全文获取类型
收费全文 | 3291篇 |
免费 | 740篇 |
国内免费 | 475篇 |
专业分类
测绘学 | 819篇 |
大气科学 | 286篇 |
地球物理 | 365篇 |
地质学 | 1788篇 |
海洋学 | 314篇 |
天文学 | 17篇 |
综合类 | 339篇 |
自然地理 | 578篇 |
出版年
2024年 | 13篇 |
2023年 | 46篇 |
2022年 | 113篇 |
2021年 | 155篇 |
2020年 | 136篇 |
2019年 | 179篇 |
2018年 | 85篇 |
2017年 | 173篇 |
2016年 | 151篇 |
2015年 | 169篇 |
2014年 | 226篇 |
2013年 | 213篇 |
2012年 | 279篇 |
2011年 | 252篇 |
2010年 | 210篇 |
2009年 | 285篇 |
2008年 | 188篇 |
2007年 | 157篇 |
2006年 | 200篇 |
2005年 | 173篇 |
2004年 | 145篇 |
2003年 | 161篇 |
2002年 | 151篇 |
2001年 | 96篇 |
2000年 | 86篇 |
1999年 | 62篇 |
1998年 | 48篇 |
1997年 | 58篇 |
1996年 | 46篇 |
1995年 | 72篇 |
1994年 | 17篇 |
1993年 | 64篇 |
1992年 | 8篇 |
1991年 | 32篇 |
1990年 | 5篇 |
1989年 | 13篇 |
1988年 | 6篇 |
1987年 | 8篇 |
1986年 | 10篇 |
1985年 | 9篇 |
1984年 | 2篇 |
1983年 | 3篇 |
1954年 | 1篇 |
排序方式: 共有4506条查询结果,搜索用时 15 毫秒
891.
本文以福建东山岛的地形图、土地利用现状图和TM影像图为数据源,应用GIS监督分类及目视判读的方法,提取景观生态因子信息,对 1996年和 2002年两个时段景观生态类型进行了分析.结果表明随着社会的发展,景观类型破碎化程度加大,农田、园地、人工建筑逐渐增加,存在着一些潜在的生态问题.在此基础上提出了东山海岛县景观生态格局调整的对策,为该区维持生态平衡,保护生物多样性,提高环境质量及实现可持续发展提供了科学依据. 相似文献
892.
Mary E. Power 《Progress in Oceanography》2006,68(2-4):125
Because food web regimes control the biomass of primary producers (e.g., plants or algae), intermediate consumers (e.g., invertebrates), and large top predators (tuna, killer whales), they are of societal as well as academic interest. Some controls over food web regimes may be internal, but many are mediated by conditions or fluxes over large spatial scales. To understand locally observed changes in food webs, we must learn more about how environmental gradients and boundaries affect the fluxes of energy, materials, or organisms through landscapes or seascapes that influence local species interactions. Marine biologists and oceanographers have overcome formidable challenges of fieldwork on the high seas to make remarkable progress towards this goal. In river drainage networks, we have opportunities to address similar questions at smaller spatial scales, in ecosystems with clear physical structure and organization. Despite these advantages, we still have much to learn about linkages between fluxes from watershed landscapes and local food webs in river networks. Longitudinal (downstream) gradients in productivity, disturbance regimes, and habitat structure exert strong effects on the organisms and energy sources of river food webs, but their effects on species interactions are just beginning to be explored. In fluid ecosystems with less obvious physical structure, like the open ocean, discerning features that control the movement of organisms and affect food web dynamics is even more challenging. In both habitats, new sensing, tracing and mapping technologies have revealed how landscape or seascape features (e.g., watershed divides, ocean fronts or circulation cells) channel, contain or concentrate organisms, energy and materials. Field experiments and direct in situ observations of basic natural history, however, remain as vital as ever in interpreting the responses of biota to these features. We need field data that quantify the many spatial and temporal scales of functional relationships that link environments, fluxes and food web interactions to understand how they will respond to intensifying anthropogenic forcing over the coming decades. 相似文献
893.
894.
CTD测量技术的现状与发展 总被引:2,自引:0,他引:2
CTD测量技术是海洋研究开发的关键技术之一。本文简述了CTD测量技术基本原理,介绍了国内外CTD技术的现状和动向,同时还探讨了CTD技术的发展趋势和关注的几点看法。 相似文献
895.
基于Web Service的系统集成技术在海洋监测系统中的应用 总被引:4,自引:0,他引:4
基于Web Service的系统集成技术是一项新的面向服务的应用集成技术,文章在介绍、分析其特点与优势的同时,提出了在海洋监测系统中应用的必要性。 相似文献
896.
Water Flow, Trophic Depletion, and Benthic Macrofauna Impoverishment in a Submarine Cave from the Western Mediterranean 总被引:1,自引:0,他引:1
Mikel Zabala Tecla Riera Josep Maria Gili Manel Barange Agustin Lobo Josep Peñuelas 《Marine Ecology》1989,10(3):271-287
Abstract. Biotic zonation and severe impoverishment of benthic macrofauna are two of the most conspicuous features of submarine caves. The prevailing explanation assumes trophic depletion caused by reduction of water flow. However, no isolation of water was found in the studied Catalan submarine cave, even in the wall microlayer. Fluorescein diffusion was so fast that it was detectable everywhere in the cave already only a few minutes after the injection. The rate of dissolution of plaster spheres was even greater in the cave than in a nearby tunnel – without benthic macrofauna impoverishment – showing a considerable water flow. The oxygen concentration of water in dialysis bags placed at varying distances from the cave walls showed that no wall microlayer gradients were present.
Biochemical gradients did not indicate any decrease in food supply. Although photosynthetic pigment concentration generally decreased inside the cave, and although the gradients were not linear but formed different patterns throughout the cave, the BOD5 , the POM, the C:N ratio, the [3 H]-thymidine incorporation rate, and the ETS activity were higher in the inner part of the cave. There was an accumulation of detritic material and bacteria in the inner parts that constitute a plausible food supply for benthic macrofauna.
If neither water motion nor food supply can be invoked, research into the causes of zonation and disappearence of benthic macrofauna is proposed to be carried out on biotic interactions and behavioural processes. 相似文献
Biochemical gradients did not indicate any decrease in food supply. Although photosynthetic pigment concentration generally decreased inside the cave, and although the gradients were not linear but formed different patterns throughout the cave, the BOD
If neither water motion nor food supply can be invoked, research into the causes of zonation and disappearence of benthic macrofauna is proposed to be carried out on biotic interactions and behavioural processes. 相似文献
897.
898.
应用GPS技术对杭州湾跨海大桥进行首级控制网测量,结果表明:(1)合理布设GPS控制网,并采用设置深桩固埋测量标志、增加有效观测时间、加强数据可靠性与精确性的检验、选择科学及合理的坐标系统等方法,可提高观测数据的质量和精度。同时,对观测数据采用Bernese高精度基线处理软件和精密星历进行基线解算、网平差和坐标转换,保证了杭州湾跨海大桥首级控制网的精度在厘米级之内,使成果的质量达到优良。(2)Bernese基线数据的闭合环及复测基线的检核表明:同步环、异步环闭合差量值及复测基线互差量值全部符合限差要求,且在允许值的1/3以内的基线环数分别占93.3%、77.7%和100%,其成果精度达到优良。(3)GPS控制网的控制点点位的中误差全部满足±20mm的技术设计要求,并且有96.6%的控制点点位中误差在限差的1/3范围以内,精度较高。使用精密星历和广播星历两种不同的数据处理方法进行数据处理,其坐标互差吻合得较好,全部互差值均在限差允许值的1/2范围以内,且使用精密星历计算的成果精度明显高于使用广播星历计算的成果精度。 相似文献
899.
方斑东风螺养殖技术研究 总被引:2,自引:0,他引:2
本工作对方斑东风螺亲螺培育及产卵、种苗培养、幼螺养成过程中的一系列技术进行了研究,并对其行为及生态习性进行了观察.实验结果表明,在方斑东风螺养殖过程中,不同饵料培养效果的优劣顺序为牡蛎 〉鱼 〉鲆饵 〉鳗饵.在培养密度较高时,玻璃缸流水培养效果要优于静水培养.但同为低密度培养时,静水与流水效果差异不大.水泥池养殖结果显示平面养殖每平方米可养壳高15mm以下幼螺1 000~2 000个,壳高15~20mm的1 000~1 200个,壳高20~30mm以上的600~800个.使用鲍笼立体养殖亦可获得很高的成活率与生长率,将壳高20mm以上的幼螺放入鲍笼,4~5层笼叠加立体养殖,每平方米可养1 667~2 333个,笼养60d的成活率达95%以上. 相似文献
900.