全文获取类型
收费全文 | 41721篇 |
免费 | 594篇 |
国内免费 | 322篇 |
专业分类
测绘学 | 962篇 |
大气科学 | 2971篇 |
地球物理 | 8026篇 |
地质学 | 14402篇 |
海洋学 | 3862篇 |
天文学 | 9754篇 |
综合类 | 74篇 |
自然地理 | 2586篇 |
出版年
2021年 | 404篇 |
2020年 | 401篇 |
2019年 | 453篇 |
2018年 | 940篇 |
2017年 | 843篇 |
2016年 | 1090篇 |
2015年 | 610篇 |
2014年 | 1003篇 |
2013年 | 2091篇 |
2012年 | 1139篇 |
2011年 | 1623篇 |
2010年 | 1476篇 |
2009年 | 1990篇 |
2008年 | 1651篇 |
2007年 | 1734篇 |
2006年 | 1594篇 |
2005年 | 1325篇 |
2004年 | 1272篇 |
2003年 | 1247篇 |
2002年 | 1201篇 |
2001年 | 1063篇 |
2000年 | 985篇 |
1999年 | 817篇 |
1998年 | 821篇 |
1997年 | 848篇 |
1996年 | 683篇 |
1995年 | 661篇 |
1994年 | 627篇 |
1993年 | 582篇 |
1992年 | 536篇 |
1991年 | 510篇 |
1990年 | 513篇 |
1989年 | 501篇 |
1988年 | 484篇 |
1987年 | 560篇 |
1986年 | 493篇 |
1985年 | 610篇 |
1984年 | 656篇 |
1983年 | 592篇 |
1982年 | 530篇 |
1981年 | 580篇 |
1980年 | 471篇 |
1979年 | 463篇 |
1978年 | 435篇 |
1977年 | 435篇 |
1976年 | 388篇 |
1975年 | 380篇 |
1974年 | 377篇 |
1973年 | 385篇 |
1971年 | 228篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
992.
Jeffrey M. Napp Christine T. Baier Richard D. Brodeur Kenneth O. Coyle Naonobu Shiga Kathy Mier 《Deep Sea Research Part II: Topical Studies in Oceanography》2002,49(26)
The southeastern Bering Sea shelf ecosystem is an important fishing ground for fin- and shellfish, and is the summer foraging grounds for many planktivorous seabirds and marine mammals. In 1997 and 1998, Northern Hemisphere climate anomalies affected the physical and biological environment of the southeastern Bering Sea shelf. The resulting anomalous conditions provided a valuable opportunity to examine how longer-term climate change might affect this productive ecosystem. We compared historical and recent zooplankton biomass and species composition data for the southeastern Bering Sea shelf to examine whether or not there was a response to the atmosphere–ocean–ice anomalies of 1997 and 1998. Summer zooplankton biomass (1954–1994) over the southeastern shelf did not exhibit a decline as previously reported for oceanic stations. In addition, zooplankton biomass in 1997 and 1998 was not appreciably different from other years in the time series. Spring concentrations of numerically abundant copepods (Acartia spp., Calanus marshallae, and Pseudocalanus spp.), however, were significantly higher during 1994–1998 than 1980–1981; spring concentrations of Metridia pacifica and Neocalanus spp. were not consistently different between the two time periods. Neocalanus spp. was the only taxon to have consistent differences in stage composition between the two time periods—CV copepodites were much more prevalent in May of the 1990s than early 1980s. Since relatively high zooplankton concentrations were observed prior to 1997, we do not attribute the high concentrations observed in the summers of 1997 and 1998 directly to the acute climate anomalies. With the present data it is not possible to distinguish between increased production (control from below) and decreased predation (control from above) to explain the recent increase in concentrations of the species examined. 相似文献
993.
994.
LI Ruijie WANG Houjie
Dr. Associate Professor Engineering College of Ocean University of Qingdao Qingdao P. R. China
Ph. D. Candidate Engineering College of Ocean University of Qingdao Qingdao P. R. China 《中国海洋工程》1999,(3)
Nonlinear effect is of importance to waves propagating from deep water to shallow water.Thenon-linearity of waves is widely discussed due to its high precision in application.But there are still someproblems in dealing with the nonlinear waves in practice.In this paper,a modified form of mild-slope equa-tion with weakly nonlinear effect is derived by use of the nonlinear dispersion relation and the steady mild-slope equation containing energy dissipation.The modified form of mild-slope equation is convenient to solvenonlinear effect of waves.The model is tested against the laboratory measurement for the case of a submergedelliptical shoal on a slope beach given by Berkhoff et al,The present numerical results are also comparedwith those obtained through linear wave theory.Better agreement is obtained as the modified mild-slope e-quation is employed.And the modified mild-slope equation can reasonably simulate the weakly nonlinear ef-fect of wave propagation from deep water to coast. 相似文献
995.
The Effect of Macrofauna on Porewater Profiles and Nutrient Fluxes in the Intertidal Zone of the Humber Estuary 总被引:3,自引:0,他引:3
R. J. G. Mortimer J. T. Davey M. D. Krom P. G. Watson P. E. Frickers R. J. Clifton 《Estuarine, Coastal and Shelf Science》1999,48(6):683
Macrofauna, nutrient fluxes, porewater chemistry and sediment characteristics were measured at six intertidal mudflat sites in the Humber Estuary, U.K., during the different seasons. Nereis diversicolor , Macoma balthica and Corophium volutator were found to be the dominant macrofauna. Salinity was the baseline control on macrofauna distribution but this was overprinted by periodic impoverishment due to sediment mobilization. High resolution gel probe porewater samplers provided direct evidence for the impact of burrows on porewater chemistry. The macrofauna modified nutrient fluxes during periods of mud flat stability. Nereis caused a decrease in silicate and phosphate effluxes but enhanced ammonia release and nitrate uptake. Macoma enhanced ammonia and nitrite release. The impact of Corophium was not possible to discern. The Humber is a large, highly dynamic macrotidal estuary in which sediment resuspension has a large impact on porewater profiles, nutrient fluxes and macrofaunal communities. Simple patterns and inter-relationships which are seen in small sheltered estuaries are not observed in the Humber. 相似文献
996.
Horiguchi T Li Z Uno S Shimizu M Shiraishi H Morita M Thompson JA Levings CD 《Marine environmental research》2004,57(1-2):75-88
Gastropods and bivalves were collected at 15 sites at Vancouver and Victoria, Canada between 24 May and 7 June, 1999, to establish tissue concentrations of butyltin and phenyltin compounds, to record imposex symptoms in gastropods, and to assess the present status of organotin contamination around Vancouver. No neogastropods (such as Nucella lima) were found around Vancouver. Neogastropod populations could have been extirpated by severe TBT contamination in Vancouver, as relatively high concentrations of TBT were detected in tissues of Mytilus trossulus from Vancouver, and the neogastropods distributed in Vancouver might be sensitive to TBT. Recovery from imposex, however, was observed in neogastropod populations from three sites at Victoria and Mission Point. TBT contamination has continued around Vancouver, arising from continuous use of TBT in antifouling paints for vessels larger than 25 m in length; however, TBT has decreased around Victoria and Mission Point. Different patterns of TBT accumulation in tissue were observed among the bivalve species from Vancouver. The highest TBT concentration detected in Tresus capax suggested some possible adverse effects. TBT was the most predominant butyltin component in almost all bivalve specimens surveyed, suggesting a low rate of TBT metabolism. Phenyltin compounds were not detected in any molluscan specimens in this study. 相似文献
997.
Jonsson H Sandnes KV Schiedek D Schneider R Grøsvik BE Goksøyr A 《Marine environmental research》2004,58(2-5):655-658
In an attempt to learn more about the cytochrome P450 (CYP) system of mussels, we used protein databases and alignment software to extract highly conserved CYP sequences. From these alignments synthetic peptides were produced and used for rabbit immunisation, which yielded polyclonal antibodies against the CYP families 2 and 4. The antibodies were evaluated with Western Blot and ELISA assays, using digestive gland microsomal samples from the mussel Mytilus edulis. Western Blots revealed immunoreactions for both antibodies. The anti-CYP2 sequence rendered one major immunopositive protein of approximately 49 kDa size, and weak signals for proteins of approximately 41 and 56 kDa size. The anti-CYP4 sequence rendered two major bands of approximately 56 and 59 kDa size, and also a weak immunoreaction with a protein of approximately 43 kDa size. ELISA rendered only weak signals even with a 1:50 dilution of IgG-purified serum. A 10-day exposure to Aroclor 1254 did not appear to affect any of the immunopositive proteins, while total PCBs in soft bodies increased from 14-40 ng/g DW in controls to 373-638 ng/g DW in exposed mussels. 相似文献
998.
In this study male largemouth bass (LMB) were exposed to the naturally occurring androgens, dihydrotestosterone (DHT) or 11-ketotestosterone (11-KT) in order to identify genes that are differentially regulated by these steroid hormones. Using subtractive hybridization on livers of fish treated with DHT against vehicle control, many novel LMB genes were cloned. These genes were added to our gene library and arrayed. Six genes were up-regulated and five were down-regulated by both androgens. But, each androgen also regulated specific genes. One gene that was identified as a potential androgen marker was spermidine-spermine-N(1)-acetyltransferase that was up-regulated by both androgens. Determining which genes are responsive to natural androgens will help to identify biochemical pathways that are impacted. 相似文献
999.
1000.