全文获取类型
收费全文 | 57271篇 |
免费 | 9615篇 |
国内免费 | 13519篇 |
专业分类
测绘学 | 4992篇 |
大气科学 | 6446篇 |
地球物理 | 10920篇 |
地质学 | 34742篇 |
海洋学 | 7375篇 |
天文学 | 2288篇 |
综合类 | 3975篇 |
自然地理 | 9667篇 |
出版年
2024年 | 306篇 |
2023年 | 840篇 |
2022年 | 1716篇 |
2021年 | 1984篇 |
2020年 | 2048篇 |
2019年 | 2350篇 |
2018年 | 1893篇 |
2017年 | 2137篇 |
2016年 | 2394篇 |
2015年 | 2549篇 |
2014年 | 3087篇 |
2013年 | 2966篇 |
2012年 | 3471篇 |
2011年 | 3572篇 |
2010年 | 3111篇 |
2009年 | 3692篇 |
2008年 | 3614篇 |
2007年 | 3998篇 |
2006年 | 3902篇 |
2005年 | 3429篇 |
2004年 | 3278篇 |
2003年 | 3089篇 |
2002年 | 2767篇 |
2001年 | 2413篇 |
2000年 | 2167篇 |
1999年 | 2027篇 |
1998年 | 1719篇 |
1997年 | 1569篇 |
1996年 | 1440篇 |
1995年 | 1202篇 |
1994年 | 1212篇 |
1993年 | 1054篇 |
1992年 | 824篇 |
1991年 | 612篇 |
1990年 | 502篇 |
1989年 | 459篇 |
1988年 | 305篇 |
1987年 | 210篇 |
1986年 | 133篇 |
1985年 | 87篇 |
1984年 | 35篇 |
1983年 | 31篇 |
1982年 | 32篇 |
1981年 | 21篇 |
1980年 | 24篇 |
1979年 | 35篇 |
1978年 | 39篇 |
1977年 | 24篇 |
1975年 | 4篇 |
1954年 | 16篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
551.
本文根据1983—1984年,1987—1988年台湾海峡中北部海域调查资料,着重讨论了该海域夏季溶解氧分布特征及其与上升流、生物活动的关系。该上升流区温度、盐度、溶解氧含量及其饱和度的特征值分别为24.4℃,34.3,4.49mlO_2/L和93%。溶解氧含量及其饱和度与温度呈正相关关系,与盐度呈负相关关系。 相似文献
552.
Evaluating fishery impacts using metrics of community structure 总被引:8,自引:14,他引:8
553.
A study of an expanding seagrass bed on the south-west coast of British Columbia, Canada involved documentation and explanation of the pattern of expansion of the vegetation as well as documentation and experimental investigation of the accompanying changes in the distribution of infaunal invertebrates. Expansion followed a major environmental change, improved water clarity initiated in 1969 when a causeway blocked access to the site for silty Fraser River water. The original eelgrass (Zostera marina) bed expanded landward over 30 m year−1, probably causing more and more water to be retained in the bed at low tide and thus improving its own habitat. More rapid expansion occurred from 1979–1983 after Z.japonica colonized at the landward edge of the eelgrass bed and in a separate intertidal area. Expansion ceased around 1983 coincident with, but probably independent of, further construction.Burrowing shrimp (Callianassa californiensis) decreased in abundance between 1977 and 1984 in areas where the two seagrasses colonized. Short-term experiment over one summer and fall showed that removal of all shoots allowed adult shrimp and tube worms to colonize the sediment while addition of shoots of Z. japonica by transplanting caused temporary decreases in shrimp abundance. After a few weeks an established shrimp population destroyed the transplants. Among reasonable alternate hypotheses for the decline in Callianassa, the effect of sediment texture can be eliminated, but either direct negative effects of seagrass, i.e. inhibition of the burrowing of adult shrimp or of settlement of juveniles, or indirect effects, i.e. the harboring of more predators of shrimp in seagrass beds, deserve further study. 相似文献
554.
555.
根据浅水波浪的底摩擦效应,对鲁南灵山湾和臼-岚山镇近岸区进行了波浪的海底摩擦力计算。波浪的海底摩擦力等值图与已知滨海锆石砂矿资料对比表明,底摩擦力大于0.18N/m^2的动力分选区有利于重矿物砂富集。据此,进一步预测了鲁南近岸带的砂矿富集区。 相似文献
556.
为了解南麂列岛海洋自然保护区潮间带贝类十多年来的动态变化,在1992年研究基础上,于2003年度再赴南麂列岛对岩、沙相贝类进行了研究.本次调查鉴定出贝类105种,其中岩相93种、沙相12种,6种为南麂列岛新记录种,对岩相、沙相贝类组合的生态特征进行了分析,岩相多样性和均匀度指数(1.25~2.20和0.54~0.70)高于沙相的(0.23~0.95和0.17~0.94).岩相贝类数量为3025.70g/m2和2660个/m2,生物量垂直分布为低潮区最高(6471.99g/m2),中潮区居中(3772.09g/m2),高潮区最小(78.35g/m2),栖息密度为中潮区最大(4245个/m2),低潮区居中(2973个/m2),高潮区最小(439个/m2).沙相贝类数量为16.81g/m2和11个/m2,低潮区(29.43g/m2和11个/m2)大于中潮区(14.01g/m2和10个/m2).通过与以往资料比对发现,岩礁贝类数量有所增加,沙滩贝类数量则明显下降,监控区贝类数量明显多于其他区域,对贝类资源变化产生影响的相关因子作了分析探讨. 相似文献
557.
由于海岸带的地理条件和海洋动力特性的复杂性以及人类开发活动对海岸带的影响,近岸海域的生态环境系统在空间分布上有很大的变异性.基于现场实测资料,建立系统的空间变异分析模型,对海岸带生态环境空间变化规律进行分析,可以充分了解不同区域的污染特征和变化趋势.应用所构建的空间变异分析模型对渤海湾天津近岸海域的水环境特性进行分析,结合对应年份的入海流量及其变化,分析了天津渤海湾海岸带污染的主因子和区域水环境的分布特征,结果可以对海洋功能区的划分和水环境综合评价结果进行验证和补充. 相似文献
558.
559.
Using an idealized ocean general circulation model, we examine the effect of “mixing hotspots” (localized regions of intense
diapycnal mixing) predicted based on internal wave-wave interaction theory (Hibiya et al., 2006) on the meridional overturning circulation of the Pacific Ocean. Although the assumed diapycnal diffusivity in the
mixing hotspots is a little larger than the predicted value, the upwelling in the mixing hotspots is not sufficient to balance
the deep-water production; out of 17 Sv of the downwelled water along the southern boundary, only 9.2 Sv is found to upwell
in the mixing hotspots. The imbalance as much as 7.8 Sv is compensated by entrainment into the surface mixed layer in the
vicinity of the downwelling region. As a result, the northward transport of the deep water crossing the equator is limited
to 5.5 Sv, much less than estimated from previous current meter moorings and hydrographic surveys. One plausible explanation
for this is that the magnitude of the meridional overturning circulation of the Pacific Ocean has been overestimated by these
observations. We raise doubts about the validity of the previous ocean general circulation models where diapycnal diffusivity
is assigned ad hoc to attain the current magnitude suggested from current meter moorings and hydrographic surveys. 相似文献
560.
An embedded cylinder is a large-diameter cylinder embedded in a soil foundation. The state of failure of such an embedded cylindrical structure shows large deflection instead of slide and overturn of the traditional gravity type of structure placed on a rubble base or foundation base. The critical value of deflection of the embedded cylindrical structure, which is the maximum allowable deflection for stability calculation of the cylinder, is a vital control value. Through investigation on deflection and soil pressures on an embedded cylinder by model experiments, the variations of the angle of rotation θ of a cylinder with effective anti-overturning ratio η and moment MH of thrust are discussed. On the basis of experimental study, the critical value of deflection of the cylindrical structure is proposed in the paper. Meanwhile, the formulas for calculating deflection of cylinders are derived. 相似文献