全文获取类型
收费全文 | 1448篇 |
免费 | 57篇 |
国内免费 | 19篇 |
专业分类
测绘学 | 28篇 |
大气科学 | 115篇 |
地球物理 | 289篇 |
地质学 | 559篇 |
海洋学 | 157篇 |
天文学 | 190篇 |
综合类 | 5篇 |
自然地理 | 181篇 |
出版年
2022年 | 8篇 |
2021年 | 15篇 |
2020年 | 20篇 |
2019年 | 15篇 |
2018年 | 42篇 |
2017年 | 34篇 |
2016年 | 57篇 |
2015年 | 34篇 |
2014年 | 48篇 |
2013年 | 84篇 |
2012年 | 47篇 |
2011年 | 84篇 |
2010年 | 66篇 |
2009年 | 65篇 |
2008年 | 84篇 |
2007年 | 66篇 |
2006年 | 47篇 |
2005年 | 50篇 |
2004年 | 48篇 |
2003年 | 63篇 |
2002年 | 48篇 |
2001年 | 43篇 |
2000年 | 25篇 |
1999年 | 24篇 |
1998年 | 31篇 |
1997年 | 24篇 |
1996年 | 24篇 |
1995年 | 18篇 |
1994年 | 18篇 |
1993年 | 18篇 |
1992年 | 22篇 |
1991年 | 18篇 |
1990年 | 18篇 |
1989年 | 17篇 |
1988年 | 13篇 |
1987年 | 14篇 |
1986年 | 7篇 |
1985年 | 9篇 |
1984年 | 21篇 |
1983年 | 13篇 |
1982年 | 15篇 |
1981年 | 15篇 |
1980年 | 12篇 |
1979年 | 10篇 |
1978年 | 11篇 |
1977年 | 7篇 |
1976年 | 8篇 |
1975年 | 8篇 |
1974年 | 8篇 |
1973年 | 9篇 |
排序方式: 共有1524条查询结果,搜索用时 15 毫秒
11.
An increasingly utilized strategy for expanding conservation in the developing world has been the promotion of protected areas
that supersede national borders. Alternatively known as transfrontier biosphere reserves, transfrontier or transboundary conservation
areas, or Peace Parks, these protected areas are aggressively advanced by conservation agencies for their purported ecological
and economic benefits. This article provides a comparative assessment of two case studies to understand the various impacts
of transboundary conservation. The Great Limpopo Transfrontier Park, which unites protected areas in South Africa, Mozambique
and Zimbabwe, is contrasted with efforts to protect jaguars along the United States–Mexico border. We argue that while these
cases are promising for the purposes of biodiversity protection, they demonstrate that transboundary conservation can minimize
political context, contributes to the hegemony of international conservation agendas, and remains closely linked to economic
neoliberalism and decentralization in the developing world.
相似文献
Brian KingEmail: |
12.
Laurence P. Madin Stephen M. Bollens Erich Horgan Mari Butler Jeffrey Runge Barbara K. Sullivan Grace Klein-Macphee Edward Durbin Ann G. Durbin Donna Van Keuren Stphane Plourde Ann Bucklin M. Elizabeth Clarke 《Deep Sea Research Part II: Topical Studies in Oceanography》1996,43(7-8)
Hydroids are typically attached, benthic cnidarians that feed on a variety of small prey. During sampling on Georges Bank in spring 1994, we found huge numbers of hydroids suspended in the plankton. They fed on young stages of copepods that are an important prey for fish, as well as on young fish themselves. Two independent methods were used to estimate feeding rates of the hydroids; both indicate that the hydroids are capable of consuming from 50% to over 100% of the daily production of young copepods. These results suggest that hydroids can have a profound effect on the population dynamics of zooplankton and young fish on Georges Bank. 相似文献
13.
An inherent tension exists between legal approaches to nature conservation and fisheries management in the European Union, as the former remains the remit of Member States while the latter is under the exclusive legislative jurisdiction of the European Community (EC). This tension is of particular importance when addressing the conservation of habitats or species that are under threat from fishing activities. This article examines recent developments in offshore marine conservation in the North-East Atlantic in light of the legislative developments and political frameworks that are currently evolving. By analysing the emergency closure of the Darwin Mounds area of cold-water corals and the UK pair-trawl ban, it becomes evident that the precautionary principle is a key factor in the tension between fisheries management and marine nature conservation, and is not always taken into account. 相似文献
14.
Christopher T. Mills Robert F. Dias Dennis Graham Kevin W. Mandernack 《Marine Chemistry》2006,98(2-4):197-209
Sediment samples ranging from 0.05 to 278 m below sea floor (mbsf) at a Northwest Pacific deep-water (5564 mbsl) site (ODP Leg 191, Site 1179) were analyzed for phospholipid fatty acids (PLFAs). Total PLFA concentrations decreased by a factor of three over the first meter of sediment and then decreased at a slower rate to approximately 30 mbsf. The sharp decrease over the first meter corresponds to the depth of nitrate and Mn(IV) reduction as indicated by pore water chemistry. PLFA-based cell numbers at site 1179 had a similar depth profile as that for Acridine orange direct cell counts previously made on ODP site 1149 sediments which have a similar water depth and lithology. The mole percentage of straight chain saturated PLFAs increases with depth, with a large shift between the 0.95 and 3.95 mbsf samples. PLFA stable carbon isotope ratios were determined for sediments from 0.05 to 4.53 mbsf and showed a general trend toward more depleted δ13C values with depth. Both of these observations may indicate a shift in the bacterial community with depth across the different redox zones inferred from pore water chemistry data. The PLFA 10me16:0, which has been attributed to the bacterial genera Desulfobacter in many marine sediments, showed the greatest isotopic depletion, decreasing from − 20 to − 35‰ over the first meter of sediment. Pore water chemistry suggested that sulfate reduction was absent or minimal over this same sediment interval. However, 10me16:0 has been shown to be produced by recently discovered anaerobic ammonium oxidizing (anammox) bacteria which are known chemoautotrophs. The increasing depletion in δ13C of 10me16:0 with the unusually lower concentration of ammonium and linear decrease of nitrate concentration is consistent with a scenario of anammox bacteria mediating the oxidation of ammonium via nitrite, an intermediate of nitrate reduction. 相似文献
15.
The wadic project: A comprehensive field evaluation of directional wave instrumentation 总被引:1,自引:0,他引:1
J. Allender T. Audunson S. F. Barstow S. Bjerken H. E. Krogstad P. Steinbakke L. Vartdal L. E. Borgman C. Graham 《Ocean Engineering》1989,16(5-6)
The results of a comprehensive field trial of nearly all commercially available directional wave measurement systems at the Edda field in the North Sea during winter 1985-86 are presented. The results summarize the accuracy of the principal engineering wave parameters from each system and the dependence on sea state. Limiting factors on system performance and operational problems are also included in the assessment. Overall experience has been good with systems utilizing widely different measurement principles returning consistent results. 相似文献
16.
C. Graham 《Coastal Engineering》1982,6(4)
This study sets out to define the basic forms in which wind speed and wave height persistence statistics may be defined for offshore engineering applications, and describes the development of a mathematical persistence model.The model incorporates some of the principles laid down by other workers, but it is fundamentally based on a new concept for parameterising persistence statistics, linking wind speed and sea state percentage probability of exceedance with the persistence average duration.North Sea measured wind and wave data have been used to calibrate and test the model. During the course of these test runs it proved necessary to fine tune the basic Weibull equation of the model, but following these adjustments the model runs were found to correlate well with the measured data.It is concluded that the model may be used to predict wind speed and wave height persistence statistics with acceptable accuracy for preliminary stage oil industry planning purposes and that the calibrated model has particular application for those areas where little measured data are currently available. 相似文献
17.
ABSTRACT Understanding temporal patterns in restored environments is important for identifying potential barriers to recovery and improved management of degraded habitats. In this paper, we use temporal beta diversity analyses to compare invertebrate community recovery trajectories in three restored agricultural stream sites under different integrated catchment plans, a native forest reference site, and two unmodified pasture control stream sites over 24 years. The restored sites diverged from their initial community composition over time and became more similar to the reference site community, which was relatively stable over time. Variation partitioning showed that prior to restoration beta diversity was primarily associated with environmental and spatial drivers, whereas post-restoration beta diversity was more influenced by temporal and environmental drivers, including changes in substrate size, fine sediments, water clarity, and nutrients, as well as temperature and flow regime. Species’ contributions to beta diversity varied between sites and years, with sensitive EPT taxa contributing more in reference and control sites. However, contributions of some EPT species, particularly mayflies, increased in restored sites post-ICM. In summary, after nearly two decades of ICM, restored stream sites show recovery towards reference conditions, yet differences persist, indicating that rehabilitation may take longer, depending on the restoration goals. 相似文献
18.
19.
Effect of native and invasive cordgrass on Macoma petalum density, growth, and isotopic signatures 总被引:1,自引:0,他引:1
Ecosystem engineers can influence community and ecosystem dynamics by controlling resources, modifying the flow of energy or biomass, or changing physical characteristics of the habitat. Invasive hybrid cordgrass (Spartina alterniflora × Spartina foliosa) is an ecosystem engineer in salt marshes in San Francisco Bay, California, U.S.A. that raises intertidal elevations and may be either increasing C4 plant carbon input into food webs or tying up carbon in a form that is not usable by consumers. A manipulative experiment compared abundance, growth, and stable isotope (δ13C and δ15N) composition of the clam Macoma petalum (=M. balthica) among native marsh, hybrid Spartina, and mudflats in central San Francisco Bay. We found higher densities (individuals m−2) of M. petalum on mudflats compared to either native or hybrid Spartina (p < 0.001). Macoma petalum shell growth was significantly greater in mudflats than in either vegetation type in 2002 (p = 0.005) but not 2003. Differences in shell growth between native and hybrid Spartina were not significant. Stable isotope results showed differences between habitats in δ13C but not δ15N. Carbon signatures of M. petalum placed in Spartina were much more depleted than the isotopic signature of Spartina. Neither native nor hybrid Spartina appears to be a significant carbon source for M. petalum in San Francisco Bay, and we found no evidence that hybrid Spartina contributes carbon to M. petalum beyond what is provided by S. foliosa, despite the hybrid's much greater biomass. Our results show that loss of mudflat habitat, rather than increased input of C4 carbon, is the greatest effect of the invasion of hybrid Spartina on M. petalum. 相似文献
20.
Population structure of Haliotis rubra from South Australia inferred from nuclear and mtDNA analyses 总被引:1,自引:0,他引:1
1Introduction ThemajorityofAustralia’sabalonefisheryex ports(5.135kt,worth$216millionin2002~2003,ABARE2004)consistofblacklipabalone(HaliotisrubraLeach,1814).AssuchH.rubrais consideredasanimportantmarineresourcewithin Australia.Likemanyabalonespecieswor… 相似文献