全文获取类型
收费全文 | 46517篇 |
免费 | 668篇 |
国内免费 | 434篇 |
专业分类
测绘学 | 924篇 |
大气科学 | 2742篇 |
地球物理 | 8644篇 |
地质学 | 17762篇 |
海洋学 | 4586篇 |
天文学 | 10959篇 |
综合类 | 107篇 |
自然地理 | 1895篇 |
出版年
2022年 | 464篇 |
2021年 | 722篇 |
2020年 | 750篇 |
2019年 | 844篇 |
2018年 | 1724篇 |
2017年 | 1597篇 |
2016年 | 1742篇 |
2015年 | 809篇 |
2014年 | 1584篇 |
2013年 | 2637篇 |
2012年 | 1796篇 |
2011年 | 2196篇 |
2010年 | 2031篇 |
2009年 | 2437篇 |
2008年 | 2078篇 |
2007年 | 2270篇 |
2006年 | 2052篇 |
2005年 | 1210篇 |
2004年 | 1139篇 |
2003年 | 1104篇 |
2002年 | 1126篇 |
2001年 | 1008篇 |
2000年 | 899篇 |
1999年 | 738篇 |
1998年 | 743篇 |
1997年 | 727篇 |
1996年 | 609篇 |
1995年 | 585篇 |
1994年 | 580篇 |
1993年 | 452篇 |
1992年 | 462篇 |
1991年 | 437篇 |
1990年 | 483篇 |
1989年 | 382篇 |
1988年 | 364篇 |
1987年 | 428篇 |
1986年 | 367篇 |
1985年 | 488篇 |
1984年 | 452篇 |
1983年 | 429篇 |
1982年 | 443篇 |
1981年 | 358篇 |
1980年 | 387篇 |
1979年 | 358篇 |
1978年 | 360篇 |
1977年 | 295篇 |
1976年 | 285篇 |
1975年 | 295篇 |
1974年 | 283篇 |
1973年 | 314篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
601.
Reproductive variability in North Sea plaice, sole, and cod 总被引:4,自引:1,他引:4
Rijnsdorp A. D.; Daan N.; van Beek F. A.; Heessen H. J. L. 《ICES Journal of Marine Science》1991,47(3):352-375
602.
Sublethal effects of cadmium on reproduction can have detrimental consequences for the long-term survival, even of species with high LC50 values. The sea mussel, Mytilus edulis, is well-known for its use in biomonitoring work due to its relatively low sensitivity towards many toxicants, including cadmium.1 Because little is known on sublethal effects of this metal in invertebrates,2–6 we have studied these effects, with emphasis on reproduction, by exposing mussels to 100 ppb cadmium in sea water. This concentration significantly inhibits follicle development in both male and female gonads, but has subsequently a stimulating effect on the spawning frequency. Due to these opposite effects the consequences of cadmium stress for gamete production are expected to be small, as the average number of gametes released by spawning animals was not significantly changed. 相似文献
603.
Textural isotopic and microfossil data from two gravity cores obtained in Saguenay Fjord, Quebec, suggest that a distinctive sandy clay bed was deposited as the result of a major landslide in the Saguenay River basin. Pb-210 dating of the cores indicate that the bed is of similar age to the magnitude 7 earthquake of February 5, 1663. The slide involved sensitive marine clays and may have occurred in two stages. Slide sediments carried into the Saguenay River channel were probably reworked and subsequently transported down the Fjord basin as two distinct cohesionless mass flows. Fine clay laminae that overlie the older mass flow bed record the modulation of depositional processes by tidal currents for several weeks after this event. 相似文献
604.
Abstract. The marine plant communities of the littoral zone in different biotopes of the Greek coasts were investigated in 1980-81. Seasonal distribution and variation of marine plant biomass were assessed. The communities of Cystoseira crinita and C. compressa were outstanding with maximum biomass during the summer months. Corallina officinalis and Pterocladia capillacea + Viva rigida communities predominated with maximum biomass in autumn and exhibited a decrease in winter, except in stressed biotopes. The species diversity and productivity of seaweeds along the Greek coasts are interpreted in relation to a number of environmental parameters. 相似文献
605.
I. N. Ivanova O. N. Mel’nikova T. A. Nivina K. V. Pokazeev 《Izvestiya Atmospheric and Oceanic Physics》2006,42(5):646-652
The air-flow velocity field near the water surface is studied in the zone of wind-wave intensification. Caused by a periodic separation of eddies, a nonzero time-averaged value of the wind velocity in the near-water streamline is detected at the leading slope of the wave. The distribution of pressure along the wave with allowance for the vertical velocity shear and disturbances produced by eddies and a periodic deceleration of the viscous layer was calculated with the aid of the Cauchy-Lagrange integral. This procedure made it possible to calculate the growth rate of the wave amplitude, whose value was found to be close to its experimental value at the initial stage of acceleration. 相似文献
606.
The dependence of the variation in the depth of the upper mixed layer (MLD) on the governing parameters (the momentum flux, the buoyancy fluxes at the ocean surface, and the density gradient in the pycnocline) is considered. It is shown that, in the spring storm season, wind mixing dominates over convective mixing. In this case, the MLD is linearly correlated with the Ekman scale calculated from the friction velocity observed approximately 12 h before the measurement of the MLD. 相似文献
607.
A. N. Dubovik 《Izvestiya Atmospheric and Oceanic Physics》2006,42(3):371-374
Properties of surface singularities and the form of wave crests of limiting gravity waves in steady-state flows of an ideal liquid are considered by analyzing the kinematic boundary condition. It is shown that, for rotational waves, the angle at the crest can have any value from 0° to 180°, while it has the only value 90° in the case of irrotational waves. Two inferences are made from Bernoulli’s integral and the properties of singularities: (i) the Stokes wave is a rotational wave and (ii) no angular points can appear on the profiles of capillary-gravity and capillary waves. 相似文献
608.
Adaptive optimal control of an autonomous underwater vehicle in the dive plane using dorsal fins 总被引:1,自引:0,他引:1
In this paper, adaptive control of low speed bio-robotic autonomous underwater vehicles (BAUVs) in the dive plane using dorsal fins is considered. It is assumed that the model parameters are completely unknown and only the depth of the vehicle is measured for feedback. Two dorsal fins are mounted in the horizontal plane on either side of the BAUV. The normal force produced by the fins, when cambered, is used for the maneuvering. The BAUV model considered here is non-minimum phase. An indirect adaptive control system is designed for the depth control using the dorsal fins. The control system consists of a gradient based identifier for online parameter estimation, an observer for state estimation, and an optimal controller. Simulation results are presented which show that the adaptive control system accomplishes precise depth control of the BAUV using dorsal fins in spite of large uncertainties in the system parameters. 相似文献
609.
I. Brandariz P. Castro M. Montes F. Penedo M.E. Sastre de Vicente 《Marine Chemistry》2006,102(3-4):291-299
Acid–base equilibrium constants of triethanolamine (TEA) have been determined by potentiometric titrations with a glass electrode, at 25 °C. Ionic strength was kept constant with only one electrolyte (using one of these salts: NaCl, KCl, MgCl2 or CaCl2), with binary mixtures of MgCl2 and CaCl2, and finally, in a solution with a composition approximately similar to that of natural seawater without sulfate. Equilibrium constants have been expressed in function of ionic strength by means of Pitzer equations and interaction parameters proposed in this theory have been obtained. It has been found that acid–base behaviour of TEA depends greatly on the salt used: basicity of TEA is decreased by CaCl2, while it is increased by the other electrolytes used in this work. 相似文献
610.
Multi-proxy evidence of long-term changes in ecosystem structure in a Danish marine estuary, linked to increased nutrient loading 总被引:3,自引:0,他引:3
Marianne Ellegaard Annemarie L. Clarke Nina Reuss Simon Drew Kaarina Weckstrm Stephen Juggins N. John Anderson Daniel J. Conley 《Estuarine, Coastal and Shelf Science》2006,68(3-4):567
This paper presents a study of changes in eutrophication over the past 100 years in a fertile estuary. The Danish estuary Mariager Fjord is a long, narrow sill-fjord with a permanently anoxic basin. In 1997 anoxia spread from the basin to the entire inner estuary, killing almost all eukaryotes and prompting debate on the causes. This paper reports a multi-proxy survey of 210Pb-dated sediment cores from the anoxic basin. Analyses of diatoms, dinoflagellates, pigments and geochemical proxies were used to determine changes in ecosystem structure over the past 100 years. The aim was to establish ‘base-line conditions’, for management purposes, of the biological structure prior to 1900, and to examine possible causes of changes observed. Geochemical proxies total nitrogen (TN), total carbon (TC) and biogenic silica (BSi) were consistently high throughout the sediment record. Increased concentrations of pigments and natural isotopes (δ13C, δ15N) suggested increasing production and nutrient loading. The main changes in the biological proxies occurred between 1915 and the 1940s, and indicated that the estuary has been somewhat eutrophic since 1900, but that the eutrophication process increased over the past 100 years. A reconstruction of TN concentration by a diatom-based transfer function supports this interpretation, with inferred TN ca. 1900 around 60 μmol l−1, and an increase in TN concentration over the past century to ca. 130 μmol l−1 by 1995. Inferred TN decreased to ca. 100 μmol l−1 by 2001, similar to present day monitoring data. 相似文献