全文获取类型
收费全文 | 69324篇 |
免费 | 18957篇 |
国内免费 | 43486篇 |
专业分类
测绘学 | 3958篇 |
大气科学 | 23930篇 |
地球物理 | 19760篇 |
地质学 | 47647篇 |
海洋学 | 18692篇 |
天文学 | 9427篇 |
综合类 | 4459篇 |
自然地理 | 3894篇 |
出版年
2022年 | 475篇 |
2021年 | 884篇 |
2020年 | 2134篇 |
2019年 | 5653篇 |
2018年 | 6402篇 |
2017年 | 6081篇 |
2016年 | 6414篇 |
2015年 | 5340篇 |
2014年 | 5142篇 |
2013年 | 6618篇 |
2012年 | 5546篇 |
2011年 | 6061篇 |
2010年 | 5606篇 |
2009年 | 5513篇 |
2008年 | 4248篇 |
2007年 | 4135篇 |
2006年 | 3916篇 |
2005年 | 3669篇 |
2004年 | 4200篇 |
2003年 | 3736篇 |
2002年 | 3353篇 |
2001年 | 2925篇 |
2000年 | 2485篇 |
1999年 | 2386篇 |
1998年 | 2496篇 |
1997年 | 2534篇 |
1996年 | 2034篇 |
1995年 | 1923篇 |
1994年 | 1793篇 |
1993年 | 1825篇 |
1992年 | 1616篇 |
1991年 | 1263篇 |
1990年 | 1150篇 |
1989年 | 1030篇 |
1988年 | 894篇 |
1987年 | 891篇 |
1986年 | 737篇 |
1985年 | 745篇 |
1984年 | 808篇 |
1983年 | 698篇 |
1982年 | 698篇 |
1981年 | 561篇 |
1980年 | 516篇 |
1979年 | 476篇 |
1978年 | 406篇 |
1977年 | 380篇 |
1976年 | 324篇 |
1975年 | 289篇 |
1974年 | 321篇 |
1973年 | 349篇 |
排序方式: 共有10000条查询结果,搜索用时 406 毫秒
951.
Diel fluctuations in dissolved free amino acids and monosaccharides in Chesapeake Bay dinoflagellate blooms 总被引:1,自引:0,他引:1
The accumulation of phytoplankton biomass in recurring summer dinoflagellate blooms of Chesapeake Bay is accompanied by large pools of dissolved organic matter (DOM). Two fractions of the DOM, free amino acids (DFAA) and monosaccharides (MONO), were measured at 3 h intervals in mixed species dinoflagellate blooms (Katodinium rotundatum, Gymnodinium spp.) and related to productivity, biomass and photoperiod. Peak chlorophyll levels for the three blooms were 28, 65 and 938 μg1−1. In general, DFAA and MONO concentrations increased with increasing biomass of bloom-forming species, reaching 203 and 844 μg1−1. MONO appeared to accumulate during the day while there was no consistent pattern for DFAA. The accumulations of DFAA and MONO in blooms indicate that bloom production might stimulate microheterotrophy, thereby enhancing carbon and nutrient cycling in bloom-impacted regions. 相似文献
952.
The northern parts of the Prathap and Laccadive Ridge system, eastern Arabian Sea, consist of three parallel basement ridge peaks at varied depths. The topographic highs are associated with either well-developed or subdued magnetic signatures. Model studies, constrained by seismic results, determine the varied nature and depth to the top of the causative basement bodies. Similarities of the geophysical signatures of the ridges and their structural resemblance perhaps point to their common origin. Hence we propose that the Prathap Ridge complex may be a part of the Chagos-Laccadive Ridge system and formed because of the Reunion hotspot activity. 相似文献
953.
Studies in marine microbiology relevant to the cultivation of lobster in Fatty Basin were made.
Biomass of bacteria and allied microorganisms in whole seawater column of the basin was very small (2×104 gC), but a large biomass was found in the sediments (3×105 gC).
The decomposition of chitin occurred chiefly in the sediments. The rate of decomposition (500 g/day) was approximately half
of the rate of production. However, the remaining production was considered not to be involved in the chitin cycle of the
basin. This hypothesis was supported by the results of the analysis of the budget of organic matter in the area.
Shell disease of lobster caused by chitinoclastic bacteria was detected, although it was not serious. A destructive yeast
parasite of crustaceans,Metschnikowia, was collected only from a crab in the basin.
This report is dedicated to Professor KenSugawara for his 70 years old anniversary. 相似文献
954.
955.
Sand dabs, an important commercial flatfish in the Bohai Sea, has recently decreased gradually in biomass, resulting from overfishing and marine pollution. Artificial culture and multiplication of sand dabs are essential and practical in China. The present paper examines the food habitof this fish to provide detailed information for its culture and multiplication. 相似文献
956.
957.
Effectsoflight,temperatureandnutrientsonphotosynthesisofBiddulphiaregiaINTRODUCTIONLight,temperatureandnutrientsareimportante... 相似文献
958.
Subrata K. Chakrabarti 《Ocean Engineering》1989,16(4)
The design and performance of an offshore structure depends largely upon the response of the structure to the environmental loading such as waves. The extreme response chosen for the design of a structure should meet its lifetime response, operational response as well as the fatigue damage. The failure of the structural member may be caused by the maximum instantaneous stress experienced by the member due to a given environment. This is considered short-term as opposed to long-term or fatigue damage. The short-term response statistics are obtained on the basis of one particular seastate. Since this seastate is invariably high, nonlinearity in the excitation and response of the structure is almost invariably present. The general nonlinear problem in the extreme response prediction is largely unsolved. Response characteristics are often obtained from the perturbation methods and equivalent linearization techniques. Unlike nonlinear problems, these methods greatly simplify the analysis for extreme values. This paper reviews the available approximation techniques in the response computation and the limits of their applicability in a design situation. Results are illustrated so that a designer may evaluate the suitability of a method in a particular design condition. 相似文献
959.
Wen Shengchang 《海洋学报(英文版)》1988,7(1):1-16
In this part ot the paper theoretical wind-wave spectra nave been derived oy (I) expressing the spectrum in series composed of exponential terms; (2) assuming that the spectrum satisfies a high order linear ordinary differential equation; (3) introducing proper parameters in the spectrum; and (4) making use of some known charateristics of wind-wave spectrum, for instance, the law governing the equilibrium range. The spectrum obtained contains the zero order moment of the spectrum m0, the peak frequency ω0 and the ratio R =ω/ω0 (ω being the mean zero-crossing frequency) as parameters. The shape of the nondimensional spectrum S(ω) = ω0S(ω)/m0(ω=ω/ω0) changes with R and theoretically reduces to a Dirac delta function δ(ω-1) when R = 1. A spectrum of simplified form is given for practical uses, in which R is replaced by a peakness factor P=S(1). 相似文献
960.