首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   99226篇
  免费   1885篇
  国内免费   896篇
测绘学   2461篇
大气科学   7041篇
地球物理   19927篇
地质学   34998篇
海洋学   8807篇
天文学   22488篇
综合类   265篇
自然地理   6020篇
  2022年   506篇
  2021年   950篇
  2020年   1054篇
  2019年   1128篇
  2018年   2485篇
  2017年   2288篇
  2016年   2942篇
  2015年   1701篇
  2014年   2802篇
  2013年   5247篇
  2012年   3019篇
  2011年   4101篇
  2010年   3543篇
  2009年   4756篇
  2008年   4257篇
  2007年   4102篇
  2006年   3866篇
  2005年   3153篇
  2004年   3102篇
  2003年   2925篇
  2002年   2761篇
  2001年   2481篇
  2000年   2369篇
  1999年   1922篇
  1998年   2022篇
  1997年   1945篇
  1996年   1590篇
  1995年   1593篇
  1994年   1387篇
  1993年   1291篇
  1992年   1224篇
  1991年   1143篇
  1990年   1215篇
  1989年   1102篇
  1988年   1015篇
  1987年   1233篇
  1986年   1051篇
  1985年   1302篇
  1984年   1417篇
  1983年   1309篇
  1982年   1252篇
  1981年   1170篇
  1980年   1033篇
  1979年   972篇
  1978年   955篇
  1977年   908篇
  1976年   854篇
  1975年   805篇
  1974年   807篇
  1973年   786篇
排序方式: 共有10000条查询结果,搜索用时 546 毫秒
991.
Polycyclic aromatic hydrocarbons (PAH) were measured in mussels (Mytilus trossulus) collected between 1990 and 2002 from 11 sites on the shores of Prince William Sound (PWS), Alaska, that were heavily oiled by the 1989 Exxon Valdez oil spill (EVOS). This study, utilizing the methods of the NOAA Status and Trends Mussel Watch Program, found that concentrations of PAH released from spill remnants have decreased dramatically with time and by 2002 were at or near the range of total PAH (TPAH) of 3-355 ng/g dry weight obtained for mussels from unoiled reference sites in PWS. Time-series TPAH data indicate a mean TPAH half-life in mussel tissues of 2.4 years with a range from 1.4 to 5.3, yielding an annual mean loss of bioaccumulated TPAH of 25%. The petroleum-derived TPAH fraction in mussel tissues has decreased with time, reflecting the decreasing release of EVOS residues in shoreline sediments. These results show that PAH from EVOS residues that remain buried in shoreline sediments after the early 1990s are in a form and at locations that have a low accessibility to mussels living in the intertidal zone.  相似文献   
992.
This paper discusses the results obtained through numerical modelling of the annual course of circulation and thermohaline field evolution in the tropical Atlantic under the impact of real wind forcing (from March 1982 to March 1983) and compares them with the calculation of seasonal variability induced by the climatic wind. Changes in the pattern of the equatorial currents and related temperature fields induced by the specific peculiarities in the wind field are analysed.Translated by Vladimir A. Puchkin.  相似文献   
993.
Coastal and marine areas the world over provide food, transportation, recreation, and energy resources to increasing numbers of people each year. As demands for these resources rise, the potential for user conflicts is radically heightened. This situation can be avoided or counteracted by instigating proactive multiple use planning. Multiple use zoning plans can only exist in a concrete management framework: marine and coastal protected areas provide just such a foundation. Nature-based or ecotourism can be encouraged in coastal protected areas aimed at achieving sustainability. Well-planned tourism provides economic and political incentives for management and for conservation, and may bring additional benefits to local communities and regional economies. Examples where nature-based tourism has been or is becoming successfully integrated into multiple use planning can be found in Quintana Roo, Mexico; the Lesser Antilles; and Australia, among other areas.  相似文献   
994.
Glauconitic minerals range from green smectite to 10Å glauconite, occur mainly in peloids (most of which were fecal pellets), and apparently were produced largely by neoformation replacing several kinds of substrate. Chamositic minerals, berthierine and chamosite, occur in Recent and ancient peloids, but mostly in ancient ooids, and they developed by alteration of a precursor Al-rich clay mineral. Ooid sheaths were built by rolling of cores on ue sea floor. Glauconitic greensands and chamositic oolitic ironstones are condensed sequences deposited as sandwaves during long periods of reduced influx of sediment. Some are associated with hardgrounds.  相似文献   
995.
The aim of this study was to compare the biological actions of selected chlorinated biphenyls (CB's) (with different degrees of chlorine substitution), using the sea urchin test system, for their ability to affect fertilisation, embryogenesis and mitotic activity. Sea urchin embryos were exposed to the test agents throughout larval development or for shorter intervals (e.g. before or after hatching). Sperm or eggs were exposed before fertilisation, thereafter the contaminants were removed from embryo cultures. The results showed maximal activities from medium-chlorinated (tetra- and hexachloro-substituted) compounds, as compared to low- and high-chlorinated biphenyls (mono-, di- and octachloro-substituted), at nominal levels ranging from 10−6m to 10−5m. This held true for the different parameters and treatment schedules. Developmental defects were only detected following post hatching exposure of embryos to CB's, whereas pre-hatching exposure failed to reveal any embryological or cytogenetic abnormalities.  相似文献   
996.
A survey of the holdfast fauna of Laminaria hyperborea was carried out at thirty-five sites around the UK, mostly along the North Sea and south coasts.Data from different age classes of holdfast from a polluted and unpolluted site indicate that volume of holdfast space is a most important measurement of sample size, a minimum size of sample being one litre and, preferably, three.Gradients of increased heavy metals were defined down the North Sea and west coasts and, by utilising measurements of kelp penetration, water clarity gradients were determined as increasing northwards on the west coast and both north and southwards on the North Sea coast from a point near Tyne and Wear county. Analysis of holdfast fauna showed numerous gradients of total numbers, species and diversity which correlate closely with the heavy metal and water clarity gradients, or with undetermined parameters analogous to latitude and longitude. Where there were increases of heavy metals or decreases of water clarity, various reciprocal relationships between suspension feeders and all other trophic groups were commonly observed. In the North Sea numbers of suspension feeders increased while numbers of all other trophic groups declined. On the west coast a heavy metal increase correlated with a decreasing species richness of all trophic categories except suspension feeders, while with total numbers only suspension feeders showed an increase. On the south coast transect neither gradient existed, and only here did all trophic categories covary, except at one very polluted site where the same reciprocal relationship was again indicated.In terms of species similarity the North Sea showed a greater pool of common species than occurred between sites of other regions and between North Sea sites with other regions, despite the widest range of heavy metal levels and wate clarity that existed there. Between-site similarity appeared to be much more dependent on geographical proximity than on the two measured environmental factors. It is suggested that effects of clarity-reduction can be measured by trophic analysis more successfully than by species presence or absence, and that changes of holdfast fauna along the North Sea coast are due largely to pollution.  相似文献   
997.
The bacterial populations of mangrove swamps of Killai backwaters (11°21–11°29N, 79°46–79°50E, South India) were studied during August 1968 (Pre monsoon period) and December (post monsoon period). The presence of these groups such as agar digesters, algin digesters, cellulose digesters, sulphate reducers etc., bring about transformation of organic matter in the mangrove swamps. The presence of denitrifiers in mangrove swamps and in association with the molluscs may bring about the precipitation of calcium carbonate by removing the acid radicals such as sulphate and nitrite, increasing alkalinity. The luminiscent bacteria such asVibrio andAeromonas were also isolated in mangrove swamps of Killai backwaters. The iron bacteria likeLeptothrix sp. andGallionella sp. were also isolated from mangrove swamps of Killai backwaters.  相似文献   
998.
999.
An automatic sample changer based upon a commercially available 20-port rotary valve has been developed for use in automated seawater nutrient analyses. All parts exposed to seawater are of stainless steel, Teflon®, or Kel-F®. The sampler has proven to be rugged and efficient in both shore and shipboard laboratories.  相似文献   
1000.
The effect of dissolved petroleum hydrocarbons in the environment on phytoplankton biomass measured as chlorophyll a was studied near the oil tanker route in the southern Bay of Bengal. In the transect from 5° N, 77° E to 5° N, 87° E the concentrations of dissolved petroleum hydrocarbons were negatively correlated with phytoplankton biomass, whereas in the 0° N, 87° E to 1° N, 79° E transect they were positively correlated with phytoplankton biomass. The mean petroleum hydrocarbon concentrations in the two transects were 12·12 ± 4·67 μg litre−1 and 11·23 ± 4·5 μg litre−1, respectively.It is surmised that the effect of dissolved petroleum hydrocarbons on phytoplankton biomass varies depending on the nature rather than the quantity of petroleum hydrocarbons present. Culture studies with unialgal Nitzschia sp. in seawater collected from selected stations in the study area as well as in artificial seawater spiked with the water-soluble petroleum hydrocarbon fraction of light Arabian Crude support this.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号