全文获取类型
收费全文 | 5174篇 |
免费 | 530篇 |
国内免费 | 1344篇 |
专业分类
测绘学 | 269篇 |
大气科学 | 429篇 |
地球物理 | 847篇 |
地质学 | 2804篇 |
海洋学 | 1295篇 |
天文学 | 93篇 |
综合类 | 232篇 |
自然地理 | 1079篇 |
出版年
2024年 | 16篇 |
2023年 | 49篇 |
2022年 | 152篇 |
2021年 | 187篇 |
2020年 | 193篇 |
2019年 | 250篇 |
2018年 | 140篇 |
2017年 | 200篇 |
2016年 | 237篇 |
2015年 | 175篇 |
2014年 | 268篇 |
2013年 | 520篇 |
2012年 | 249篇 |
2011年 | 335篇 |
2010年 | 285篇 |
2009年 | 345篇 |
2008年 | 396篇 |
2007年 | 325篇 |
2006年 | 337篇 |
2005年 | 276篇 |
2004年 | 226篇 |
2003年 | 193篇 |
2002年 | 224篇 |
2001年 | 180篇 |
2000年 | 182篇 |
1999年 | 162篇 |
1998年 | 132篇 |
1997年 | 123篇 |
1996年 | 110篇 |
1995年 | 90篇 |
1994年 | 92篇 |
1993年 | 72篇 |
1992年 | 66篇 |
1991年 | 48篇 |
1990年 | 27篇 |
1989年 | 31篇 |
1988年 | 15篇 |
1987年 | 22篇 |
1986年 | 20篇 |
1985年 | 26篇 |
1984年 | 21篇 |
1983年 | 17篇 |
1982年 | 8篇 |
1981年 | 9篇 |
1980年 | 7篇 |
1979年 | 2篇 |
1978年 | 8篇 |
排序方式: 共有7048条查询结果,搜索用时 218 毫秒
51.
Tan Jiahua Ma Wei Pan Bin Associate Professor Shanghai Jiao Tong University Shanghai Lecturer shanghai Jiao Tong University Shanghai 《中国海洋工程》1992,(1)
The basis, process and results of the demonstration of the main dimensions of a 75000t floating production and storage vessel are discussed in this paper. A simple but reliable orthogonal design method is applied in the main dimension optimization. The ideas of gradual approximation and feedback from various aspects are put into effect. During the demonstration, in order to make the model tally with the actual situation, the draft design is closely related to the computational analysis, so that the demonstration model can be verified at any time; the handling of the overall system is closely related to the research of each item, which is beneficial not only to the mastery of various regularities, but also to the balance of decisions. Finally, according to the computational results and the regularities obtained from analysis, the main dimensions are determined. 相似文献
52.
53.
地学在近海渔业管理中的应用虽是个刚刚兴起的研究领域,但已向人们展示了它令人振奋的成果。通过分析地学成果的获取、表达及地球物理技术的进展情况,介绍了加拿大地学专家如何成功地将地学成果应用于指导近海渔业捕捞,以及如何利用地学知识评价不同捕捞器材对环境的影响。 相似文献
54.
55.
长江冲淡水区细菌生产力研究 总被引:11,自引:3,他引:11
为了解细菌在长江口冲淡水区生态系营养动力学过程中的重要作用,笔者于1997年10月10日至20日,1998年5月14日至6月1日在观测海区以及在绿华山海域设置大水体围隔生态系实验装置进行细菌生产力的现场观测研究.结果表明,秋季观测海区平均细菌生产力(C)为(1.44±1.30)μg/(dm3·h),高值出现在测区中部的A3,B3和C3站.春季测区表层细菌生产力(2.43±1.22)μg/(dm3·h)高于底层(1.01±0.43)μg/(dm3·h),高值出现在测区中部的A3和B断面诸站.秋、春季平均细菌生产力相当于浮游植物初级生产力的23%.秋季和春季表层细菌数量分别为(5.22×108±0.88×108)个/dm3和(1.97×108±1.10×108)个/dm3.1998年5月18日至6月1日在围隔实验点的自然海区中细菌生产力变幅范围为0.13~5.79μg/(dm3·h),平均值为(2.47±1.60)μg/(dm3·h).围隔装置内加可溶性磷(PO43-)实验,春季细菌生产力由1.28μg/(dm3·h)增长至32.20μg/(dm3·h),其增长幅度低于秋季1.43~43.47μg/(dm3·h).油污染实验中细菌生产力由6.61μg/(dm3·h)增长至37.97μg/(dm3·h),呈逐日上升趋势. 相似文献
56.
57.
Erika Sternberg Catherine Jeandel Juan-Carlos Miquel Beat Gasser Marc Souhaut Roseanna Arraes-Mescoff Roger Francois 《Marine Chemistry》2007,105(3-4):281-295
Biogenic barium, mostly in the barite (BaSO4) form, has been proposed as a tracer for export production in the ocean. Here we report on biogenic barium (Baxs) and particulate organic carbon (POC) fluxes from sediment traps deployed at the DYFAMED site in the Northwestern Mediterranean Sea. Baxs fluxes display average values of 37 ± 45 and 50 ± 58 μg/m2/d at 200 and 1000 m respectively, and are linearly correlated to POC fluxes (mean values of 7.9 ± 9.3 and 6.8 ± 6.8 mg C/m2/d at 200 and 1000 m). Export production estimates, calculated using published Baxs- or POC-based algorithms, all fall below or close to the lower limit of potential export values proposed in the literature. This work clearly demonstrates the usefulness of Baxs as a tracer of oceanic export production in the Northwestern Mediterranean Sea. However, development of a quantitative export production proxy requires a clear understanding of the underlying cause(s) for the observed spatial variations in the relationship between Baxs and POC fluxes. The present study confirms that the processes leading to barite formation differ between margin and open-ocean sites and probably account for much of the regional variability in the POC/Baxs ratio. 相似文献
58.
Atmospherically-promoted photosynthetic activity in a well-mixed ecosystem: Significance of wet deposition events of nitrogen compounds 总被引:1,自引:0,他引:1
C. Boulart P. Flament V. Gentilhomme K. Deboudt C. Migon F. Lizon M. Schapira A. Lefebvre 《Estuarine, Coastal and Shelf Science》2006,69(3-4):449
Wet atmospheric deposition of dissolved N, P and Si species is studied in well-mixed coastal ecosystem to evaluate its potential to stimulate photosynthetic activities in nutrient-depleted conditions. Our results show that, during spring, seawater is greatly depleted in major nutrients: Dissolved Inorganic Nitrogen (DIN), Dissolved Inorganic Phosphorus (DIP) and Silicic acid (Si), in parallel with an increase of phytoplanktonic biomass. In spring (March–May) and summer (June–September), wet atmospheric deposition is the predominant source (>60%, relative to riverine contribution) for nitrates and ammonium inputs to this N-limited coastal ecosystem. During winter (October–February), riverine inputs of DIN predominate (>80%) and are annually the most important source of DIP (>90%). This situation allows us to calculate the possibility for a significant contribution to primary production in May 2003, from atmospheric deposition (total input for DIN ≈300 kg km−2 month−1). Based on usual Redfield ratios and assuming that all of the atmospheric-derived N (AD-N) in rainwater is bioavailable for phytoplankton growth, we can estimate new production due to AD-N of 950 mg C m−2 month−1, during this period of depletion in the water column. During the same episode (May 2003), photosynthetic activity rate, considered as gross primary production, was estimated to approximately 30 300 mg C m−2 month−1. Calculation indicates that new photosynthetic activity due to wet atmospheric inputs of nitrogen could be up to 3%. 相似文献
59.
Managing marine fisheries using output controls in the form of individual transferable quotas (ITQs) can be an attractive alternative to more traditional input controls. There are now a number of examples of where ITQ-managed fisheries have been able to reduce the impact of the major management problems in global fisheries, namely, gross over-capitalisation and effort. However, ITQs are not the perfect management tool and one of the lesser known consequences of ITQ-managed fisheries where ITQs consist of a harvest right is the implicit relationship between ITQ property rights and rights of access to the fishing grounds. This implicit spatial right to the grounds can provide obstacles in the way of allocating water-space within fishing grounds for alternative uses such as marine-protected areas, large-scale aquaculture, and wind farms. These lesser-known consequences of ITQ-managed fisheries are discussed here. 相似文献
60.
Yusuke Okazaki Osamu Seki Takeshi Nakatsuka Tatsuhiko Sakamoto Minoru Ikehara Kozo Takahashi 《Journal of Oceanography》2006,62(5):639-648
Cycladophora davisiana, a radiolarian species dwelling at mesopelagic depths, is known as a representative glacial fauna due to its unique distribution
during glacial periods. In the present ocean, abundant production of C. davisiana is only observed in the Okhotsk Sea, indicating an adaptation of C. davisiana for seasonal sea-ice covered conditions. We found pronounced abundant production of C. davisiana during the early to middle Holocene in the Okhotsk Sea, suggesting more favorable conditions for C. davisiana than the present Okhotsk Sea. In order to clarify the reason, oceanographic conditions during the Holocene were reconstructed
based on biomarkers, lithogenic grains including ice-rafted debris (IRD), biogenic opal, and total organic carbon (TOC) in
two sediment cores from the Okhotsk Sea. These indicators suggest that the pronounced C. davisiana production may be attributed to: 1) a supply to mesopelagic depths under intensified stratification of fine organic particles
derived from coccolithophorids, bacteria, and detrital materials; and 2) cold, well-ventilated intermediate water formation. 相似文献