全文获取类型
收费全文 | 37141篇 |
免费 | 1067篇 |
国内免费 | 930篇 |
专业分类
测绘学 | 938篇 |
大气科学 | 2845篇 |
地球物理 | 7620篇 |
地质学 | 13583篇 |
海洋学 | 3331篇 |
天文学 | 8170篇 |
综合类 | 241篇 |
自然地理 | 2410篇 |
出版年
2022年 | 272篇 |
2021年 | 446篇 |
2020年 | 427篇 |
2019年 | 483篇 |
2018年 | 906篇 |
2017年 | 865篇 |
2016年 | 1034篇 |
2015年 | 703篇 |
2014年 | 1029篇 |
2013年 | 1866篇 |
2012年 | 1336篇 |
2011年 | 1777篇 |
2010年 | 1561篇 |
2009年 | 2019篇 |
2008年 | 1696篇 |
2007年 | 1763篇 |
2006年 | 1701篇 |
2005年 | 1225篇 |
2004年 | 1142篇 |
2003年 | 1036篇 |
2002年 | 1002篇 |
2001年 | 845篇 |
2000年 | 824篇 |
1999年 | 671篇 |
1998年 | 716篇 |
1997年 | 689篇 |
1996年 | 570篇 |
1995年 | 561篇 |
1994年 | 479篇 |
1993年 | 418篇 |
1992年 | 416篇 |
1991年 | 385篇 |
1990年 | 455篇 |
1989年 | 372篇 |
1988年 | 354篇 |
1987年 | 437篇 |
1986年 | 343篇 |
1985年 | 430篇 |
1984年 | 531篇 |
1983年 | 451篇 |
1982年 | 452篇 |
1981年 | 401篇 |
1980年 | 418篇 |
1979年 | 358篇 |
1978年 | 344篇 |
1977年 | 340篇 |
1976年 | 309篇 |
1975年 | 296篇 |
1974年 | 312篇 |
1973年 | 340篇 |
排序方式: 共有10000条查询结果,搜索用时 281 毫秒
411.
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
412.
413.
414.
415.
416.
The influence of salinity on the solubility of Zn and Cd sulphides in the Scheldt estuary 总被引:2,自引:0,他引:2
In the estuary of the river Scheldt, where an oxygen gradient exists in addition to the salinity gradient, redox processes will be of major importance for trace metal mobilisation. In this study, the influence of salinity and pH on the redox processes of dissolved Zn and Cd sulphides is investigated together with the effects on the ratio of the dissolved Zn and Cd concentrations. The speciation of these metals is calculated with the chemical equilibrium programme
+. Zn sulphides are oxidised at lower oxygen concentrations than Cd sulphides, due to lower stability constants, causing a sudden increase or peak in the dissolved Zn/Cd ratio. The formation of dissolved Cd chloride complexes when oxidation occurs at high salinities (S=15) increases the mobility of Cd, causing a decrease in the Zn/Cd peak of the total dissolved concentrations. The peak is three to four times smaller at S=15 than when oxidation occurs at S=2. The simple model calculations compare very well with field data. The Scheldt estuary is suitable to illustrate these calculations. In the 1970s, the anoxic part of the estuary reached S=15–20, but since the early 1980s it has dropped to S=2–10. Historic data on metals in the estuary from 1978, 1987 and the 1990s were used to compare with the equilibrium calculations. The increase of the dissolved Zn/Cd peak at low salinity as a consequence, of the decreasing anoxic region is confirmed well by the data. The good agreement between model calculations and field data is a proof of the extreme importance of redox processes for the solubility of Zn and Cd sulphides in the estuary. 相似文献
417.
Bori L. Olla Allen J. Bejda Anne L. Studholme Walter H. Pearson 《Marine environmental research》1984,13(2):121-139
The burrowing and emergence behavior of sand worms, Nereis (Neanthes) virens Sars, in sediment contaminated with sublethal concentrations of Prudhoe Bay crude oil ranging from 74 to 5222 ppm, was studied in the laboratory. Initial burrowing in oiled sediment did not differ from that observed in unoiled sediment. Emergence of exposed worms was related to the oil concentration and the extent of weathering of oiled sediment. Sand worms buried in unoiled sediment did not emerge. Worms dug from oiled sediment after 12 hours' exposure and placed on unoiled sediment were impaired, resulting in a significant increase in time to burrow. However, after 12h, they recovered and burrowed at normal rates. Worms held in oiled sediment for 96 h also recovered. Possible causes for recovery are discussed. Results suggest that oil-induced aberrations may increase vulnerability to predation. 相似文献
418.
Copper in a series of samples from the northwestern Atlantic has been determined by anodic stripping voltammetry (ASV) and atomic absorption spectrometry after preconcentration by cobalt—APDC coprecipitation. Samples from a transect across the continental shelf directly off the New York Bight showed a linear relationship between total copper and salinity (31–34‰) and less than 5% of the total copper was detectable by ASV. A slope water station showed less than 9% of the total copper to be ASV detectable in the upper 780m, below which the ASV detectable copper increased to 70% at 3000 m. The Sargasso Sea profiles showed less than 7% ASV detectable copper in the upper 150 m; the fraction of the total copper detected by ASV increased to about 80% of the total copper at greater depths. The reduced availability of copper to the ASV measurement in coastal waters and open ocean surface waters appears to be related to recent contact with waters where biological productivity occurs. 相似文献
419.
420.
Abstract. The emergence of the Isthmus of Panama subdivided the amphi-American biota. In the present study, Pacific and Atlantic populations of four cognate pairs of crabs were used to discern whether exposure to different thermal regimes in habitats, in the putative absence of gene flow, has resulted in physiological divergence. Populations that potentially form a common genetic pool were also used; these were populations of the Atlantic Panama cognate that occur in Belize and Florida. Decreases in water temperature occur periodically in Pacific Panama and Florida, but not in Atlantic Panama or Belize. In this study, physiological divergence in oxygen uptake was assessed in response to repeated exposure to either control and decreased temperature or control and increased temperature. Results indicate that, in only some of the genera tested, exposure to decreases in habitat temperature has resulted in divergence. Partial support is found for the corollary that adaptation to an environment with periods of decreased temperature results in reduced compensation during exposure to elevated temperature. 相似文献