全文获取类型
收费全文 | 39665篇 |
免费 | 830篇 |
国内免费 | 461篇 |
专业分类
测绘学 | 1413篇 |
大气科学 | 2973篇 |
地球物理 | 8014篇 |
地质学 | 14080篇 |
海洋学 | 3197篇 |
天文学 | 8750篇 |
综合类 | 159篇 |
自然地理 | 2370篇 |
出版年
2021年 | 356篇 |
2020年 | 383篇 |
2019年 | 433篇 |
2018年 | 973篇 |
2017年 | 934篇 |
2016年 | 1232篇 |
2015年 | 769篇 |
2014年 | 1140篇 |
2013年 | 2136篇 |
2012年 | 1269篇 |
2011年 | 1560篇 |
2010年 | 1335篇 |
2009年 | 1701篇 |
2008年 | 1507篇 |
2007年 | 1423篇 |
2006年 | 1471篇 |
2005年 | 1198篇 |
2004年 | 1135篇 |
2003年 | 1106篇 |
2002年 | 1097篇 |
2001年 | 938篇 |
2000年 | 905篇 |
1999年 | 809篇 |
1998年 | 753篇 |
1997年 | 722篇 |
1996年 | 672篇 |
1995年 | 645篇 |
1994年 | 630篇 |
1993年 | 530篇 |
1992年 | 491篇 |
1991年 | 516篇 |
1990年 | 507篇 |
1989年 | 463篇 |
1988年 | 443篇 |
1987年 | 546篇 |
1986年 | 451篇 |
1985年 | 601篇 |
1984年 | 660篇 |
1983年 | 599篇 |
1982年 | 564篇 |
1981年 | 485篇 |
1980年 | 467篇 |
1979年 | 428篇 |
1978年 | 418篇 |
1977年 | 397篇 |
1976年 | 350篇 |
1975年 | 342篇 |
1974年 | 346篇 |
1973年 | 384篇 |
1972年 | 243篇 |
排序方式: 共有10000条查询结果,搜索用时 203 毫秒
951.
A multimetric fish index, the Estuarine Fish Community Index (EFCI) of Harrison and Whitfield (2004), was applied to data
collected for 190 South African estuaries. Estuaries spanned three biogeographic regions and included three distinct estuarine
typologies. The EFCI is based on 14 metrics or measures that represent four broad fish community attributes: species diversity
and composition, species abundance, nursery function, and trophic integrity. Metric reference conditions and scoring criteria
were developed separately for each estuary type within each zoogeographic region. The final EFCI was applied to each estuary
by comparing its fish community with the appropriate reference. Index values ranged between 18 (very poor) and 66 (very good).
A comparison of the EFCI with independent measures of estuarine condition revealed that the index was able to effectively
differentiate between poor and good quality sites. Applying the EFCI to estuaries in which multiple samples were taken also
showed that the index is reproducible. The EFCI is both a robust and sensitive method for assessing the ecological condition
of estuarine systems; it is also an effective communication tool for converting ecological information into an easily understood
format for managers, policy makers, and the general public. 相似文献
952.
An ephemeral estuarine turbidity maximum (ETM) occurs at high water in the macrotidal Taf estuary (SW Wales, United Kingdom).
A new mechanism of ETM formation, due to resuspension and advection of material by flood tidal currents, is observed that
differs from classical mechanisms of gravitational circulation and tidal pumping. The flood tide advances across intertidal
sand flats in the main body of the estuary, progressively entraining material from the rippled sands. Resuspension creates,
a turbid front that has suspended sediment concentrations (SSC) of about 4,000 mg I−1 by the time it reaches its landward limit which is also the landward limit of salt penetration. This turbid body constitutes
the ETM. Deposition occurs at high slack water but the ETM retains SSC values up to 800 mg I−1, 1–2 orders of magnitude greater than ambient SSC values in the river and estuarine waters on either side. The ETM retreats
down the estuary during the ebb; some material is deposited thinly across emergent intertidal flats and some is flushed out
of the estuary. A new ETM is generated by the next flood tide. Both location and SSC of the ETM scale on Q/R3 where Q is tidal range and R is river discharge. The greatest expression of the ETM occurs when a spring tide coincides with
low river discharge. It does not form during high river discharge conditions and is poorly developed on neap tides. Particles
in the ETM have effective densities (120–160 kg m−3) that are 3–4 times less than those in the main part of the estuary at high water. High chlorophyll concentrations in the
ETM suggest that flocs probably originate from biological production in the estuary, including production on the intertidal
sand flats. 相似文献
953.
954.
The effects of advection, dispersion, and biological processes on nitrogen and phytoplankton dynamics after a storm event
in December 2002 are investigated in an estuary located on the northern New South Wales coast, Australia. Salinity observations
for 16 d after the storm are used to estimate hydrodynamic transports for a one-dimensional box model. A biological model
with nitrogen limited phytoplankton growth, mussel grazing, and a phytoplankton mortality term is forced by the calculated
transports. The model captured important aspects of the temporal and spatial dynamics of the bloom. A quantitative analysis
of hydrodynamic and biological processes shows that increased phytoplankton biomass due to elevated nitrogen loads after the
storm was not primarily regulated by advection or dispersion in spite of an increase in river flow from <1 to 928×103 m3 d−1. Of the dissolved nitrogen that entered the surface layer of the estuary in the 16 d following the storm event, the model
estimated that 28% was lost through exchange with the ocean or bottom layers, while 15% was removed by the grazing of just
one mussel species,Xenostrobus securis, on phytoplankton, and 50% was lost through other biological phytoplankton loss processes.X. securis grazing remained an important loss process even when the estimated biological parameters in the model were varied by factors
of ± 2. The intertidal mangrove pneumatophore habitat ofX. securis allows filtering of the upper water column from the lateral boundaries when the water column is vertically stratified, exerting
top-down control on phytoplankton biomass. 相似文献
955.
Pensacola Bay, Florida, was in the strong northeast quadrant of Hurricane Ivan when it made landfall on September 16, 2004
as a category 3 hurricane on the Saffir-Simpson scale. We present data describing the timeline and maximum height of the storm
surge, the extent of flooding of coastal land, and the magnitude of the freshwater inflow pulse that followed the storm. We
computed the magnitude of tidal flushing associated with the surge using a tidal prism model. We also evaluated hurricane
effects on water quality using water quality surveys conducted 20 and 50 d after the storm, which we compared with a survey
14 d before landfall. We evaluated the scale of hurricane effects relative to normal variability using a 5-yr monthly record.
Ivan's 3.5 m storm surge inundated 165 km2 of land, increasing the surface area of Pensacola Bay by 50% and its volume by 230%. The model suggests that 60% of the Bay's
volume was flushed, initially increasing the average salinity of Bay waters from 23 to 30 and lowering nutrient and chlorophylla concentrations. Additional computations suggest that wind forcing was sufficient to completely mix the water column during
the storm. Freshwater discharge from the largest river increased twentyfold during the subsequent 4 d, stimulating a modest
phytoplankton bloom (chlorophyll up to 18 μg l−1) and maintaining hypoxia for several months. Although the immediate physical perturbation was extreme, the water quality
effects that persisted beyond the first several days were within the normal range of variability for this system. In terms
of water quality and phytoplankton productivity effects, this ecosystem appears to be quite resilient in the face of a severe
hurricane effect. 相似文献
956.
957.
958.
The release of methane from crater sites following meteorite impact is a possible consequence of the thermal alteration of organic matter, or tapping of reservoired gas of biogenic or abiogenic origin. At least the latter is feasible on Mars. Methane and higher hydrocarbons are susceptible to polymerization and precipitation by radioactive minerals. Where such minerals are present in impact target rocks, the craters can be a preferred site for carbon concentration, and the formation of complex organic molecules. On icy bodies, such as Titan and Europa, methane released by impact could be a fuel for prebiotic chemistry involving other forms of irradiation. 相似文献
959.
Darren F. Mark John Parnell Simon P. Kelley Sarah C. Sherlock 《Journal of Geochemical Exploration》2006,89(1-3):259
Integration of fluid inclusion analysis with high spatial resolution Ar–Ar dating of K-feldspar cements has been used to resolve and reconstruct palaeo-fluid flow. Fluid inclusion analysis allows discrimination of distinct cement phases, thereby identifying discrete episodes of fluid flow. Ar–Ar dating of the same cements via high spatial resolution laserprobe establishes absolute age constraints on the framework previously constructed. Integration of these two datasets yields temperature–composition–time data. 相似文献
960.