全文获取类型
收费全文 | 1470421篇 |
免费 | 133536篇 |
国内免费 | 1526篇 |
专业分类
1605483篇 |
出版年
2021年 | 18115篇 |
2019年 | 16293篇 |
2018年 | 18781篇 |
2017年 | 17487篇 |
2016年 | 29010篇 |
2015年 | 43138篇 |
2014年 | 51175篇 |
2013年 | 77623篇 |
2012年 | 42054篇 |
2011年 | 31805篇 |
2010年 | 45818篇 |
2009年 | 46434篇 |
2008年 | 29226篇 |
2007年 | 27627篇 |
2006年 | 31817篇 |
2005年 | 32566篇 |
2004年 | 31656篇 |
2003年 | 28971篇 |
2002年 | 26951篇 |
2001年 | 43777篇 |
2000年 | 41047篇 |
1999年 | 38531篇 |
1998年 | 26426篇 |
1997年 | 26359篇 |
1996年 | 25574篇 |
1995年 | 23713篇 |
1994年 | 23419篇 |
1993年 | 22612篇 |
1992年 | 32056篇 |
1991年 | 30209篇 |
1990年 | 28645篇 |
1989年 | 29199篇 |
1988年 | 26681篇 |
1987年 | 25015篇 |
1986年 | 23765篇 |
1985年 | 25414篇 |
1984年 | 24549篇 |
1983年 | 21513篇 |
1982年 | 21300篇 |
1981年 | 20482篇 |
1980年 | 19113篇 |
1979年 | 20456篇 |
1978年 | 18890篇 |
1977年 | 18101篇 |
1976年 | 16970篇 |
1975年 | 16555篇 |
1974年 | 17151篇 |
1973年 | 17474篇 |
1972年 | 14919篇 |
1971年 | 13554篇 |
排序方式: 共有10000条查询结果,搜索用时 6 毫秒
991.
Maria João Feio Trefor B. Reynoldson Verónica Ferreira Manuel Augusto S. Graça 《Hydrobiologia》2007,579(1):55-68
We sampled macroinvertebrates at 75 locations in the Mondego river catchment, Central Portugal, and developed a predictive
model for water quality assessment of this basin, based on the Reference Condition Approach. Sampling was done from June to
September 2001. Fifty-five sites were identified as “Reference sites” and 20 sites were used as “Test sites” to test the model.
At each site we also measured 40 habitat variables to characterize water physics and chemistry, habitat type, land use, stream
hydrology and geographic location. Macroinvertebrates were generally identified to species or genus level; a total of 207
taxa were found. By Unweighted Pair Group Method with Arithmetic mean (UPGMA) clustering and analysis of species contribution
to similarities percentage (SIMPER), two groups of reference sites were established. Using Discriminant Analysis (stepwise
forward), four variables correctly predicted 78% of the reference sites to the appropriate group: stream order, pool quality,
substrate quality and current velocity. Test sites’ environmental quality was established from their relative distance to
reference sites, in MDS ordination space, using a series of bands (BEAST methodology). The model performed well at upstream
sites, but at downstream sites it was compromised by the lack of reference sites. As with the English RIVPACS predictive model,
the Mondego model should be continually improved with the addition of new reference sites. The adaptation of the Mondego model
methodology to the Water Framework Directive is possible and would consist mainly of the integration of the WFD typology and
increasing the number of ellipses that define quality bands.
Handling editor: K. Martens 相似文献
992.
A new cestode species, Diphyllobothrium klebanovskii sp. n., is described on the basis of experimental examination and morphological study of 51 specimens expelled from men in lower part of the Amur river and 18 specimens expelled from golden hamsters. The larval stage of this cestode is plerocercoid of type F, which is localized in muscles of Oncorhynchus keta and O. gorbuscha. D. klebanovskii sp. n. is a marine species, it differs from D. latum and other species of the genus Diphyllobothrium. 相似文献
993.
994.
995.
996.
997.
998.
Zingiber anamalayanum (Zingiberaceae), a new species from the southern western Ghats, India is described and illustrated. A phenetic analyses to assess the similarity indices and the uniqueness of the new species have been performed. A diagnostic key to Zingiber spp. in south India is provided. 相似文献
999.
1000.