全文获取类型
收费全文 | 335篇 |
免费 | 48篇 |
国内免费 | 60篇 |
专业分类
测绘学 | 8篇 |
大气科学 | 86篇 |
地球物理 | 76篇 |
地质学 | 59篇 |
海洋学 | 56篇 |
天文学 | 104篇 |
综合类 | 17篇 |
自然地理 | 37篇 |
出版年
2024年 | 2篇 |
2023年 | 1篇 |
2022年 | 4篇 |
2021年 | 5篇 |
2020年 | 10篇 |
2019年 | 4篇 |
2018年 | 7篇 |
2017年 | 10篇 |
2016年 | 11篇 |
2015年 | 16篇 |
2014年 | 21篇 |
2013年 | 15篇 |
2012年 | 16篇 |
2011年 | 24篇 |
2010年 | 16篇 |
2009年 | 24篇 |
2008年 | 18篇 |
2007年 | 28篇 |
2006年 | 24篇 |
2005年 | 18篇 |
2004年 | 10篇 |
2003年 | 27篇 |
2002年 | 20篇 |
2001年 | 18篇 |
2000年 | 13篇 |
1999年 | 13篇 |
1998年 | 14篇 |
1997年 | 10篇 |
1996年 | 8篇 |
1995年 | 11篇 |
1994年 | 10篇 |
1993年 | 5篇 |
1991年 | 3篇 |
1990年 | 2篇 |
1989年 | 2篇 |
1987年 | 2篇 |
1980年 | 1篇 |
排序方式: 共有443条查询结果,搜索用时 31 毫秒
361.
二苯甲酮-3 (BP-3)、4-甲基苄亚基樟脑(4-MBC)和4-甲氧基肉桂酸-2-乙基己酯(EHMC)是三种常用的有机紫外吸收剂, 在水环境中被频繁检出, 对水生生态系统安全构成潜在威胁。为探究三种有机紫外吸收剂对菲律宾蛤仔(Ruditapes philippinarum)鳃组织抗氧化响应和相关细胞凋亡基因的影响, 将蛤仔分别暴露于环境相关浓度的三种紫外吸收剂溶液中, 检测鳃组织抗氧化酶活性和细胞凋亡相关基因转录水平, 并通过第二代整合生物标志物响应法(IBRv2)对三种紫外吸收剂的生物毒性进行比较分析。结果显示, 三种紫外吸收剂短期暴露会诱导抗氧化响应提高抗氧化能力, 而长期高浓度暴露会导致抗氧化能力的降低。BP-3、4-MBC和EHMC可能通过启动线粒体途径和fas介导的死亡受体途径诱导菲律宾蛤仔鳃组织产生细胞凋亡。通过IBRv2分析发现, 在环境常见浓度1 μg/L的暴露水平下, 短期(1 d, 7 d)暴露时, BP-3对菲律宾蛤仔鳃组织表现出的综合毒性效应最强, 而随着暴露时间的延长(28 d), 三种紫外吸收剂表现出的综合毒性效应相近。研究结果为水环境中有机紫外吸收剂的生态风险评估提供了参考数据。 相似文献
362.
G. Beskin V. Debur V. Plokhotnichenko S. Karpov A. Biryukov L. Chmyreva A. Pozanenko K. Hurley 《Astrophysics and Space Science》2007,308(1-4):477-479
The region of SGR 1806-20 localization was observed during its gamma-ray activity in 2001. The observations have been performed
on the 6-meter telescope of the Special Astrophysical Observatory, using the Panoramic Photometer-Polarimeter (PPP). The search
for variability was performed on the 10−6–10 s time scale, and its results were compared to the properties of corresponding X-ray flares.
This work has been supported by the Russian Foundation for Basic Research (grant No 04-02-17555), Russian Academy of Sciences
(program “Evolution of Stars and Galaxies”), and by the Russian Science Support Foundation. The authors would also like to
thank the anonymous referee for his/her valuable comments. 相似文献
363.
364.
This experiment was conducted to examine the effects of salinity fluctuation frequency on the growth, molting rate and hemolymph
20-hydroxyecdysone (20-HE) concentration for juvenile Fenneropenaeus chinensis with an initial wet body weight of 1.115 g±0.012 g. The salinity of the control treatment (represented by D0) was 28 throughout
the experiment, whereas treatments D2, D4, D6 and D8 were subjected to different salinity fluctuation frequencies of 2, 4,
6 and 8 d, respectively. The range of salinity fluctuation was 4. The salinity in treatments D2, D4, D6 and D8 was kept at
28 for 2, 4, 6 and 8 d, respectively; each original salinity was decreased abruptly to salinity 24, which first lasted for
another 2 d and was then raised to its initial value 28. This constituted a salinity fluctuation cycle and afterwards the
cycle repeated. It was found that: 1) The weight gain of shrimp in treatment D4 was 202.9% and significantly higher than that
in treatment D0 (P<0.05). The molting rate in treatment D2 was the lowest, while that in treatment D6 was the highest and significantly higher
than those in treatments D0 and D2 (P<0.05). 2) The hemolymph 20-HE concentration of shrimps in all treatments was at low levels and increased first gradually
during the inter-molt period and then increased sharply. It reached a peak value during the pre-molt stage, and then decreased
abruptly during the post-molt stage when the lowest level occurred. 3) During the post-molt stage, the hemolymph 20-HE concentration
in treatment D2 was 7.47 pg μL−1 and significantly higher than that in treatment D0 (P<0.05). There were, however, no significant differences in hemolymph 20-HE concentration among all five treatments during
the inter-molt stage (P>0.05). During the pre-molt stage, the hemolymph 20-HE concentration in treatment D6 was significantly higher than that in
treatment D0 (P<0.05), whereas the hemolymph 20-HE concentration in treatment D2 was significantly lower than that in other treatments (P<0.05). Adequate salinity fluctuation promoted the molting rate through increasing hemolymph 20-HE concentration at the pre-molt
stage. 相似文献
365.
366.
Marco A. Liñán‐Cabello Laura A. Flores‐Ramírez Tania Zenteno‐Savin Norma O. Olguín‐Monroy Ramón Sosa‐Avalos Manuel Patiño‐Barragan Aramis Olivos‐Ortiz 《Marine Ecology》2010,31(3):407-417
The physiological responses of the coral Pocillopora capitata to environmental conditions common in winter and summer were studied in 2007 during February–March (winter) and June–July (summer) at La Boquita reef (Manzanillo, Colima, Mexico). Shallow and deep sampling stations were established at different distances from a small jetty built next to the Juluapan Lagoon. We analyzed superoxide radicals () and lipid peroxidants (TBARS); the activities of superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx), glutathione reductase (GR), and glutathione‐S‐transferase (GST); chlorophyll a (Chl a), zooxanthellae density (ZD); and mycosporine‐like amino acids (MAAs). Our results showed that the , TBARS, CAT, GST, MAAs, and Chl a, levels were significantly higher in summer (P < 0.05); no seasonal difference was found for GPx, GR or ZD. We found significant differences (P < 0.05) in winter only for Chl a and ZD at shallow sites and, in contrast, for at deeper sites. The results of this study indicate that increasing temperature and radiation associated with seasonal changes (from winter to summer), the efficiency of the enzymes GST, CR and GPX, and the production of MAAs together form a powerful mechanism for P. capitata to offset the detrimental effects of environmental change. 相似文献
367.
A new seasonal and annual dataset describing Arctic sea ice extents for 1901–2015 was constructed by individually re-calibrating sea ice data sources from the three Arctic regions (North American, Nordic and Siberian) using the corresponding surface air temperature trends for the pre-satellite era (1901–1978), so that the strong relationship between seasonal sea ice extent and surface air temperature observed for the satellite era (1979-present) also applies to the pre-satellite era. According to this new dataset, the recent period of Arctic sea ice retreat since the 1970s followed a period of sea ice growth after the mid-1940s, which in turn followed a period of sea ice retreat after the 1910s. Arctic sea ice is a key component of the Arctic hydrological cycle, through both its freshwater storage role and its influence on oceanic and atmospheric circulation. Therefore, these new insights have significance for our understanding of Arctic hydrology.
EDITOR D. KoutsoyiannisASSOCIATE EDITOR not assigned 相似文献
368.
369.
370.
《国际泥沙研究》2020,35(3):287-294
The Suquia River,the largest urban river in Cordoba(Argentina),has been severely polluted for decades.Actions must be taken to restore its environmental quality by managing riparian zones for increased water-self purification.The current study aimed to characterize organic matter(OM) dynamics and humic substances(HS) spectrochemical properties along the lower-middle basin of the Suquia River.Riparian soil(0-20 cm) and sediment(0-10 cm) samples were collected from a reference location(S1)and four polluted sites(S2-S5) during a low-flow period.The contents of soil and sedimentary OM and HS fractions were analyzed by wet oxidation,as well as HS Fourier transform infrared(FT-IR) and ultraviolet-visible(UV-Vis) spectrochemical properties.The OM and HS fractions from riparian soil were high upstream of Cordoba City(S1 and S2,50.2-50.4 g/kg OM) and within a 50 km downstream location(S5,30.9 g/kg OM) owing to a surplus of fresh plant biomass-carbon(C) inputs.Highly heterogeneous sediment samples did not show any significant differences among sites(P 0.05).The lowest values of the ratio of absorbances at 465 and 665 nm(E4/E6)(1.78) and the Δ log K(0.15) coefficient(a measure of HS maturity degree) were obtained downstream of Cordoba City,for both riparian soil and sediment,indicating that HS were enriched by more condensed aromatic structures within highly degraded portions of the river.All samples exhibited similar IR spectra,implying overlapping recalcitrant-C structures at the functional group level,but with different absorbance intensity.Data from the current study constitute a baseline for understanding the chemical nature of HS from sediment and riparian soil along the Suquia River and can be used as a reference for future studies tracking OM compositional changes over time. 相似文献