首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   3435篇
  免费   277篇
  国内免费   1055篇
  2024年   6篇
  2023年   96篇
  2022年   119篇
  2021年   166篇
  2020年   200篇
  2019年   206篇
  2018年   181篇
  2017年   216篇
  2016年   208篇
  2015年   202篇
  2014年   204篇
  2013年   293篇
  2012年   167篇
  2011年   196篇
  2010年   117篇
  2009年   148篇
  2008年   148篇
  2007年   172篇
  2006年   160篇
  2005年   155篇
  2004年   116篇
  2003年   126篇
  2002年   123篇
  2001年   107篇
  2000年   91篇
  1999年   68篇
  1998年   70篇
  1997年   59篇
  1996年   73篇
  1995年   63篇
  1994年   59篇
  1993年   46篇
  1992年   64篇
  1991年   52篇
  1990年   50篇
  1989年   41篇
  1988年   24篇
  1987年   24篇
  1986年   18篇
  1985年   22篇
  1984年   13篇
  1983年   13篇
  1982年   31篇
  1981年   10篇
  1980年   10篇
  1979年   9篇
  1978年   9篇
  1977年   6篇
  1976年   2篇
  1973年   6篇
排序方式: 共有4767条查询结果,搜索用时 375 毫秒
101.
Y. B. Ho 《Hydrobiologia》1981,77(1):43-47
The amounts of tissue nitrogen, phosphorus, potassium, sodium, calcium and iron were estimated in the green alga Ulva lactuca L. collected from 9 rural and 14 urban littoral sites in the waters around Hong Kong Island during 1978 and 1979. The mean levels of tissue nitrogen and phosphorus were respectively 65% and 87% more in urban sites than in rural ones. Very significant correlation (r = 0.920) was found between the logarithmic concentration of seawater inorganic nitrogen and that of tissue nitrogen. The same applied to soluble reactive phosphorus in seawater and tissue phosphorus (r = 0.886). The levels of potassium, sodium and calcium in the alga were relatively uniform. However considerable variation in the level of iron was detected. The potential use of Ulva as an indicator species for eutrophication is discussed.  相似文献   
102.
N. Lair 《Hydrobiologia》1980,73(1-3):153-160
In the heated affluents of nuclear power plants of the river Loire, rotifers are abundant. Cosmopolitan species are numerically dominant, but a tropical fauna is also present, among which the genus Brachionus, representing 20% of the total species, is best represented. From a comparison between water upstream and downstream of the power plants, it further appears that in downstream warmed-up waters, some species show an important development, but not in colder upstream waters.  相似文献   
103.
Levels of cadmium and zinc in various components of Williamson Ditch (an industrially contaminated stream flowing into Palestine Lake), Trimble Creek (a stream draining Palestine Lake) and the Tippecance River (a river receiving Trimble Creek) were determined. Water, sediment, plant, fish and clam samples were analyzed for cadmium and zinc content by atomic absorption spectrophotometry. Unweighted mean metal concentrations in Trimble Creek were the following: water, 51 µg Zn/1 and 4.2 µg Cd/1; sediment, 592 µg Zn/g and 48.8,µg Cd/g; plants, 375 µm Zn/g and 7.91 µg Cd/g; fish, 145 µg Zn/g and 6.02 µg Cd/g. These concentrations were generally lower than those found in Williamson Ditch and higher than those found in the Tippecanoe River or background levels previously reported for other aquatic ecosystems.  相似文献   
104.
Bacterial periphyton formed during 48 hours was studied by glass slide method and direct counting in Vltava River in Praha, Czechoslovakia. At water temperatures 8–11°C the numbers of rods ranged between 24,000 and 336,000 per 1 cm2 and those of cocci between 30,000 and 228,000. The relation rods: cocci ranged between 0,9 and 2,4 with an average value of 1,7, whereas in a fishpond this average was 0,5. Among the periphyton 81,3% bacterial cells were active. The rods: cocci relation seems to be a good indicator of water pollution by organic matter, but numbers distinguishing the individual saprobic levels cannot be given yet.
Zusammenfassung Es wurde der bakterielle Bewuchs von Glassplatten im Vltava-Fluss in Praha untersucht. Zweitägige Exposition erwies sich als am meisten geeignet, um den Bewuchs mikroskopisch durch direkte Zählung quantitativ zu erfassen. In Abhängigkeit von der Tiefe, Wassertemperatur, Verunreinigungsgrad, Sonnenstrahlung u.a. entwickelte sich der Bewuchs quantitativ unterschiedlich und zeigte auch eine unterschiedliche Relation von Stäbchen zu Kokken. Im beta-mesosaproben Bereich wurden im Herbst (Wassertemperaturen 8–11 °C) 24.000 bis 336.000 Stäbchen und 30.000 bis 228.000 Kokken per 1 cm2 gefunden, mit den Mittelwerten 163.100 Stäbchen und 104.100 Kokken. Die Vergleichswerte von einem Fischteich in Motol lagen für Stäbchen im gleichen Bereich, während die Kokken, bis 254.000 erreichten.Die Relation Stäbchen: Kokken variierte im Vltava-Fluss zwischen 0,9 (nur einmal) und 2,4 mit dem Mittelwert 1,7, im Fischteich zwischen 0,4 ufnd 0,7 mit dem Mittelwert 0,5. Es hat sich erwiesen, dass diese Relation brauchbare Angaben über die organische Verunreinigung gibt. Leider liegen bisher zu wenige Ergebnisse vor, um eine Skale gegenüber den Saprobitätsstufen zu errichten.Unter den Bewuchsbakterien gab es in der Vltava durchschnittlich 81,3% aktive Zellen, wie durch Fluoreszenzanalyse festgestellt wurde.Fast alle Daten indizieren eine schwächere Verunreinigung des Wassers am linken Ufer als am rechten.
  相似文献   
105.
Urban and periurban forests, which are integrated within the concept of Green Infrastructure, provide important Ecosystem Services, including air purification. In this study, we quantified the Ecosystem Service of particulate matter (PM10) and Ozone (O3) removal from urban and periurban forests in ten metropolitan cities in Italy, and its total monetary value. In order to gain a better understanding of how Ecosystem Services can be regulated on a wider scale, the vegetation ecosystem types were grouped into Physiognomic-Structural Categories of Vegetation according to morphofunctional criteria. The pollution removal was mapped using a remote sensing and GIS approach, by applying a deposition model and a stomatal flux model. We estimated, for the ten metropolitan cities, an overall pollution abatement of 7150 Mg of PM10 and 30,014 Mg of O3 in the year 2003, which was an extremely hot year. Our findings indicate that structural characteristics (i.e. Leaf Area Index) and functional diversity, linked to stomatal conductance, exert a marked influence on the provision of the regulating Ecosystem Services, whose total monetary value was estimated to be equal to 47 and 297 million USD for PM10 and O3 removal, respectively. This study represent the first national-scale assessment of the Ecosystem Services of air pollution removal in Europe, thus providing information that may be useful to stakeholders to manage Green Infrastructure more efficiently.  相似文献   
106.
New thiophene (213) and thienopyrimidine (1527) derivatives have been synthesized. Twenty three compounds were screened against five cell lines namely; hepatocellular carcinoma (liver) HepG-2, epidermoid carcinoma (larynx) Hep-2, mammary gland (breast) MCF-7, human prostate cancer PC-3 and epithelioid cervix carcinoma HeLa. The results revealed that compounds 15,16,17,24 and 25 showed the highest antitumor activity against all tested cell lines compared to Doxorubicin. In order to explain the expected mode of action of the observed anticancer activity, compounds 15,16,17,24 and 25 were selected to screen their DNA binding affinity and enzyme inhibitory activity against DNA polymerase, thymidylate synthase and tyrosine kinase. The results revealed that the tested compounds showed good DNA binding affinity as well as good inhibitory activity against the three enzymes which might explain the observed anticancer activity of the target compounds.  相似文献   
107.
Twenty-five study sites were established along a 57-km-long transect in order to estimate the impact of an oil refinery, mainly emitting sulphur dioxide (24000 t yr−1), on forest soil (F/H-horizon) chemistry and microbiology. The study demonstrated the existence of a pollution gradient which was best represented by the logarithm of the concentration of vanadium in the analyzed F/H soil layer. Of the soil microbial characteristics measured, including length of fungal hyphae, soil respiration, microbial biomass C and N, and percentage mass loss of Scots pine (Pinus sylvestris) needle litter, only fungal hyphal length was suppressed by the pollution load. No reduction in basic cations (Ca, Mg, K, and Na) in the F/H-horizon, or enrichment of soluble aluminum in the F/H-horizon of the Scots pine forest could be detected to result from the deposition.  相似文献   
108.

1. 1. The objective of this paper is to investigate the indoor environment from the viewpoint of interaction between physical environment and the human responses. The field survey has been conducted over 1 year.

2. 2. A continuous measurement has been carried out for 1 week and distribution of variables have been measured for 1 day.

3. 3. The attitude of workers was investigated by a questionnaire.

4. 4. As the result, average luminance represented more than 1000 lx in the new building, in contrast with less tha 300 lx in the existing building.

5. 5. There was a significant difference of the occupants' response to the light environment between the two buildings.

6. 6. Measured thermal conditions are on the edge of the ASHRAE comfort envelope in summer, and in the neighborhood of the lower dry limit of the envelope in spring.

7. 7. The occupants' evaluations were remarkably changed before and after the moving. The office environment is better than that of the factory.

Author Keywords: Office; post occupancy evaluation; physical environmental condition; occupants' evaluation  相似文献   

109.
A series of novel HIV-1 protease inhibitors has been designed and synthesized, which contained morpholine derivatives as the P2 ligands and hydrophobic cyclopropyl as the P1 ligand at the meantime in this study, with the aim of improving the interactions between the active sites of HIV-1 protease and the inhibitors. Twenty-eight compounds were synthesized and assessed, among which inhibitors m18 and m1 exhibited excellent inhibitory effect on the activity of HIV-1 protease with IC50 value of 47 nM and 53 nM, respectively. The molecular modeling of m1 revealed possible hydrogen bondings or van der Waals between the inhibitor and the protease, worthy of in-depth study.  相似文献   
110.
Air quality thresholds for O3 for the protection of human health and vegetation set by the European Union (EU) have been exceeded in Europe regularly in the 1990s. Because target reductions for oxides of nitrogen (NOx) set for the year 2000 are unlikely to be achieved, these O3 exceedances are likely to continue into the next millenium. Improvements of plant tolerance towards O3 are being investigated but very little work has been done to explore NOx tolerance and plant acclimation to NO2 and NO. However, it is clear that within the populations of some plant species there is wide variation, and some individuals can fix NOx and use the nitrogen directly from the atmosphere, rather than rely upon, for example, root uptake of nitrate. It is possible that individuals capable of fixing NOx could be selected for a range of species, and genotypes with high rates of uptake could be of value as crops or for forestation in polluted areas (e.g. landscaping in the vicinity of motorways) to reduce tropospheric concentrations of NOx significantly and also to decrease the potential for O3 production.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号