.
1000.
Catalytic cathodic stripping voltammetry (CSV) preceded by adsorptive collection of complexes of 1-nitroso-2-napthol (NN) can be used to determine iron in seawater. It is shown here that iron(II) is effectively masked in the presence of 2,2-dipyridyl (Dp) so that iron(III) is measured selectively. The concentration of iron(II) is then calculated as the difference between the concentrations of reactive iron (FeR) in the absence and presence of 2 μM Dp, FeR being defined as that which was complexed by 20 μM NN at pH 6.9 in the presence of 1.8 mM H2O2 and 5 ppm sodium dodecyl sulphate. A 30 min reaction time was allowed for Dp to react with iron(II) in seawater prior to the determination of reactive iron(III) using the same conditions as used for FeR. Detection limits of 0.08 nM, 0.077 nM and 0.12 nM were obtained for FeR, iron(III) and iron(II), respectively, using a 60 s deposition time.The method was utilised to determine the redox speciation of iron in the northern North Sea. Concentrations of FeR ranged between 0.8 and 3.5 nM with nutrient-like depth profiles. Iron(II) was found to be present at concentrations up to 1.2 nM, the highest concentrations occurring in the upper 20 m of the water column. 相似文献