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   29179篇
  免费   182篇
  国内免费   170篇
安全科学   542篇
废物处理   1647篇
环保管理   3702篇
综合类   4363篇
基础理论   7892篇
环境理论   7篇
污染及防治   7029篇
评价与监测   2247篇
社会与环境   2003篇
灾害及防治   99篇
  2022年   178篇
  2021年   160篇
  2020年   110篇
  2019年   132篇
  2018年   1678篇
  2017年   1562篇
  2016年   1553篇
  2015年   432篇
  2014年   466篇
  2013年   1497篇
  2012年   1008篇
  2011年   2090篇
  2010年   1307篇
  2009年   1283篇
  2008年   1671篇
  2007年   2034篇
  2006年   807篇
  2005年   694篇
  2004年   684篇
  2003年   770篇
  2002年   727篇
  2001年   886篇
  2000年   610篇
  1999年   376篇
  1998年   246篇
  1997年   244篇
  1996年   239篇
  1995年   302篇
  1994年   266篇
  1993年   220篇
  1992年   234篇
  1991年   248篇
  1990年   234篇
  1989年   245篇
  1988年   211篇
  1987年   194篇
  1986年   143篇
  1985年   179篇
  1984年   214篇
  1983年   216篇
  1982年   199篇
  1981年   166篇
  1980年   148篇
  1979年   163篇
  1978年   127篇
  1977年   133篇
  1976年   119篇
  1975年   138篇
  1974年   149篇
  1973年   118篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
61.
62.
63.
Fisheries management is the practice of analyzing and selecting options to maintain or alter the structure, dynamics, and interaction of habitat, aquatic biota, and man to achieve human goals and objectives. The theory of fisheries management is: managers or decision makers attempt to maximize renewable `output' from an aquatic resource by choosing from among a set of decision options and applying a set of actions that generate an array of outputs. Outputs may be defined as a tangible catch, a fishing experience, an existence value, or anything else produced or supported by renewable aquatic resources. Overall output is always a mix of tangible and intangible elements. However defined, management goals and objectives are essential components of fisheries management or any other field of renewable natural resource management. Reaching consensus on management goals and objectives has never been a simple task. Beyond the broad and often conflicting goals of an agency, managers must decide who should set specific management objectives — agency personnel, the public, or a combination of the two. Historically, rhetoric aside, fisheries managers in North America nearly always have consulted with professionals in governmental roles to set management objectives. In a strongly pluralistic society, this often resulted in protracted political and legal conflict. Increasingly, there are calls for use of risk assessment to help solve such ecological policy and management problems commonly encountered in fisheries management. The basic concepts of ecological risk assessment may be simple, but the jargon and details are not. Risk assessment (and similar analytical tools) is a concept that has evoked strong reactions whenever it has been used. In spite of the difficulties of defining problems and setting management objectives for complex ecological policy questions, use of risk assessment to help solve ecological problems is widely supported. Ecological risk assessment will be most useful (and objective) in political deliberations when the policy debate revolves around largely technical concerns. To the extent that risk assessment forces policy debate and disagreement toward fundamental differences rather than superficial ones, it will be useful in decision making.  相似文献   
64.
65.
The diagnosis of Freeman–Sheldon syndrome was made by ultrasonographic evaluation of a 20-week fetus with a positive family history. The ultrasonographic features were abnormalities of the extremities and mouth.  相似文献   
66.
67.
Microorganisms are able to synthesize three different polyamides by enzymatic processes independently from ribosomal protein biosynthesis: poly(gamma-D-glutamic acid), poly(epsilon-L-lysine) and multi-L-arginyl-poly(L-aspartic acid) which is also referred to as cyanophycin. These polyamides, which occur mainly in Bacillus spp. (and only a few other eubacteria and the nematocysts of Cnidaria, in Streptomyces albulus or in cyanobacteria, respectively), have recently attracted considerable interest of the chemical industry and may be suitable for various applications. This review summarizes our current knowledge on the occurrence, biosynthesis, physiological functions, and biodegradation as well as on the properties and putative applications of these polyamides. Emphasis is placed on the enzymology of the polymerization and on the genes encoding the polymerizing enzymes, which have only recently become available for cyanophycin synthetases. Prospects for novel production processes. in particular for cyanophycin, are also presented.  相似文献   
68.
69.
70.
The nature of the cosmic redshift is one of the most fundamental questions in modern science. Hubble's discovery of the apparent Expansion of the Universe is derived from observations on a small number of galaxies at very low redshifts. Today, quasar redshifts have a range more than 1000 times greater than those in Hubble's sample, and represent more than 100 times as many objects. A recent comprehensive compilation of published measurements provides the basis for a study indicating that quasar observations are not in good agreement with the original predictions of the Expanding Universe theory, but are well fit by the predictions of an alternative theory having fewer adjustable parameters.  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