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   8627篇
  免费   347篇
  国内免费   95篇
测绘学   209篇
大气科学   585篇
地球物理   2022篇
地质学   3041篇
海洋学   798篇
天文学   1345篇
综合类   19篇
自然地理   1050篇
  2021年   90篇
  2020年   114篇
  2019年   118篇
  2018年   174篇
  2017年   166篇
  2016年   234篇
  2015年   200篇
  2014年   202篇
  2013年   489篇
  2012年   256篇
  2011年   315篇
  2010年   293篇
  2009年   353篇
  2008年   333篇
  2007年   294篇
  2006年   326篇
  2005年   256篇
  2004年   306篇
  2003年   281篇
  2002年   279篇
  2001年   185篇
  2000年   184篇
  1999年   146篇
  1998年   146篇
  1997年   109篇
  1996年   123篇
  1995年   120篇
  1994年   135篇
  1993年   118篇
  1992年   116篇
  1991年   110篇
  1990年   106篇
  1989年   92篇
  1988年   92篇
  1987年   126篇
  1986年   104篇
  1985年   175篇
  1984年   199篇
  1983年   155篇
  1982年   132篇
  1981年   141篇
  1980年   112篇
  1979年   130篇
  1978年   125篇
  1977年   115篇
  1976年   106篇
  1975年   100篇
  1974年   73篇
  1973年   86篇
  1972年   53篇
排序方式: 共有9069条查询结果,搜索用时 31 毫秒
111.
112.
113.
114.
A growing number of studies concerning organic chemical pollutant induction of cytochrome P450 mixed function oxidases in fish stimulate a need for common terminology. Similar names for proteins might be based on similarities in function, in regulation, and in their structure. Under the current guidelines for P450 proteins, classification is based on measured or inferred amino acid sequence. At present, a single teleost hydrocarbon-inducible P450 (rainbow trout) can conclusively be termed 1A1: a second (scup P450E) can be so termed on the basis of partial sequence. The lack of primary sequence information makes it difficult to assign a specific identity to hydrocarbon inducible P450s in other fish. Nevertheless, the sum of information on catalytic activities, introduction and immunological cross-reactivities provides a weight of evidence sufficient to assume (but not prove) gene family (I) and subfamily (A) identities for hydrocarbon-inducible proteins in many species. Reference to these proteins as P4501 or IA (or CYPI or IA) in fish species where sequence data are lacking is suggested as more appropriate than reference to them as P450IAI (or CYPIAI). Additional primary sequence data are required to further define orthologous relationships among P450IA genes in fish, and to determine whether IAI is a correct designation for a hydrocarbon-inducible P450 in many species.  相似文献   
115.
The recent benthic meiofaunal foraminiferal assemblage from the continental slope (590-2 003 m) off Cape Hatteras, North Carolina exhibits high species richness and evenness, moderate diversity values, and lacks numerically dominant species. The preserved planktic assemblage has relatively low species richness, high evenness, low diversity, and a few numerically dominant species. Approximately 9% of the benthic species are those that typically live within continental shelf depth ranges. The benthic assemblage abundances and diversities do not follow depth patterns or geophysical characteristics. No biogeographic boundary can be described within the study area for meiofaunal foraminifera. Oxygen limitation does not appear to be a factor affecting the benthos of the North Carolina continental slope based upon the community structure of the benthic foraminifera, if total assemblage is assumed to reflect the recently living community. The high carbonate content of sediments in the area may be explained by foraminiferal tests. Within the study area, the foraminiferal assemblages are uniform, and probably reflect relative consistency of primary environmental variables as well as dynamic downslope transport and high influx of material from the water column in the vicinity where the Gulf Stream and the Western Boundary Undercurrent cross.  相似文献   
116.
117.
The production and distribution of biological material in wind-driven coastal upwelling systems are of global importance, yet they remain poorly understood. Production is frequently presumed to be proportional to upwelling rate, yet high winds can lead to advective losses from continental shelves, where many species at higher trophic levels reside. An idealized mixed-layer conveyor (MLC) model of biological production from constant upwelling winds demonstrated previously that the amount of new production available to shelf species increased with upwelling at low winds, but declined at high winds [Botsford, L.W., Lawrence, C.A., Dever, E.P., Hastings, A., Largier, J., 2003. Wind strength and biological productivity in upwelling systems: an idealized study. Fisheries Oceanography 12, 245–259]. Here we analyze the response of this model to time-varying winds for parameter values and observed winds from the Wind Events and Shelf Transport (WEST) study region. We compare this response to the conventional view that the results of upwelling are proportional to upwelled volume. Most new production per volume upwelled available to shelf species occurs following rapid increases in shelf transit time due to decreases in wind (i.e. relaxations). However, on synoptic, event time-scales shelf production is positively correlated with upwelling rate. This is primarily due to the effect of synchronous periods of low values in these time series, paradoxically due to wind relaxations. On inter-annual time-scales, computing model production from wind forcing from 20 previous years shows that these synchronous periods of low values have little effect on correlations between upwelling and production. Comparison of model production from 20 years of wind data over a range of shelf widths shows that upwelling rate will predict biological production well only in locations where cross-shelf transit times are greater than the time required for phytoplankton or zooplankton production. For stronger mean winds (narrower shelves), annual production falls below the peak of constant wind prediction [Botsford et al., 2003. Wind strength and biological productivity in upwelling systems: an idealized study. Fisheries Oceanography 12, 245–259], then as winds increase further (shelves become narrower) production does not decline as steeply as the constant wind prediction.  相似文献   
118.
119.
Bathynerita naticoidea (Gastropoda: Neritidae) and Methanoaricia dendrobranchiata (Polychaeta: Orbiniidae) are two of the most abundant invertebrates associated with cold‐seep mussel beds in the Gulf of Mexico. At the methane seep known as Brine Pool NR‐1 (27 °43.415 N, 91 °16.756 W; 650 m depth), which is surrounded by a broad band of mussels (Bathymodiolus childressi), these species have distinctly different patterns of abundance, with the gastropod being found mostly at the outer edge of the mussel bed (average density in November 2003: 817 individuals·m−2 in outer zone, 20·m−2 in inner zone) and the polychaete being found almost exclusively near the inner edge (average density in November 2003: 3155 individuals·m−2 in inner zone, 0·m−2 in outer zone), adjacent to the brine pool itself. The salinity of the brine pool exceeds 120, so we hypothesized that M. dendrobranchiata should be more tolerant of high salinities than B. naticoidea. The opposite proved to be true. The gastropods were capable of withstanding salinities at least as high as 85, whereas the polychaetes died at salinities higher than 75. Both species were osmoconformers over the range of salinities (35–75) tested. Behavioral responses of B. naticoidea to salinities of 50, 60, and 70 were investigated in inverted vertical haloclines. Gastropods generally did not enter water of salinity greater than 60, but tolerated short periods at 60. Behavioral avoidance of brine should limit the vertical distribution of B. naticoidea in the inner zone to the top 2.5–5 cm of the mussel bed. Behavior is also a likely (though unproven) mechanism for controlling horizontal distribution of this species across the mussel bed. Methanoaricia dendrobranchiata can tolerate short excursions into the brine, but probably avoids hypersaline conditions by aggregating on the tops of the mussels.  相似文献   
120.
In this paper, we focus on the status and trends of the current Korean fisheries management regime. Specifically, this paper briefly introduces the Korean conventional fisheries management regime (KCFMR) and discusses its problems and limitations. In describing policy evolution, this paper finds reasons why the Korean government has chosen a TAC system, an output control approach, besides input control approaches in force for almost a century. This paper also describes the evolution of the Korean TAC system, which is carrying out a pivotal role in Korean fisheries development, and analyzes problems of the Korean TAC system. Finally, this paper gives a perspective on expanding the Korean TAC system toward Output Control Systems (OCSs) such as Individual Quotas (IQs), Individual Vessel Quotas (IVQs), Individual Transferable Traps (ITTs), Community Quotas (CQs), and Individual Transferable Quotas (ITQs).  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