全文获取类型
收费全文 | 7452篇 |
免费 | 1414篇 |
国内免费 | 2112篇 |
专业分类
测绘学 | 430篇 |
大气科学 | 1267篇 |
地球物理 | 620篇 |
地质学 | 5208篇 |
海洋学 | 1215篇 |
天文学 | 28篇 |
综合类 | 637篇 |
自然地理 | 1573篇 |
出版年
2024年 | 60篇 |
2023年 | 186篇 |
2022年 | 316篇 |
2021年 | 378篇 |
2020年 | 359篇 |
2019年 | 398篇 |
2018年 | 308篇 |
2017年 | 315篇 |
2016年 | 381篇 |
2015年 | 372篇 |
2014年 | 459篇 |
2013年 | 433篇 |
2012年 | 418篇 |
2011年 | 482篇 |
2010年 | 410篇 |
2009年 | 442篇 |
2008年 | 457篇 |
2007年 | 490篇 |
2006年 | 514篇 |
2005年 | 443篇 |
2004年 | 463篇 |
2003年 | 377篇 |
2002年 | 369篇 |
2001年 | 347篇 |
2000年 | 294篇 |
1999年 | 272篇 |
1998年 | 247篇 |
1997年 | 185篇 |
1996年 | 179篇 |
1995年 | 133篇 |
1994年 | 109篇 |
1993年 | 100篇 |
1992年 | 62篇 |
1991年 | 66篇 |
1990年 | 43篇 |
1989年 | 28篇 |
1988年 | 22篇 |
1987年 | 11篇 |
1986年 | 12篇 |
1985年 | 11篇 |
1984年 | 7篇 |
1983年 | 6篇 |
1982年 | 9篇 |
1981年 | 2篇 |
1979年 | 1篇 |
1978年 | 1篇 |
1954年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 93 毫秒
971.
972.
The coupling between biogenic reworking activity and reactive organic matter patterns within deposits is poorly understood and often ignored. In this study, we examined how common experimental treatments of sediment affect the burrowing behavior of the polychaete Nephtys incisa and how these effects may interact with reactive organic matter distributions to alter diagenetic transport – reaction balances. Sediment and animals were recovered from a subtidal site in central Long Island Sound, USA. The upper 15 cm of the sediment was sectioned into sub-intervals, and each interval separately sieved and homogenized. Three initial distributions of sediment and organic substrate reactivity were setup in a series of microcosms: (1) a reconstituted natural pattern with surface-derived sediment overlying sediment obtained from progressively deeper material to a depth of 15 cm (Natural); (2) a 15 cm thick sediment layer composed only of surface-derived sediment (Rich); and (3) a 15 cm thick layer composed of uniformally mixed sediment from the original 15 cm sediment profile (Averaged). The two last treatments are comparable to that used in microcosms in many previous studies of bioturbation and interspecific functional interaction experiments. Sediment grain size distributions were 97.5% silt-clay and showed no depth dependent patterns. Sediment porosity gradients were slightly altered by the treatments. Nepthys were reintroduced and aquariums were X-rayed regularly over 5 months to visualize and quantify spatial and temporal dynamics of burrows. The burrowing behaviour of adult populations having similar total biovolume, biomass, abundance, and individual sizes differed substantially as a function of treatment. Burrows in sediment with natural property gradients were much shallower and less dense than those in microcosms with altered gradients. The burrow volume/biovolume ratio was also lower in the substrate with natural organic reactivity gradients. Variation in food resources or in sediment mechanical properties associated with treatments, the latter in part coupled to remineralization processes such as exopolymer production, may explain the burrowing responses. In addition to demonstrating how species may respond to physical sedimentation events (substrate homogenization) and patterns of reactive organic matter redistribution, these experiments suggest that infaunal species interactions in microcosms, including the absolute and relative fluxes of remineralized solutes, may be subject to artifacts depending on exactly how sediments are introduced experimentally. Nonlocal transport and cylinder microenvironment transport – reaction models readily demonstrate how the multiple interactions between burrowing patterns and remineralization rate distributions can alter relative flux balances, decomposition pathways, and time to steady state. 相似文献
973.
Stakeholder consultation is being adopted as standard practice in the planning and management of natural resource management programs. While the utility of stakeholder participation has been investigated for the evaluation and implementation phases of natural resource management programs, few studies have examined the utility of stakeholder consultation during the initial phases of developing such programs. This paper presents a case study from a project developing a marine and coastal monitoring program for the Pilbara and Kimberley region of northern Western Australia. Via a series of workshops held in the region, stakeholders were asked to prioritise future research needs using several voting procedures. During the analyses of the results from the different voting procedures, it became apparent that there were high levels of inconsistency, poor correlation, and contradiction, between participants’ responses. Despite the rigour of the selection process used to identify ‘suitable’ stakeholders for the workshops, these results show that stakeholders did not have the technical or broader contextual knowledge about marine ecosystems to effectively and objectively contribute to the research prioritisation and planning process. Based on the outcomes of this study, we argue that project designers need to be clear about why they are involving stakeholders in a project, particularly in light of the costs involved (financial, time, resources, costs to the stakeholder) in stakeholder consultation. Stakeholder involvement may be appropriate in later stages of developing natural resource management programs (implementation and management), however, stakeholder involvement is not appropriate in the initial phases of such programs, where scientific expertise is essential in formulating scientific concepts and frameworks. 相似文献
974.
鞍带石斑鱼工厂化育苗研究 总被引:3,自引:0,他引:3
作者设计了一种新的鞍带石斑鱼(Epinephelus lanceolatus)工厂化人工育苗方法。测定了鞍带石斑鱼人工育苗过程中水环境因子和育苗水体浮游生物的种类、数量变化。育苗期间水温28.0~30.7℃;盐度为27.0~32.0;pH值为7.39~7.90;溶氧量6mg/L以上;中午池面平均光照3315.9lx;记录了1d的光照、水温、溶解氧和pH的变化;育苗池前18d只加水不换水,氨氮不断上升,最低值0.24mg/L,最高值2.53mg/L,育苗后期采取换水措施,氨氮迅速下降;育苗期间,水体除正常投喂的轮虫(Brachionus plicatilis)和桡足类外,池中出现浮游植物12种、浮游动物5种,浮游植物平均生物量6.92×106个/L,原生动物平均生物量0.74×106个/L。经过34d的培养,得到平均全长2.45cm鱼种,单位水体育苗密度300尾/m3,育苗成活率为2.73%。结果表明,用有效微生物群(effective microorganisms,简称EM)+高级虾片调节水质的模式,可以保持育苗水质稳定,成功培育出鞍带石斑鱼幼鱼。 相似文献
975.
976.
977.
978.
基于支持向量机的京津冀城市群热环境时空形态模拟 总被引:1,自引:0,他引:1
城市群热环境作为区域生态重要组成部分,已成为近年来的研究热点。而如何选择针对城市群这种复杂地地貌特征的热环境量化工具一直是亟待解决的技术难点,基于此本研究提出了一种解决多样本、非线性、非平稳及高维函数拟合的计算方法,并建立了基于支持向量机(SVM)的京津冀城市群热环境曲面模型来揭示城市群热环境的时空形态变化。研究结果表明:① SVM模型在刻画多核心、多种土地利用类型城市群热环境的空间分布方面具有理论与实践可行性,能够根据热环境的整体空间布局通过高斯核函数进行局部优化差值,最大限度减少缺省值对模型拟合结果的影响。相比于对照方法可以模拟出更高精度的复杂地貌特征城市群热岛空间分布格局;② 在SVM模型曲面拟合的过程中,拟合精度和拟合时间是衡量拟合结果的重要指标,而原始影像的分辨率则是影响该指标的决定性因素;③ 2003-2013年区域内北京市与天津市的城市热岛效应变化最为明显,热岛面积分别增加7091 km2与4196 km2,空间上呈现出逐年接近连片发展趋势,热岛重心移动轨迹具有明显的时空分异性。北京城市热岛特征为东南部地区异速增长,西部地区缓慢增长;天津城市热岛特征为以城市中心为圆心向周围扩展。本研究进一步丰富了城市群热环境评测的定量方法,可以在实践上对城市群的城市规划、城市建设、环境保护和区域可持续发展等提供定量化、可视化的决策支持。 相似文献
979.
980.
Sediment cores were collected from the subaqueous delta of the Changjiang Estuary. Sediment grain-size profiles and their fractal dimensions were analyzed, to elucidate responses to long-term sedimentary processes. In addition, the environmental sensitive populations of grain size have been extracted. The sediment cores can be divided into two parts, according to the sedimentary structures present. The upper part (0-12 cm) is interpreted as being the active layer, which is influenced frequently by changes i... 相似文献