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   2449篇
  免费   367篇
  国内免费   592篇
测绘学   156篇
大气科学   196篇
地球物理   477篇
地质学   1243篇
海洋学   388篇
天文学   13篇
综合类   178篇
自然地理   757篇
  2024年   26篇
  2023年   53篇
  2022年   112篇
  2021年   122篇
  2020年   150篇
  2019年   149篇
  2018年   130篇
  2017年   112篇
  2016年   163篇
  2015年   128篇
  2014年   170篇
  2013年   184篇
  2012年   168篇
  2011年   157篇
  2010年   134篇
  2009年   148篇
  2008年   127篇
  2007年   147篇
  2006年   131篇
  2005年   148篇
  2004年   129篇
  2003年   93篇
  2002年   78篇
  2001年   68篇
  2000年   74篇
  1999年   49篇
  1998年   37篇
  1997年   38篇
  1996年   25篇
  1995年   26篇
  1994年   31篇
  1993年   24篇
  1992年   13篇
  1991年   13篇
  1990年   9篇
  1989年   12篇
  1988年   11篇
  1987年   4篇
  1986年   1篇
  1985年   3篇
  1984年   5篇
  1983年   2篇
  1982年   1篇
  1981年   1篇
  1979年   1篇
  1978年   1篇
排序方式: 共有3408条查询结果,搜索用时 15 毫秒
71.
Since the 1980s, there have been continuous increases in the coverage of marine protected areas (MPAs) in China, and a total of 158 MPAs have been declared. The MPA system in China is characterized by (1) decentralised designation and management with reduced control from the central government; (2) a dominance of de jure fully protected MPAs that are often implemented as de facto multiple-use areas; and (3) a lack of objective evaluation processes. To improve China's MPA system requires an appropriate integration of fully protected and multiple-use MPAs, and an approach that balances the advantages of top-down and bottom-up approaches.  相似文献   
72.
Three Tunisian coastal and lagoon sites, variously affected by human activities, and differing according to their degree of communication with the sea, were studied. The total organic matter content and the granulometry of the sediment were measured and the macro-invertebrates were sampled and identified. Currently due to the lack of an efficient single biotic index for quality assessment, indications from these measurements can contribute to enrich our knowledge of the southern Mediterranean areas. Several biotic indices (AMBI, BENTIX, BOPA, BOPA modified, I2EC, ITI), most of which have been used in the context of the European Water Framework Directive, and some other structural parameters were used. The results show that the Bizerte lagoon, an almost closed area characterized mainly by extreme salinity/temperature and with a strong fluctuation of nutrients, appears biologically more impoverished. The Dkhila coast, which is more open to the sea, however, appears to be less dominated by the principal species. Biotic indices showed that the most polluted areas included the region exposed to urban inputs of Bizerte, the mouth of Hamdoun wadi and the harbour areas. In these locations, benthic macrofauna is dominated by opportunistic species like Capitella capitata, Scolelepis fuliginosa, Prionospio malmgreni, Polydora sp., Cirratulus cirratus and Cirriformia tentaculata. Other stations, which are farther from the sources of disturbance such as the mouths of the principal Wadis, the south of the Bizerte ship canal and the south of the Tunis bay, appear to be in good ecological condition and hosted a variety of sensitive species, Other stations were classified differently by the biotic indices used mainly because of the lack of a real inter-calibration of the various classification methodologies used by these indices.  相似文献   
73.
Systematic planning for conservation is highly regarded but relies on spatially explicit data that are lacking in many areas of conservation concern. The decision support tool Marxan is applied to a reef system in the central Philippines where 30 marine protected areas (MPAs) have been established in communities without much use of biophysical data. The intent was to explore how Marxan might assist with the legally required expansion to protect 15% of marine waters, and how existing MPAs might affect that process. Results show that biophysical information alone did not provide much guidance in identifying patterns of conservation importance in areas where the data are poor. Socioeconomic data were needed to distinguish among possible areas for protection; but here, as elsewhere in marine environments, the availability of such data was very limited. In the final analysis, local knowledge and integrated understanding of socioeconomic realities may offer the best spatially explicit information. The 30 existing MPAs, which encompassed a small proportion of the reef system, did not limit future options in developing a suite of MPAs on a broader scale. Rather, they appeared to generate the support for MPAs that is obligatory for any larger zoning effort. In summary, establishing MPAs based on community-driven criteria has biological and social value, but efforts should be made to collect ecological and socioeconomic data to guide the continued creation of MPAs.  相似文献   
74.
In Belgium a long process (1999–2005) led to the designation of several marine protected areas (MPAs). In order to analyse the designation process, the ‘policy arrangement approach’ was used as an analytical tool. Attention was given to four dimensions (actors and coalitions, arguments, rules and resources). Particular attention was paid to the switch from an authoritative to a more deliberated policy style in reaction to conflicts with stakeholders. This switch in policy style contributed to a successful legal designation, but does not guarantee the best protection of marine biodiversity. This case study linked science (setting ecological goals) to policy (analysis of a policy process) and led to some more general conclusions on the designation of MPAs and the role of participation in this process.  相似文献   
75.
南海及其邻近海域SST系统的点概率密度谱特征   总被引:3,自引:0,他引:3  
采用相空间点概率密度谱分析方法对南海及其邻近海域NCEPreanalysisSST候平均资料进行了分析,发现南海与其邻近海域的SST系统的结构均具有混沌特征,南海与孟加拉湾SST系统的耦合谱分布几乎一致;年变化最显著,年际变化次之,季节内变化相对较弱。邻近西太平洋SST系统中的准3a耦合振荡相当显著。对于季节内振荡,西太平洋SST系统强于孟加拉湾和南海的SST系统。结果还表明季节内振荡(30~60d)是一种耦合振荡。  相似文献   
76.
根据采集的3 500 km浅地层剖面资料,对闽北近岸海域末次盛冰期古河道体系的平面展布特征(宽度、深度变化特征)和垂向剖面反射特征(反射界面、层序内部反射特征)进行了详细的描述,系统地分析了研究区在末次盛冰期古河道体系的平面展布和古河道典型断面反射特征,初步推测了其起源及形成时的地质环境。结果表明:①研究区内存在一个古河道体系,结合当时古地形及地理位置综合分析,古河道分布与当时海底的地形变化大致对应,推测其中一条走向大致为EW向的古河道可能来源于闽江,另外一条走向为SN向的古河道可能来源于长江支流,具体起源尚待进一步探讨;②进一步验证了自末次盛冰期以来,研究区从发生海退裸露地表发育古河道到再次遭受海侵形成浅海环境的历史变迁。  相似文献   
77.
作者采用动态实验法与静态实验法相结合的方法,研究了三门湾脊尾白虾(Exopalaemon carinicauda)、口虾蛄(Oratosquilla oratoria)和日本(Charybdis japonica)在不同季节的基础水温即驯化水温(8~29℃)和温升速率(0.5~15.0℃/h)下的热耐受能力。结果表明,驯化水温和温升速率对各实验动物的热耐受性均有显著影响。实验动物的热耐受性与驯化水温总体上呈显著正相关,而温升速率对热耐受性的影响具有物种特异性,并受驯化水温制约;在不同驯化水温下,各实验动物的热耐受性随温升速率增大呈不同变化趋势。各实验动物的24 h高起始致死温度受驯化水温的影响显著,随着驯化水温从8℃升高到29℃,脊尾白虾、日本和口虾蛄的24hUILT50分别从24.2、34.6、24.9℃显著增大到35.3、37.4和34.4℃。结合3种实验动物的最大临界温度分析,它们的热耐受能力依次为:日本脊尾白虾口虾蛄。研究结果可为探究三门湾水域潜在的热污染状况及其生态环境效应提供科学依据。  相似文献   
78.
Seeing the ocean through the eyes of seabirds could help meet the challenges of managing common-pool marine resources both in protected and unprotected areas. First, seabirds are top-predators, exposed to all threats affecting the oceans, and this makes them ideal sentinel organisms for monitoring changes within marine ecosystems. Second, seabirds cross both ecological and political boundaries, and following their movements should help making interdependencies within and between marine ecosystems more visible. Third, seabirds are conspicuous and often charismatic animals, which interact differently with different groups of stakeholders and provide the opportunity to acknowledge and discuss each other's values and interests. In this paper, we present these research avenues using a seabirds’ view, for tackling marine conservation and management issues, and we give operational examples of implementation based on our work in the English Channel.  相似文献   
79.
There is an on-going process to establish Marine Conservation Zones (MCZs) in England, to form part of a coherent and representative network of marine protected areas under national and EU legislation. From 2009 to 2011, the MCZ process included strong participatory elements. Four regional multi-sector stakeholder groups developed MCZ recommendations collaboratively, in line with ecological guidance provided by the Government's nature conservation advisers. This guidance was based on Government policy principles, including that MCZs should be designated based on ‘best available evidence’. This paper analyses the multi-dimensional conflicts that emerged within the stakeholder group in south-west England, which were magnified by uncertainty about future MCZ management. In September 2011, after working through these conflicts through trade-offs and negotiations, the stakeholder groups jointly recommended 127 MCZs to Government. The process subsequently shifted to a top-down approach, with further stakeholder engagement limited to bilateral consultation. There was a concurrent shift in policy, from a broad-scale network-level focus towards single-feature conservation. A lengthy series of evidence reviews concluded that the existing evidence at the time was insufficient to progress with the designation of most sites, marking a clear departure from the policy principle of proceeding with the designation of a representative network based on ‘best available evidence’, and effectively undermining the work carried out by stakeholder groups. Though MCZ designation was originally timetabled for 2012, in November 2013 just 27 of the recommended 127 MCZs were designated in a first tranche. At the time, no clear timetable was in place for subsequent tranches.  相似文献   
80.
金枪鱼类是中西太平洋海域重要的经济鱼种,其中鲣产量约占到总产量的50%。本研究利用1995-2010年16年的中西太平洋(20°S~20°N,120°E~155°W)鲣围网生产统计数据和Niño3.4海区(5°S~5°N,120°~170°W)海表温度异常数据,对这16年鲣产量最高的十大渔区(5°×5°)进行时空格局分析,讨论渔场分布差异及CPUE与ENSO指数的关系。结果表明:16年间十大作业渔区主要分布在5°S~5°N、130°~175°E区域,这十大渔区产量占总产量的比重达47.5%,其中5°S~0°、155°~160°E,0°~5°N、130°~135°E,0°~5°N、135°~140°E及5°S~0°、160°~165°E等4个渔区产量占高产渔区产量的比重均超过10%,是中西太平洋重要的鲣产区。高产渔区的分布受海表温度影响较大,在厄尔尼诺时期,高产渔区分布明显偏东,主要分布在155°~180°E海域;在拉尼娜时期,高产渔区分布明显偏西,主要分布在130°~160°E海域。  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