全文获取类型
收费全文 | 73109篇 |
免费 | 707篇 |
国内免费 | 565篇 |
专业分类
测绘学 | 1962篇 |
大气科学 | 4473篇 |
地球物理 | 13418篇 |
地质学 | 29043篇 |
海洋学 | 6170篇 |
天文学 | 16257篇 |
综合类 | 274篇 |
自然地理 | 2784篇 |
出版年
2022年 | 536篇 |
2021年 | 833篇 |
2020年 | 897篇 |
2019年 | 1020篇 |
2018年 | 4160篇 |
2017年 | 3725篇 |
2016年 | 3259篇 |
2015年 | 1072篇 |
2014年 | 1952篇 |
2013年 | 3109篇 |
2012年 | 2889篇 |
2011年 | 4560篇 |
2010年 | 4071篇 |
2009年 | 4707篇 |
2008年 | 3966篇 |
2007年 | 4569篇 |
2006年 | 2679篇 |
2005年 | 1853篇 |
2004年 | 1799篇 |
2003年 | 1770篇 |
2002年 | 1585篇 |
2001年 | 1319篇 |
2000年 | 1235篇 |
1999年 | 930篇 |
1998年 | 925篇 |
1997年 | 944篇 |
1996年 | 704篇 |
1995年 | 722篇 |
1994年 | 712篇 |
1993年 | 577篇 |
1992年 | 572篇 |
1991年 | 526篇 |
1990年 | 616篇 |
1989年 | 518篇 |
1988年 | 490篇 |
1987年 | 514篇 |
1986年 | 413篇 |
1985年 | 596篇 |
1984年 | 619篇 |
1983年 | 598篇 |
1982年 | 564篇 |
1981年 | 500篇 |
1980年 | 523篇 |
1979年 | 406篇 |
1978年 | 433篇 |
1977年 | 398篇 |
1976年 | 347篇 |
1975年 | 358篇 |
1974年 | 338篇 |
1973年 | 359篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
851.
Joana F.M.F. Cardoso Denis Langlet Jos F. Loff Ana R. Martins Johannes IJ. Witte Paulo T. Santos Henk W. van der Veer 《Journal of Sea Research》2007,57(4):303-315
The Pacific oyster Crassostrea gigas was introduced in Europe for commercial purposes in the mid 1960s. It was initially thought that low winter temperatures would restrain this species' reproduction and settlement; however, its present distribution in areas where no introduction has taken place suggests that natural invasion and expansion has occurred. Along the European coast, wild populations of Pacific oysters are already found from northern Germany to southern Portugal. Whether C. gigas will continue to further expand through northern waters will depend on its physiological performance. In this study, the performance of wild oyster populations has been studied in terms of growth and reproduction at three stations: La Rochelle (France; 46°N), Yerseke (Oosterschelde estuary, The Netherlands, 51°N), and Texel (Wadden Sea estuary, The Netherlands, 53°N). The French population had the lowest somatic-shell mass ratio and an increase in maximum shell length, somatic and gonadal mass was observed from France to the Netherlands. In addition, mean oocyte diameter decreased significantly from south to north. The combination of increasing gonadal mass and decreasing oocyte volume suggests an increasing reproductive output in terms of egg numbers from France to The Netherlands. Differences in temperature between locations will at least be partly responsible for the observed patterns; however, other environmental factors (such as food availability, predation pressure, sediment type and/or seston concentration) cannot be excluded. Since smaller eggs (oocytes) are thought to have a longer development time, the environmental conditions along the Dutch coast may result in increased larval dispersal and possibly in further population expansion. 相似文献
852.
In the highly productive region off central Chile, the structure and temporal and spatial variability of planktonic assemblages, and the factors that determine changes in this structure are poorly understood. In the region, wind-driven upwelling, heating by solar radiation and freshwater inputs are highly seasonal processes, which, together with higher frequency events, can promote changes in the planktonic communities, especially in the upper layer. This study focuses on the structure of nano- through to micro-planktonic assemblages (2-200 μm) of unicellular organisms (protists) in surface waters (0-30 m) during different hydrographic conditions. Samples were taken from a fixed shelf station off Concepción (COPAS time series Station 18) on eight occasions between September 2003 and August 2004. The nano-plankton flagellate-dominated fraction was numerically important during the whole period. Maxima in flagellate abundance and biomass occurred during the upwelling period (November-April samplings) but these maxima appear to be unrelated to the degree of water column stratification. The micro-plankton diatom-dominated fraction was usually the largest component in terms of biomass during the study period and the diatoms made important numerical contributions during the upwelling period, with maxima in abundance and biomass when water column stability was lowest. The dominant genera and morphotypes in each functional group were found throughout the study period, with maxima in abundance and biomass co-occurring under similar environmental upwelling conditions. The mean macro-nutrient concentrations (nitrate and silicate) were relatively high in the top 30 m during both upwelling and non-upwelling periods, and did not explain the maxima in plankton or functional group replacements. The persistence of the dominant taxa in the planktonic assemblages suggests a high degree of flexibility, though probably not at the specific level, to withstand the highly variable environmental conditions in this upwelling area. 相似文献
853.
Generalization of the approximation for the autocovariance function of estimate error, used frequently in four-dimensional analysis of the ocean's hydrophysical fields, is suggested for cross-covariance functions. It is shown that the principal properties of the covariance function of error estimate are not then violated. Approximation accuracy is studied through numerical experimentation using the two-dimensional equation for passive admixture transport as an example.Translated by V. Puchkin. 相似文献
854.
An instrumented tetrapod was deployed for three weeks on the Dry Tortugas Bank at a depth of 26 m in February 1995. Bottom
roughness was dominated by shrimp burrows and worm mounds with rms roughness amplitudes ranging from 0.47 to 1.75 cm. Logarithmic
velocity profiles show apparent total roughness heights ranging from 0.30 to 1.45 cm, values consistent with observed biological
roughness. The bed sediments were weakly bound by an algal crust at the sediment–water interface. When this bound layer was
scraped away by a mooring that was accidentally dragged, sharp-crested wave-induced ripples appeared within the resulting
swath. We conclude that physically induced roughness is biologically suppressed, but if dominant, would be significantly higher
than the prevailing biological roughness. 相似文献
855.
This paper focuses on the numerical modelling of tsunami propagation in the open Black Sea. Two types of numerical models
are discussed: a model for the radial propagation of long waves and an evolutionary finite-difference prognostic model. Experimentally
derived numerical data on the model source of tsunamis are reported.
Translated by Vladimir A. Puchkin. 相似文献
856.
V. A. Kalmykov 《Physical Oceanography》1996,7(2):91-97
Zakharov's equation is applied to study numerically the evolution of wave amplitudes. It is demonstrated that, owing to Krasitskii's
new kernel functions, the system of surface waves remains Hamiltonian, in contrast to the former approaches used by Yuen,
Lake, and Zakharov. If surface waves are wind-generated, they grow and become stochastic. If the effect of the waves on the
wind is considered, then the wave amplitude evolution, similar to the no-wind case, continues to be recurrent.
Translated by Vladimir A. Puchkin. 相似文献
857.
858.
Whether or not there are extensional detachment faults in the Alboran basement can be tested directly because a part of the Alboran Basin is now emerged. These detachments, related to crustal thinning beneath the Alboran Basin, occurred from the Aquitanian to Tortonian. The resulting extensional geometries can be described in general terms. During the Serravalian a considerable southwest extension of the basin took place, accompanied by south-southeast extension in the northern Gibraltar Arc. Other detachments affected by Serravalian extension can be found. The spreading of the Alboran was nearly coeval with roughly westward migration of the Gibraltar mountain front. 相似文献
859.
Wave records at seven different locations within a groin field have been analysed by both statistical and spectral approaches to study the general wave climate. The wave heights and periods from the wave records were obtained by both upcross and downcross methods for the statistical approach. The variation of different wave height and wave period parameters with respect to the time of measurement at a particular location and its variation at different locations within the groin field at a particular instant of time are presented and discussed in detail in this paper. It is generally found that the wave heights follow the Weibull distribution. 相似文献
860.