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   1086篇
  免费   147篇
  国内免费   291篇
测绘学   17篇
大气科学   12篇
地球物理   157篇
地质学   164篇
海洋学   691篇
天文学   3篇
综合类   111篇
自然地理   369篇
  2024年   6篇
  2023年   31篇
  2022年   56篇
  2021年   56篇
  2020年   49篇
  2019年   49篇
  2018年   52篇
  2017年   58篇
  2016年   69篇
  2015年   60篇
  2014年   84篇
  2013年   96篇
  2012年   95篇
  2011年   92篇
  2010年   76篇
  2009年   71篇
  2008年   69篇
  2007年   83篇
  2006年   71篇
  2005年   62篇
  2004年   46篇
  2003年   39篇
  2002年   32篇
  2001年   22篇
  2000年   19篇
  1999年   20篇
  1998年   6篇
  1997年   10篇
  1996年   9篇
  1995年   3篇
  1994年   5篇
  1993年   6篇
  1992年   3篇
  1990年   1篇
  1988年   1篇
  1987年   1篇
  1985年   3篇
  1984年   5篇
  1983年   2篇
  1982年   2篇
  1981年   3篇
  1978年   1篇
排序方式: 共有1524条查询结果,搜索用时 31 毫秒
21.
Genetic diversity of cultivated populations was investigated using assay of vertical slabpolyacrylamide gel electrophoresis in 4 species of shrimps, which were Penaeus japonicus, Penaeus monodon , Penaeus vannamei and Metapenaeus ensis, The results showed that the mean number of alle-les per locus (A) is 1.3±0.2, 1.3±0.1, 1.3±0.1 and 1.3 ±0.1 respectively; the percentage of polymorphic loci (P0.95) is 12.5, 6.7, 20 and 23.5 respectively; the expected heterozygosity (He) is 0.042±0.034,0.042±0.031, 0.094±0.042 and 0.097±0.047 respectively; and the observed heterozygosity (H0)is 0.029±0.024, 0.028±0.023,0.154±0.082 and 0.150±0.084 respectively. The difference of genetic diversity is obvious in 4 species of shrimps. The degree of genetic diversity is M. ensis > P. vannamei > P. japonicus > P. monodon. In short, the lower level of genetic diversity is estimated in 4 species of shrimps.  相似文献   
22.
1Introduction ThemajorityofAustralia’sabalonefisheryex ports(5.135kt,worth$216millionin2002~2003,ABARE2004)consistofblacklipabalone(HaliotisrubraLeach,1814).AssuchH.rubrais consideredasanimportantmarineresourcewithin Australia.Likemanyabalonespecieswor…  相似文献   
23.
Rare species, habitat diversity and functional redundancy in marine benthos   总被引:1,自引:0,他引:1  
Macro-ecological theories relating species richness, abundance, range size, biological traits and environmental tolerance have rarely been tested in marine soft-sediments, despite the spatial extent of these habitats and the inherent richness of resident communities. This study examines the contribution of rare species to marine soft-sediment communities from New Zealand, focussing on the relationships of range size with abundance, environment, habitat diversity and life history traits. 54% of the 351 species sampled exhibited restricted ranges (found at ≤ 2 sites). In contrast to many terrestrial systems, we observed only a weak positive relationship between abundance and frequency of occurrence. Restricted-range species were not randomly distributed, with their distribution related to habitat characteristics, suggesting an important link between habitat diversity and rarity. They exhibited a similar range of traits to the total observed species pool, suggesting that they are not only important to biodiversity but could play a role in stability. Restricted range species were generally not small and this, together with the number of different biological traits represented, suggests that rare species are important to the functioning of marine systems. Thus, our results highlight the importance of considering rare species in habitat-based approaches to conservation.  相似文献   
24.
2001年夏季胶州湾浮游植物群落结构的特征   总被引:7,自引:2,他引:7  
本文根据 2 0 0 1年 8月 2 0~ 2 2日期间对胶州湾海域进行的大面积调查资料 ,对浮游植物的群落结构进行了初步研究。结果表明 ,浮游植物群落主要由沿岸暖水性种类组成 ,以硅藻为主 ,还有少量的甲藻、金藻和绿藻。湾中央出现的种类最多 ,湾口其次 ,湾边缘最少。细胞数量的高值区出现在B断面和 A5站周围的水域 ,低值区出现 D断面和 E断面。此外 ,对群落多样性的研究表明 ,Shannon指数在湾北部的多数站位高于湾南部水域 ,在 D2和 B1站出现 2个低值区 ;Margalef指数在湾内东部和 D4站周围水域明显高于湾内西部和湾口水域 ;均匀度则在 B1和 D2站周围水域偏高。浮游植物群落同营养盐的相关分析表明 ,调查期氨盐和磷酸盐是影响浮游植物生长的主要营养盐 ,中肋骨条藻、浮动弯角藻和旋链角毛藻等几种优势种类决定了这一关系  相似文献   
25.
The response of phytoplankton assemblage structure to terrestrial nutrient inputs was examined for the Gulf of Kalloni in the Northern Aegean Sea, a productive semi-enclosed coastal marine ecosystem. The study was focused on a typical annual cycle, and emphasis was placed on the comparative analysis between blooms developing after significant nutrient inputs from the watershed, and naturally occurring blooms. Baseline information was collected on a monthly basis from a network of stations located in the oligotrophic open sea and the interior and more productive part of the embayment. Intensive sampling was also carried out along a gradient in the vicinity of a river which was the most important source of freshwater and nutrient input for the Gulf. Phytoplankton assemblage structure was analyzed from 188 samples using diversity indices (Shannon and Average Taxonomic Distinctness), multivariate plotting methods (NMDS), multivariate statistics (PERMANOVA), and canonical correspondence analysis (CCA). Three characteristic assemblages were recognized: (1) an autumn assemblage developed under nutrient depleted conditions, having low diversity due to the dominance of two small diatoms, (2) a winter bloom of the potentially toxic species Pseudo-nitzschia calliantha occurring immediately after a nutrient peak and characterized by very low diversity, and (3) a naturally occurring early summer bloom of centric diatoms with relatively high diversity. The results of the study support the view that moderate nutrient inputs may have a beneficial effect on the functioning of coastal ecosystems, stimulating the taxonomic diversity through the growth of different taxonomic groups and taxa. On the other hand, a sudden pulse of high nutrient concentrations may greatly affect the natural succession of organisms, have a negative effect on the diversity through the dominance of a single species, and can increase the possibility of a harmful algal bloom development.  相似文献   
26.
27.
根据自行分离的翘嘴鳜微卫星序列(GenBank登录号:DQ789247-DQ789306)设计并合成20对微卫星引物,对鳜属鱼类4个物种即翘嘴鳜、大眼鳜、斑鳜和暗鳜共80个个体进行了物种鉴定分析.结果表明,20对微卫星引物共检测到293个等位基因,大小在80-301bp之间.它们在4个物种中的多态性位点百分率分别为90%、75%、85%和85%,累积个体识别率和非父排除率均达到0.9999,属于高识别力的微卫星遗传标记系统,可以用来进行鳜属鱼类物种的鉴定分析.UPGMA聚类分析表明,翘嘴鳜与暗鳜之间亲缘关系最近,可归属于第Ⅰ类;大眼鳜为第Ⅱ类;斑鳜独自为第Ⅲ类.本研究可为鳜属鱼类的分类及进化关系、群体遗传结构分析等提供理论支持,为野生原种鳜类遗传多样性的监测和评估以及鳜类优良的种质资源得到合理保护和开发利用奠定基础.  相似文献   
28.
Evaluating fishery impacts using metrics of community structure   总被引:8,自引:14,他引:8  
  相似文献   
29.
广西地区文蛤的遗传多样性研究   总被引:13,自引:3,他引:13  
以随机扩增多态性DNA(RAPD)研究广西的山口、北海、犀牛角、东兴的文蛤种群的遗传多样性。结果表明,在筛选的60个引物中,23个呈阳性反应,其中10个引物可产生105条稳定、清晰的带。山口、北海、犀牛角、东兴的文蛤种群的遗传多样性指数分别为0.1995,0.2039,0.1773,0.1936;多态位点比例分别为86.32%,90.63%,81.05%,88.00%,4个种群间的遗传距离在0.050~0.070之间,相邻种群的遗传差距较小。除犀牛角种群以外,其它3个种群的遗传变异水平较高。  相似文献   
30.
1 IntroductionManila clam (Ruditapes philippinarum) is aworldwide distributed species. In China, it is foundfrom the northern to southern seashores (Wang etal., 1993; Zhuang, 2001). This species is eura-line with the salinity tolerance ranging from 20 to4…  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