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   32971篇
  免费   5811篇
  国内免费   7899篇
测绘学   2589篇
大气科学   6796篇
地球物理   8135篇
地质学   16034篇
海洋学   3911篇
天文学   1382篇
综合类   3611篇
自然地理   4223篇
  2024年   175篇
  2023年   572篇
  2022年   1479篇
  2021年   1750篇
  2020年   1390篇
  2019年   1649篇
  2018年   1792篇
  2017年   1643篇
  2016年   1926篇
  2015年   1677篇
  2014年   2060篇
  2013年   1926篇
  2012年   1870篇
  2011年   1947篇
  2010年   2061篇
  2009年   1920篇
  2008年   1786篇
  2007年   1682篇
  2006年   1315篇
  2005年   1241篇
  2004年   941篇
  2003年   946篇
  2002年   905篇
  2001年   942篇
  2000年   1084篇
  1999年   1482篇
  1998年   1189篇
  1997年   1124篇
  1996年   1037篇
  1995年   908篇
  1994年   788篇
  1993年   712篇
  1992年   563篇
  1991年   450篇
  1990年   330篇
  1989年   310篇
  1988年   270篇
  1987年   150篇
  1986年   143篇
  1985年   95篇
  1984年   99篇
  1983年   75篇
  1982年   65篇
  1981年   58篇
  1980年   41篇
  1979年   34篇
  1978年   11篇
  1977年   11篇
  1976年   6篇
  1958年   30篇
排序方式: 共有10000条查询结果,搜索用时 11 毫秒
971.
Ma  Jun  Song  Jinming  Li  Xuegang  Yuan  Huamao  Li  Ning  Duan  Liqin  Wang  Qidong 《中国海洋湖沼学报》2020,38(4):1215-1224
An investigation was carried out in the Y3 seamount area of the Western Pacific Ocean in December 2014,and the distribution of dissolved inorganic carbon(DIC) and its relationship with environmental factors in this area were explored.The results show that DIC concentration was higher in the adjacent waters of the Y3 seamount area,and the uplift of DIC isolines at the stations was close to the seamount.Meanwhile,interaction between the North Equatorial Current(NEC) and the Y3 seamount affected the DIC distribution i.e.,the upwelling in the same direction of the NEC was obvious,resulting in a decreasing trend of average concentration of DIC in the 200 m water column from the top to the two side s in this direction but in the cross direction.The DIC concentration increased with the water depth increase,and its distribution was affected by various environmental factors.In the surface water,high temperature was a decisive factor for the decrease of the DIC concentration,but the photosynthesis of phytoplankton showing only a weak influence.In the North Pacific Tropic Water(NPTW),DIC production rate from organic matter decomposition was higher than that of DIC consumption by phytoplankton photosynthesis,leading to a continual increase of DIC.In the North Pacific Intermediate Water(NPIW),organic matter decomposition played a leading role in the increase of DIC.In the deep water,decomposition of organic matter weakened,and the dissolution of CaCO_3 controlled the carbonate system,and DIC had the smallest variation range.  相似文献   
972.
Li  Mengna  Chen  Hao  Wang  Minxiao  Zhong  Zhaoshan  Zhou  Li  Li  Chaolun 《中国海洋湖沼学报》2020,38(4):1292-1303
Deep-sea mussels of the subfamily Bathymodiolinae are common and numerically dominant species widely distributed in cold seeps and hydrothermal vents.During long-time evolution,deep-sea mussels have evolved to be well adapted to the local environment of cold seeps and hydrothermal vents by various ways,especially by establishing endosymbiosis with chemotrophic bacteria.However,biological processes underlying the establishment and maintenance of symbiosis between host mussels and symbionts are largely unclear.In the present study,Gigantidasplatifrons genes possibly involved in the symbio sis with methane oxidation symbionts were identified and characterized by Lipopoly saccharide(LPS) pull-down and in situ hybridization.Five immune related proteins including Toll-like receptor 2(TLR2),integrin,vacuolar sorting protein(VSP),matrix metalloproteinase 1(MMP1),and leucine-rich repeat(LRR-1) were identified by LPS pull-down assay.These five proteins were all conserved in either molecular sequences or functional domains and known to be key molecules in host immune recognition,phagocyto sis,and ly so some-mediated digestion.Furthermore,in situ hybridization of LRR-1,TLR2 and VSP genes was conducted to investigate their expression patterns in gill tissues of G.platifrons.Consequently,LRR-1,TLR2,and VSP genes were found expressed exclusively in the bacteriocytes of G.platifrons.Therefore,it was suggested that TLR2,integrin,VSP,MMP1,and LRR-1 might be crucial molecules in the symbiosis between G.platifrons and methane oxidation bacteria by participating in symbio sis-related immune processes.  相似文献   
973.
Zhong  Zhaoshan  Wang  Minxiao  Chen  Hao  Zheng  Ping  Li  Chaolun 《中国海洋湖沼学报》2020,38(4):1304-1318
Gigantidas platifrons(Bivalvia,Mytilidae),the dominant species at the Formosa cold seep,relies on methanotrophic symbionts dwelling in its gills for nutrition.The reproductive patterns of G.platifrons provide fundamental information for understanding the population recruitment of this species.However,we know very little about important processes in reproduction,such as gametogenesis and symbiotic bacteria transmission.To this end,we described the developmental patterns of the gonads from nine surveys and juvenile length-distribution from one-year larval traps and detected bacteria in gonad from G.platifrons samples.Our results show that G.platifrons is a functionally dioecious species.The reproduction of G.platifrons is discontinuous,with spawning maturity peak around the fourth quarter of the year.The seasonal reproduction of G.platifrons was further supported by the unimodal shell length distribution of the trapped juvenile mussels.Given the small oocyte size(48.99-70.14 μm),which was comparable to that of coastal mussels,we proposed that G.platifrons developed via a free-living,planktotrophic larval stage before settlement.The blooms at the water surface can also supply the development of the planktonic larvae of G.platifrons.Meanwhile,no bacteria were observed in gonads,suggesting a horizontal symbiont transfer mode in this mussel.Collectively,these results provide fundamental biological information for an improved understanding of the early life history of G.platifrons in the Formosa cold seep.  相似文献   
974.
Liu  Changshui  Zang  Kun  Li  Shihao  Li  Fuhua  Ma  Qingjun 《中国海洋湖沼学报》2020,38(6):1891-1899
Journal of Oceanology and Limnology - Thymidylate synthase (TS) is a key enzyme in the de novo biosynthesis of thymidine monophosphate, serving as a well-known drug target in chemotherapy against...  相似文献   
975.
近几年,菏泽市国土资源事业发展迅速,土地收益逐年增加、土地整理项目效益日渐显现、土地市场秩序逐渐规范、涉矿项目由单纯的黏土逐渐将成为华东地区唯一一块未开发的整装煤田。随着体制调整逐步到位,财务部门面临新的形势,横向衍生出具有专业职能的事业单位,纵向延伸到县乡,国土资源财务管理范围和难度加大。为此,必须进一步规范国土资源财务管理,加强各项资金的监督与管理,更好地发挥国土资源在经济社会发展中的保障作用。  相似文献   
976.
We investigated seasonal variations in cyanobacterial biomass and the forms of its dominant population(M. aeruginosa) and their correlation with environmental factors in the water source area of Chaohu City,China from December 2011 to October 2012. The results show that species belonging to the phylum Cyanophyta occupied the maximum proportion of phytoplankton biomass,and that the dominant population in the water source area of Chaohu City was M. aeruginosa. The variation in cyanobacterial biomass from March to August 2012 was well fitted to the logistic growth model. The growth rate of cyanobacteria was the highest in June,and the biomass of cyanobacteria reached a maximum in August. From February to March 2012,the main form of M.aeruginosa was the single-cell form; M.aeruginosa colonies began to appear from April,and blooms appeared on the water surface in May. The maximum diameter of the colonies was recorded in July,and then gradually decreased from August. The diameter range of M. aeruginosa colonies was 18.37–237.77 μm,and most of the colonies were distributed in the range 20–200 μm,comprising 95.5% of the total number of samples. Temperature and photosynthetically active radiation may be the most important factors that influenced the annual variation in M. aeruginosa biomass and forms. The suitable temperature for cyanobacterial growth was in the range of 15–30°C. In natural water bodies,photosynthetically active radiation had a significant positive influence on the colonial diameter of M.aeruginosa(P0.01).  相似文献   
977.
Phenotypic and genetic parameters for growth-related traits in the half-smooth tongue sole, Cynoglossus semilaevis, were estimated in 22 full-sib families produced by normal and neo-male breeding stocks. As phenotypic males with female genotypes, neo-males are harmful in C. semilaevis aquaculture because they reduce overall production. The present study evaluated the difference in the growth-related traits: total length (TL), body weight (BW) and square root of body weight (SQ_BW) at the age of 570 days between normal and neo-male offspring (neo-males used as male parents). The difference in the proportion of females between normal and neo-male offspring was also assessed. Based on the linear mixed model, restricted maximum likelihood (REML) and best linear unbiased prediction (BLUP) were used to estimate various (co)variance components and estimated breeding values (EBVs) of growth-related traits. As a result, all the mean values of the three studied traits were significantly larger in normal offspring than in neo-male offspring. Additionally, the female proportion was significantly larger in normal offspring than in neo-male offspring. Heritability was 0.128±0.066 2 for TL, 0.128±0.065 5 for BW and 0.132±0.062 9 for SQ_BW, all of which were low level heritabilities. The correlation coefficients of EBVs and phenotypic values of the target traits were 0.516 for TL, 0.524 for BW and 0.506 for SQ_BW, all of which were highly significant (P <0.01). Genetic correlations among TL, BW and SQ_BW were positive high (0.921–0.969) and higher than those of phenotype (0.711–0.748), both of which had low standard errors (0.063–0.123 for genotype, and 0.010–0.018 for phenotype). Compared with normal offspring, neo-male offspring have lower breeding values for each studied trait through EBVs comparison. Therefore, neo-male offspring should not be used as broodstock in a C. semilaevis breeding programs.  相似文献   
978.
The Drake Passage is located between the Antarctic Peninsula and Tierra del Fuego in the south of South America. Surface seawater samples were collected at seven sites in the Drake Passage during the austral summer of 2012. The 16S rRNA sequences were analyzed from 187 isolated bacterial strains. Three phyla, 29 genera and 56 species were identified. The three phyla were Actinobacteria, Firmicutes and Proteobacteria; the Proteobacteria included α-Proteobacteria, β-Proteobacteria and γ-Proteobacteria. γ-Proteobacteria, Actinobacteria and Firmicutes were the dominant class or phyla in terms of quantity and species. Gram-positive bacteria (Actinobacteria and Firmicutes) accounted for 57.8% of all types identified. There were nine dominant genera, including Curtobacterium, Staphylococcus, and Halomonas, and 14 dominant species including Curtobacterium flaccumfaciens, Curtobacterium pusillum, and Staphylococcus sciuri. Of the strains identified, 87.2% were catalase positive or weakly positive.  相似文献   
979.
Currently, there is limited literature dealing with the length of stay of circular migrants in urban areas (LSCMU), although many studies have discussed the phenomenon of migration and the determinants of migration. This study attempts to fill that gap using survey data from the mountainous areas of Chongqing, Southwest China. From a comparative perspective, this study divided workers into two groups (a young group aged between 16 and 35 years and an old group aged between 36 and 65 years). The average LSCMU values for the young and old groups were 225 days and 174 days, respectively. Two multi-regression models were used to estimate the determinants of LSCMU in the two groups. The results showed that LSCMU was closely related to individual factors in both groups, including gender, age and job training. Family and community factors, including household size, arable land per capita and the distance from market, had much weaker effects on the dependent variable, especially in the older group. It was noticeable that job training had significant positive effects on LSCMU in both groups; these findings have special implications for the urbanization process in China.  相似文献   
980.
As an effective livelihood approach to alleviate poverty without rural population migration, ethnic tourism has become the primary choice of economic development in ethnic areas worldwide in addition to traditional livelihood approaches. This article applies the theories of livelihood to study the community evolution driven by tourism livelihood and examine three mountainous tourism communities in different stages of tourist area life cycle. Drawing on the methods of GIS spatial analysis, semi-structured interviews and questionnaires, this article proposes a sustainable livelihood framework for ethnic tourism to explore the evolution of ethnic tourism communities by identifying changes in livelihood assets (natural, financial, social, cultural and human capitals) in the process of tourism development. The results show that the development of ethnic tourism has led to changes in the increase of building land, and the diversification of land use functions with a trend of shifting from meeting local villagers' living needs to satisfying tourists, income composition and uneven distribution of tourism income spatially. Ethnic tourism also led to the deterioration of traditional social management structure, collapse of neighboring relationship, the over- commercialization and staged authenticity of ethnic culture, as well as the gradual vanish of agricultural knowledge with a trend of increasing modern business knowledge and higher education. In addition, these changes, involving livelihood assets from natural, economic, human, social and cultural aspects are interrelated and interactive, which form new evolution characters of ethnic community. This study reveals the conflicts over livelihood approaches which have formed new vulnerabilities to impact on sustainable evolution of ethnic communities. This research provides implications for achieving the sustainable development of ethnic communities with the driving force of tourism livelihood.  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