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   661篇
  免费   62篇
  国内免费   93篇
测绘学   7篇
大气科学   17篇
地球物理   131篇
地质学   154篇
海洋学   362篇
天文学   2篇
综合类   16篇
自然地理   127篇
  2024年   2篇
  2023年   7篇
  2022年   13篇
  2021年   19篇
  2020年   20篇
  2019年   24篇
  2018年   24篇
  2017年   23篇
  2016年   37篇
  2015年   25篇
  2014年   35篇
  2013年   42篇
  2012年   15篇
  2011年   25篇
  2010年   28篇
  2009年   24篇
  2008年   40篇
  2007年   40篇
  2006年   32篇
  2005年   32篇
  2004年   29篇
  2003年   33篇
  2002年   18篇
  2001年   21篇
  2000年   27篇
  1999年   20篇
  1998年   28篇
  1997年   19篇
  1996年   26篇
  1995年   22篇
  1994年   17篇
  1993年   10篇
  1992年   9篇
  1991年   11篇
  1990年   1篇
  1989年   6篇
  1988年   4篇
  1986年   2篇
  1984年   3篇
  1983年   2篇
  1982年   1篇
排序方式: 共有816条查询结果,搜索用时 15 毫秒
21.
王辉  刘小宇  郭建科  孙才志 《地理研究》2016,35(6):1193-1202
具有百余年历史的美国国家公园志愿者服务已成为美国国家公园管理的重要部分,志愿者服务有效实现了美国国家公园的全民管理思路。通过实地考察和访谈记录梳理,就国家公园志愿者服务的相关概念、服务动机、服务决策、服务内容、服务支持和服务机制深入剖析,从庞大有序的服务系统、管理与运行、功能实现三方面分析国家公园志愿者服务机制;提炼出美国国家公园志愿者服务的四个支持:宣传支持、技术支持、学习支持和立法支持。并以美国海峡群岛国家公园为例进行论证。最后提出国家公园志愿者服务和中国国家公园志愿服务尚需研究的诸多内容,任重而道远。  相似文献   
22.
The male of Abergasilus amplexus Hewitt, 1978 is described for the first time. Previous records of A. amplexus are discussed and a new locality, Te Whanga Lagoon, Chatham Islands, is reported.  相似文献   
23.
This work investigated the biology of Paromola cuvieri in the Atlantic Ocean, including reproductive patterns, sex structure, depth distribution and egg development. Females were more abundant than males, mainly at depths greater than 600 m. Males were dominant in size classes larger than 110 mm carapace width (CW), especially over 120 mm, for which only males were observed. The mean length of both sexes decreased with depth. P. cuvieri appears to have continuous reproduction throughout the year. Large females tend to spawn mainly between summer and winter, but smaller females spawn during the summer. Four maturity categories were identified based on the ovary colour, and these categories were histologically validated. Three categories of testes were identified according to their colour and morphology. Sexual maturity was estimated at sizes between 71.6 and 74.0 mm CW for females and at 91.0 mm CW for males. The relative growth showed changes along the ontogeny, at 73.6 mm CW (females) and 91.3–92.2 mm CW (males). Egg development appears to not be completely synchronous, and two colour patterns can be observed simultaneously. Four stages of egg development were found: eggs undivided and fully filled with yolk in orange egg masses; eggs with a free region of yolk visible in orange or red egg masses; embryos with slightly pigmented, crescent-shaped eyes in brown egg masses; and embryos with visible pigmented structures, enlarged eyes, segmented appendages and abdomen in brown egg masses. The mean number of external eggs carried by females in stage I was estimated to be 315,753±19,267. Three species of barnacle were observed on the exoskeleton: Poecilasma aurantia, Poecilasma crassa and Heteralepas microstoma (Cirripedia, Thoracica).  相似文献   
24.
The underwater light regime of a Mediterranean coastal lagoon (Albufera des Grau, Balearic Islands) was studied during four years in order to characterise the spatial and temporal variations in the light attenuation coefficient (K) and to assess the relative contribution of the different water components to total light attenuation.  相似文献   
25.
运用面向对象的软件开发技术,结合珊瑚礁地貌学、计算机图像学和计算机信息管理技术,在Windows平台上研制开发出南沙群岛珊瑚礁地貌图像信息管理系统。该系统集文件管理、数据库管理和图形、图像管理于一体,具有信息检索输出、信息编辑和信息维护等功能,以图形用户介面交互,达到"所见即所得"的效果。  相似文献   
26.
Socio-economic development of small-island fishing communities is greatly dependent on local coastal and marine resources. However, illegal fishing and aggressive practices in insular ecosystems lead to over-exploitation and environmental deterioration. Moreover, a lack of scientific data increases uncertainty and prevents the adequate monitoring of marine resources. This paper focuses on the integration of local fishing communities into decision-making processes with the aim of promoting artisanal fishing on the Island of Tenerife (the Canary Islands), as a way to preserve the marine ecosystem and socio-economic development of traditional cofradias (fishers' organisations). A qualitative methodological framework, based on participatory problem-solution trees and focus groups, was used to identify the main factors impeding the sustainable development of the artisanal fishing sector on the island and to elaborate collective proposals with policy implications. The fishing community involved identified four main issues that are maintaining an unsustainable island fishery: 1) Over-exploitation; 2) Poor self-management of cofradias and commercialisation problems; 3) Fisher individualism and low co-management strategies, and 4) Illegal fishing increase vs. artisanal fishing decline. Results show the required policy enhancements to tackle them and the need to adapt regulations to the local situation.  相似文献   
27.
We examined whether adults of three species of sea urchins species (Diadema antillarum, Arbacia lixula, and Paracentrotus lividus) exhibit a consistent depth-dependent partitioning pattern on rocky reefs of the Canarian Archipelago (eastern Atlantic). Hydrodynamic experiments were carried out to quantify the resistance to flow-induced dislodgement in these three species. We tested the model that different morphology can result in habitat partitioning among these sea urchins. Abundances of D. antillarum increased with depth. In contrast, A. lixula and P. lividus showed the opposite zonation pattern, coexisting in high abundances in the shallowest depths (<5 m), and occurring in low densities in the deepest part of reefs (>7 m). Both A. lixula and P. lividus had greater adhesion-surface to body-height ratios than D. antillarum. Similarly, A. lixula and P. lividus showed a greater ability to resist flow-induced dislodgement compared with D. antillarum. The mean “velocity of dislodgement” was 300% and 50% greater for A. lixula and P. lividus, respectively, relative to D. antillarum, for any particular size. As a result, A. lixula and P. lividus are better fitted to life in high-flow environments than D. antillarum. We conclude that the risk of dislodgement by water motion likely play a relevant role in the vertical distribution patterns of these sea urchins in the eastern Atlantic.  相似文献   
28.
The relationship between dissolved cadmium (Cd) and phosphate (PO4) was examined at three stations in the subtropical area near the Ryukyu Islands in May 1999. Preformed PO4 was obtained using the Redfield ratio in order to separate the surface water and the other layers in this study area. Almost 0 μM (−0.043 μM to 0.094 μM) was estimated in the layers above 300 m and 250 m at Sts. 1 and 3 and at St. 2, respectively. Up to these depths, water was considered to be uniform, and these layers were defined as the surface water in this study area. In the surface water, the slopes of the regression lines of the Cd-PO4 plot were 0.162, 0.156, and 0.226 (nM/μM) at Sts. 1, 2, and 3, respectively, and these values were much closer to the estimated regenerated ratio of Cd to PO4 from the Apparent Oxygen Utilization (AOU)-Cd/PO4 plots, which was 0.197 (nM/μM) in this study area. Below surface layers, the slopes of the Cd-PO4 plot changed to 0.371, 0.352, and 0.362 (nM//μM) at Sts. 1, 2, and 3, respectively. In the relationships between Cd and PO4, clear deviations or kinks were observed at three stations at a PO4 concentration of approximately 0.2 μM in the plot, which was attributable to the discontinuity of surface water and the other layers across the North Pacific subtropical mode water. In studies of the interaction between surface water and biogenic particles concerning the Cd/PO4 ratio, separate analyses of seawater (surface water and the other layers) should be carried out to obtain the individual surface water ratio because the Cd/PO4 ratio in the surface water is expected to differ from that of the underlying water. Furthermore, the biological fractionation of these constituents is based on the surface water ratio. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   
29.
Measurements of surface partial pressure of CO2 and water column alkalinity, pHT, nutrients, oxygen, fluorescence and hydrography were carried out, south of the Canary Islands during September 1998. Cyclonic and anticyclonic eddies were alternatively observed from the northwestern area to the central area of the Canary Islands. Nutrient pumping and vertical uplifting of the deep chlorophyll maximum by cyclonic eddies were also ascertained by upward displacement of dissolved inorganic carbon. A model was applied to determine the net inorganic carbon balance in the cyclonic eddy. The fluxes were determined considering both the diffusive and convective contributions from the upward pumping and the corresponding horizontal transport of water outside the area. An increase in the total inorganic carbon concentration in the upper layers inside the eddy field of 133 mmol C m− 2 d− 1 was determined. The upward flux of inorganic carbon decreased the effect of the increased primary production on the carbon dioxide chemistry. The reduced fCO2 inside the cyclonic eddy, 15 μatm lower than that observed in non-affected surface water, was explained by thermodynamic aspects, biological activity, eddy upward pumping and diffusion and air–sea water exchange effects.  相似文献   
30.
A. Dem  ny  A. Ahijado  R. Casillas  T. W. Vennemann 《Lithos》1998,44(3-4):101-115
Fuerteventura—the second largest of the Canary Islands consists of Mesozoic sediments, submarine volcanic rocks, dike swarms and plutons of the Basal Complex, and younger subaerial basaltic and trachytic series. Carbonatites are found in two Basal Complex exposures: the Betancuria Massif in the central part of the island and the Esquinzo area in the north. values of the carbonatites increase progressively from south to north of the island. This phenomenon is attributed to different degrees of assimilation of sedimentary carbonate. Homogeneous, typically magmatic values for carbonatites which have preserved primary igneous textures and minerals suggest a well-mixed reservoir where changes in values result from the storage of carbonate magmas at different structural levels. The magma storage allowed assimilation of sediment to varying degrees before final emplacement of carbonatites. Shifts in towards more positive and negative values from presumed primary compositions are observed in the carbonatites. On the basis of the oxygen isotope compositions of calcite, mica and K-feldspar, and the hydrogen isotope compositions of micas, the changes in the values of the carbonatites can be related to fluid/rock interactions.  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