全文获取类型
收费全文 | 86180篇 |
免费 | 1667篇 |
国内免费 | 641篇 |
专业分类
测绘学 | 2456篇 |
大气科学 | 6660篇 |
地球物理 | 17414篇 |
地质学 | 29335篇 |
海洋学 | 7242篇 |
天文学 | 20012篇 |
综合类 | 214篇 |
自然地理 | 5155篇 |
出版年
2021年 | 671篇 |
2020年 | 759篇 |
2019年 | 819篇 |
2018年 | 1896篇 |
2017年 | 1807篇 |
2016年 | 2319篇 |
2015年 | 1470篇 |
2014年 | 2301篇 |
2013年 | 4448篇 |
2012年 | 2408篇 |
2011年 | 3101篇 |
2010年 | 2877篇 |
2009年 | 3770篇 |
2008年 | 3327篇 |
2007年 | 3301篇 |
2006年 | 3107篇 |
2005年 | 2598篇 |
2004年 | 2575篇 |
2003年 | 2398篇 |
2002年 | 2362篇 |
2001年 | 2102篇 |
2000年 | 1970篇 |
1999年 | 1787篇 |
1998年 | 1719篇 |
1997年 | 1738篇 |
1996年 | 1423篇 |
1995年 | 1391篇 |
1994年 | 1312篇 |
1993年 | 1180篇 |
1992年 | 1087篇 |
1991年 | 1073篇 |
1990年 | 1160篇 |
1989年 | 1069篇 |
1988年 | 981篇 |
1987年 | 1154篇 |
1986年 | 980篇 |
1985年 | 1248篇 |
1984年 | 1480篇 |
1983年 | 1358篇 |
1982年 | 1309篇 |
1981年 | 1227篇 |
1980年 | 1066篇 |
1979年 | 1043篇 |
1978年 | 1046篇 |
1977年 | 939篇 |
1976年 | 869篇 |
1975年 | 821篇 |
1974年 | 861篇 |
1973年 | 895篇 |
1972年 | 563篇 |
排序方式: 共有10000条查询结果,搜索用时 562 毫秒
971.
Harpacticoida taxocen diversity in the silty-sandy littoral zone of Kandalaksha Bay of the White Sea
E. S. Chertoprud M. V. Chertoprud D. V. Kondar P. N. Kornev A. A. Udalov 《Oceanology》2006,46(4):492-500
The fauna composition, ecological groups, and life forms of Harpacticoids of soft littoral grounds from 15 sites off the Karelian coast in Kandalaksha Bay of the White Sea were studied. Six types of harpacticoid taxocens with different sets of dominating species are described. On silty sands with clear signs of desalination, the Stenchelia palustris taxocen is common; medium-grained sands are characterized by the Heterolaophonte minuta, Paraleptastacus kliei — Huntemannia jadensis, and Amphiascoides nanus — Scottopsyllus minor taxocens; while on coarse sands, the Ameira parvula and Ameira scotti — Mesochra lilljeborgi taxocens have developed. The diversity of the taxocens is partly explained by the differences in the grain-size composition and salinity at different sites. In addition, groups of various taxocens dwelling on the same type of the sediments were found in different minor bights of the coastline. 相似文献
972.
This paper investigates mercury contamination in recent marine sediment of the Tyrrhenian continental shelf near the mouths of three rivers draining an area with cinnabar deposits (M. Amiata, Italy). By means of chemical leaching, two fractions of the total mercury were distinguished: a ‘non-leachable’ fraction, consisting of mercury held in relatively stable forms and a ‘leachable’ fraction, composed of forms that are more weakly bound to the sediments. The two Hg fractions are correlated to the organic matter content, the distribution of which is strictly dependent on the sediment surface area.In order to eliminate the effects of grain size variations among the samples, the Hg concentrations were normalized to the unit of surface area (ng/m2). Following this procedure, the ‘non-leachable’ Hg was found to be concentrated mostly in a belt along the coast, while the ‘leachable’ Hg fraction was shown to accumulate largely in the zones with high depositional dynamics near the mouths of the rivers.Hypotheses to account for the discrepancy between the low concentrations of ‘leachable’ Hg in the sediment of the outer continental shelf and the high concentrations in the fauna of the same area, reported in previous papers, are presented. 相似文献
973.
Santos MM ten Hallers-Tjabbes CC Vieira N Boon JP Porte C 《Marine environmental research》2002,54(3-5):661-665
Normal and imposex-affected female Buccinum undatum were sampled from the open North Sea at three locations, one with low, and two with high shipping densities. Cytochrome P450 components and P450 aromatase activity were determined in the microsomal fractions isolated from pooled digestive gland/gonads. Cytochrome P450 aromatase activity was significantly higher (P < 0.05) in normal females collected in the low shipping density area (1,325 +/- 295 fmol/h/mg protein) than levels from imposex animals from a high shipping density area (620 +/- 287 fmol/h/mg protein). A negative correlation was found between aromatase activity and organotin body burden (r = -0.99). Levels of CYP450, cytochrome b5 and NADPH cytochrome c reductase activity did not show differences among groups. This is the first field evidence of depressed aromatase activity in imposex affected females, although additional research under laboratory controlled conditions is required to fully understand the mechanisms underlying the development of imposex in this species. 相似文献
974.
Community-based coastal resource management projects facilitated by non-governmental organizations (NGOs) in the Philippines have explored approaches to decrease poverty-driven over-exploitation of coastal resources. There has been little reporting and analysis of such activities until recently. Nine case studies have recently been published and this paper presents a brief summary of these, along with an analysis of the trends and themes identified. Though diverse and covering the Philippines from Mindanao to Luzon, the case studies highlight the value of community commitment and participation in decisions regarding, and in the implementation of, resource management in ways that consider not only the bio-physical aspects of resource management but the social, economic and legal implications. Experience in working with local government has been both successful and limited. Successful projects raise hopes for further progress through education and organization of communities to improve livelihoods and protect coastal resources. 相似文献
975.
This paper deals with the solution of the problem of the dynamic response of the leg of a tension leg platform subjected to an axial, suddenly applied load at one end, considering a highly simplified model but taking into account several complicating factors. The problem is solved by means of a non-harmonic Fourier expansion in terms of eigenfunctions obtained from a non-regular Sturm-Liouville system. 相似文献
976.
C. L. Smith K. J. Richards M. J. R. Fasham 《Deep Sea Research Part I: Oceanographic Research Papers》1996,43(11-12)
A coupled QuasiGeostrophic mixed-layer ECOsystem model (QGECO) is used to investigate the impact of the underlying mesoscale eddy field on the spatial and temporal scales of biological production and on overall rates of primary productivity. The model exhibits temporal trends in the biological and physical fields similar to those observed in the North Atlantic; i.e. the mixed layer shallows in spring causing a rapid increase in phytoplankton concentrations and a corresponding decline in nutrient levels. Heterogeneity is produced in the mixed layer through Ekman pumping velocities resulting from the interaction of windstress and surface currents. This variability impacts on biological production in two ways. Firstly, spatial variations in the depth of the mixed layer affect the photosynthetically active radiation (PAR) availability and hence production rates, and secondly, eddy enhanced exchange between the surface water and those at depth bring additional nutrients into the euphotic zone. These processes result in significant spatial and temporal heterogeneity in the ecosystem distributions.Investigation of the spatial heterogeneity of the biological system finds variability to be significantly greater than that of the mixed layer. The relationship between the eddy field and the ecosystem is investigated. The structure and correlation of the biogeochernical fields change with time. The biological fields are found to have a shorter horizontal scale, but whiter spectrum than the underlying eddy field.Overwinter conditions are found to have a profound effect on the variability, size and timing of the following spring bloom event. Variations in the nitrate levels are primarily responsible for the variability in the biological system in the first year. In subsequent years the variation in the overwintering population is found to be dominant. 相似文献
977.
Quality control in mud coring 总被引:1,自引:0,他引:1
W. R. Parker 《Geo-Marine Letters》1991,11(3-4):132-137
Consideration of factors influencing the interaction between a corer and sediment suggest the likelihood of significant and
anisotropic straining of the particle framework during coring. Direct observations allow the coring process to be discretised,
the relative motion of corer, core and sediment to be visualized and confirm that the development of differences between corer
penetration and core length is not a continuous process. Records suggest the possibility of dislocation of the relative positions
of porewater and particle framework, which may lead to the disequilibration of sorbed species. 相似文献
978.
An instrument which measures the velocity and direction of benthic water currents at depths of up to 80 m is described. Characteristics of this meter include: (i) readings are not affected by water flow during sinking or retrieval; (ii) it is portable; (iii) it operates unattended; (iv) it is inexpensive; and (v) it requires only simple maintenance. A timing circuit which operates two time periods in succession is used to activate and de-activate a revolution counter and a fluidfilled compass which are mounted on a special frame and vane system. In the field, the instrument accurately measured average current velocities of up to 6 m s−1. Data are presented which illustrate the usefulness of this instrument in describing the velocity and direction of benthic currents along the east coast of Australia and demonstrate a positive correlation between benthic current velocity and catch rates of the commercially-exploited spanner crab Ranina ranina. 相似文献
979.
Distributions of physical, chemical and biological characteristics were recorded for coastal waters off south-west Ireland during the summers of 1985–1987. A number of thermal fronts were identified from both shipboard measurements and satellite imagery. Upwelling was shown to be an important process in the area and enhanced phytoplankton biomass and productivity were associated with areas of upwelling and the fronts. Phytoplankton species composition, although varying between cruises, was associated with differences in hydrographic features. 相似文献
980.