全文获取类型
收费全文 | 86239篇 |
免费 | 1143篇 |
国内免费 | 695篇 |
专业分类
测绘学 | 2456篇 |
大气科学 | 6270篇 |
地球物理 | 16378篇 |
地质学 | 32766篇 |
海洋学 | 7061篇 |
天文学 | 18542篇 |
综合类 | 319篇 |
自然地理 | 4285篇 |
出版年
2021年 | 626篇 |
2020年 | 623篇 |
2019年 | 727篇 |
2018年 | 4036篇 |
2017年 | 3728篇 |
2016年 | 3241篇 |
2015年 | 1202篇 |
2014年 | 1825篇 |
2013年 | 3469篇 |
2012年 | 2859篇 |
2011年 | 4882篇 |
2010年 | 4447篇 |
2009年 | 5173篇 |
2008年 | 4298篇 |
2007年 | 4883篇 |
2006年 | 2566篇 |
2005年 | 2493篇 |
2004年 | 2265篇 |
2003年 | 2302篇 |
2002年 | 2067篇 |
2001年 | 1671篇 |
2000年 | 1623篇 |
1999年 | 1366篇 |
1998年 | 1330篇 |
1997年 | 1326篇 |
1996年 | 1143篇 |
1995年 | 1096篇 |
1994年 | 1037篇 |
1993年 | 931篇 |
1992年 | 847篇 |
1991年 | 842篇 |
1990年 | 848篇 |
1989年 | 819篇 |
1988年 | 769篇 |
1987年 | 904篇 |
1986年 | 798篇 |
1985年 | 986篇 |
1984年 | 1044篇 |
1983年 | 987篇 |
1982年 | 907篇 |
1981年 | 893篇 |
1980年 | 805篇 |
1979年 | 723篇 |
1978年 | 692篇 |
1977年 | 693篇 |
1976年 | 615篇 |
1975年 | 593篇 |
1974年 | 621篇 |
1973年 | 657篇 |
1972年 | 381篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
A. Winter K.O. Coyle G. Swartzman 《Deep Sea Research Part II: Topical Studies in Oceanography》2007,54(23-26):2869
Cohort abundance of walleye pollock (Theragra chalcogramma) is subject to strong interannual variation in the eastern Bering Sea, and this variation is known to be determined largely at the age-0 stage. We estimated the spatial distributions and densities of age-0 walleye pollock in five nursery areas around the eastern Bering shelf in three successive years (1997–1999) from acoustic survey data. Concurrently, we calculated estimates of the spatial distribution of euphausiids, a major prey of age-0 walleye pollock, and estimates of spatial overlap of groundfish predators with the age-0 walleye pollock. The analyses showed that all nursery areas had low densities of age-0 walleye pollock in 1997, which ultimately produced the weakest adult year-class. In the intermediate year of 1998, age-0 densities were low to medium, and in 1999, which produced the strongest of the three adult year-classes, all nursery areas had medium to high age-0 walleye pollock densities. Euphausiid distributions had a consistently positive spatial relationship with age-0 walleye pollock. Groundfish predator density ratios were positively related to age-0 walleye pollock density when age-0 walleye pollock were displaced relatively northward. Our results suggest that abundance of age-0 walleye pollock, and hence of adult cohorts in the eastern Bering Sea, can be predictable from a concise set of indicators: the densities of age-0 walleye pollock at nursery areas in mid- to late-summer, their spatial relationship to euphausiids and groundfish predators, and the latitudinal trend of their distributions. The 3 years 1997–1999 had significant differences of physical conditions in the eastern Bering Sea, and represent an advantageous framework for testing these hypotheses. 相似文献
992.
993.
Spatio-temporal variability of small copepods (especially Oithona plumifera) in shallow nearshore waters off the south coast of South Africa 总被引:1,自引:0,他引:1
Francesca Porri Christopher D. McQuaid William P. Froneman 《Estuarine, Coastal and Shelf Science》2007,72(4):711-720
Although small copepods are one of the main dietary sources for many commercially important fish, their role in the pelagic trophic dynamics has traditionally been underestimated due to the methodology commonly used in plankton sampling. Temporal variation in abundance of adults and nauplii of small copepods (particularly Oithona plumifera) in nearshore waters on the south coast of South Africa was investigated fortnightly over 14 months at site (km) and location (100 m) scales. Sampling was within <500 m of the shore, where depth was ca. 10 m, using vertical hauls of an 80-μm mesh plankton net from 1 m above the seabed to the surface. Twenty-seven adult copepod taxa were recorded, but Oithona spp. was consistently the most abundant. Taxon richness was 7–19 on each sampling occasion. There was strong temporal variation (Oithona varied between 0 and 2300 m−3), but much of this was short-term variability (e.g. between consecutive sampling sessions), with no seasonality or other long-term discernable patterns. There were periods of consistently low numbers, but very high numbers often followed samples with low abundances. Nor was there spatial structure at the location scale, though numbers differed between sites. Despite considerable variability at the location scale within sites, Kenton consistently showed higher densities than High Rocks. Separate analyses, with Bonferroni adjustment, showed that this difference was significant on eight out of 21 occasions for Oithona, six for other pelagic copepods and three for nauplii. This suggests that hydrodynamics favour aggregation of plankton at Kenton. A high degree of short-term variability, with a tendency for aggregation of small zooplankton at certain sites has implications for both pelagic processes and food-web links between the benthic and pelagic environments. 相似文献
994.
The regular wave interaction with a twin concentric porous circular cylinder system consisting of an inner impermeable cylinder and an outer perforated cylinder was studied through physical model and numerical model studies. The experiments were carried out on the twin concentric cylinder model in a wave flume to study the wave runup and rundown at the leading and trailing edges of the perforated cylinder. It was found that the maximum wave runup on the perforated cylinder is almost same as the incident wave height. The experimental results were used to develop the predictive formulae for the wave runup and rundown on the perforated cylinder, which can be easily used for design applications. The wave runup profiles around the perforated cylinder for different values of ka and porosities were studied numerically using Green's Identity Method. The results of the numerical study are presented and compared with the experimental measurements. 相似文献
995.
We collected surface water along the 142nd E meridian from Tasmania to Antarctica in December 1999. We measured temperature,
salinity and total chlorophyll a; additionally, we collected suspended particle size fractions and used fluorometric analysis to determine the quantity of
chlorophyll a in each of four cell size classes: picoplankton (<3 μm), two nanoplankton fractions (3–10 μm and 10–20 μm) and microplankton
(> 20 μm). Changes in temperature and salinity show that we crossed 6 water masses separated by 5 fronts. We found low abundance
(<0.2 mg m−3) of chlorophyll in all size classes, with the exception of higher values near the continent (0.2 to 0.4 mg m−3). Lowest chlorophyll values (<0.1 mg m−3) were found in the Polar Frontal Zone (51° to 54°S). Microplankton made up the largest portion of total chlorophyll throughout
most of the region. We conclude that biomass of all phytoplankton fractions, especially pico-and nanoplankton, was constrained
by limiting factors, most probably iron, throughout the region and that ecosystem dynamics within a zone are not circumpolar
but are regionalized within sectors. 相似文献
996.
S. D. Nikolaev 《Oceanology》2006,46(4):513-525
The objective of this study was to present details of the evolution of the thermal field of the ocean during the last Cenozoic glacial megacycle. The main causes of the phenomena observed are discussed based on the generalization of original data accumulated during many years and numerous publications. It is shown that, in the Eocene and Early Oligocene, the climatic conditions in the World Ocean were uniform and featured no evident zonal differences, the meridional temperature gradients were low, and the mean global temperature was close to the present-day value. Starting from the Late Oligocene and throughout the entire Neogene, the meridional contrasts in the temperature and hydrological conditions have increased, and, in the Late Pliocene, the climatic pattern resembled the present-day situation. The general tendency in the climate changes since the Eocene until the Pliocene consisted of its latitudinal differentiation and growing contrasts rather than of general climate cooling. In the Pleistocene, with the onset of the glaciation in the Northern Hemisphere, the character of the temperature field changed and variations in the mean global temperature values began to dominate. The factors that result from the climate changes observed are discussed and the causes are shown that lead to the differences between the author’s data and the existing concepts. 相似文献
997.
K. A. Korotenko 《Oceanology》2007,47(3):313-324
A new high-resolution (<2 km) version of the DieCAST fourth-accuracy-order model for the ocean circulation is proposed for the study of the general circulation, mesoscale structures, and their variability in the Adriatic Sea. The model uses mean seasonal data on the temperature, salinity, buoyancy fluxes, and wind. The data of the COAMPS system with a 4-km resolution were used for the simulation of the sea response to the effects of various winds: Sirocco, Maestro, and two types of boras. The mean monthly runoffs from 38 rivers and mean daily runoffs from 12 main rivers throughout the year were given in the model. The conditions at the open boundary of the Strait of Otranto were given on the basis of the hierarchy of two coarser models for the Adriatic and Mediterranean seas. Due to the extremely weak dissipation and the high resolution (the mesh size is less than the baroclinic radius of deformation, 5–10 km), the model allows one to trace the development of a baroclinic instability along the Italian coast, to simulate mesoscale structures associated with the instability, and to estimate the scales of the structures. Mesoscale filaments, meanders, mushroom-like currents, fronts, and intrusions known from satellite observations were simulated and explained. The scenario of the anomalous upwelling near the Italian coast observed in the summer of 2003 was also simulated and analyzed. 相似文献
998.
Interannual variability in Nordic seas primary production 总被引:1,自引:0,他引:1
999.
1000.
A simple and effective method of heating divers in extreme cold water has been developed. The local heating concept utilizes a uniformly distributed granular mixture of magnesium and iron particles packed in small sachets. Upon activation by sea water, the two metal mixture behaves as a multitude of short-circuited electrolytic cells, producing thermal energy rather than electrical energy. The 45 mm-square sachets may be placed where heating is required on the diver's body, thus giving rise to the term “local heating”, obviously, the heating system requires no pumping device, or distribution network. Heating rate and duration of output of the sachets are controlled by particle size, and mixture ratio of the constituent Mg and Fe particles.This paper described the development, testing and performance of the heating sachets. Results of live tests in different dive situations are also presented and discussed. 相似文献