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991.
依据宁夏环境地质、气候条件、水氟含量、营养状况等诸多因素,分析了地方性氟中毒的流行特征。在回顾地方性氟病防治研究进展基础上,指出只探求毒理剂量的氟引起机体微观变化,缺乏多学科间交叉印证,难以进一步阐明发病机理。应遵循生态学原理,从环境与人类关系的作用、反作用和相互作用出发,采取相应的治本、干预、抗氟的防治措施,可取得标本兼治的效果。
相似文献
992.
993.
Phytoplankton and its relationships with physical and chemical variables were analysed over a 2-year period in Hueihue and Linao Bays (southern Chile). Samples were collected on a monthly basis from May 1991 to May 1993. The growth rate of a single stock of hatchery-produced oysters (Ostrea chilensis) distributed between these two bays was also monitored monthly. The growth rate of oysters maintained at Linao Bay (site associated with a fish farm) was significantly higher, at every depth, compared with the oysters maintained at Hueihue Bay. Temperature and salinity values and their annual fluctuations were similar at both locations, presenting higher variations at the surface (1 m) due to the influence of air temperature and local precipitation than at depth. However, significant differences in phytoplankton abundance and composition were found between the two locations. The phytoplankton community was characterized by high biomass values during spring and summer, the dominance of diatoms and an inverse relationship between temperature and species diversity. However, phytoplankton cell abundance, biovolume and chlorophyll a concentration were significantly higher at Linao at every depth sampled. Cell abundance ranged between 4.49 × 104 and 7.45 × 106 cells · l?1 in Hueihue and between 6.48 × 104 and 8.71 × 106 cells ·1?1 in Linao. The influence of temperature on chlorophyll a concentration was significant except at 8 m at both locations. The instantaneous oyster growth rate was found to be positively correlated with temperature, chlorophyll a concentration and particulate organic matter at both locations. A significant negative relationship between oyster growth and amount of particulate inorganic matter was found; this confirms the important role played by seston composition in oyster growth. The demonstration of variation in oyster growth rate associated with differences in food availability between the two locations provides insight into the ecological role played by fish farms in southern Chile. The results suggest a strong link between food availability and oyster growth. 相似文献
994.
A histological method is described allowing the identification of macro-algal fragments from stomach contents of fish. The rhodophytes Porphyra endiviifolium and Neuroglossum ligulatum were determined from the stomach contents of the Antarctic fish Notothenia coriiceps . The results show P. endiviifolium to be a more important part in the diet of this fish than previously expected. On the basis of the depth distribution of the identified algae it is suggested that N. coriiceps can migrate to very shallow waters for feeding. 相似文献
995.
996.
区域持续农业的景观生态研究 总被引:9,自引:1,他引:8
从提出背景,概念内涵,本质特性以及实践内容来看,持续农业具有鲜明的生态内涵,研究离不开生态学的指导;从系统主体,出发点、尺度、动态来看,区域持续农业在更大程度上属于景观生态学的研究范畴,因此 生态学可作为区域持续珠理论基础,而主观生态学应用方向的景观生态规划便可实现区域持续农业的空间途径,最后,文章以中国北方农牧过渡北山地的景观利用方向进行案例实践。 相似文献
997.
We interpret the historical activity of comet 55P/Tempel–Tuttle in terms of the observed characteristics of present-day short
period comets. In this respect, it is now realized that such comets are liable to undergo significant outburst and mantle
loss events at intervals separated by of order a few hundred years. On this basis one might well expect comet 55P/Tempel–Tuttle
to have undergone several outbursts since its earliest sighing in 1366. The limited absolute magnitude data available for
55P/Tempel–Tuttle is not inconsistent with the suggestion that the comet underwent outbursts during its 1699 and 1865 perihelion
returns. If the outbursts of comet 55P/Tempel–Tuttle are interpreted in terms of mantle loss events then the bright, electrophonic
sound producing fireballs reported during the great Leonid meteor storm of 1833 may have been due to the Earth sampling mantle
material ejected during the outburst of 1699.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
998.
Ages and elevations of ash layers correlated with late Tertiary and Pleistocene eruptions in the western US and present stream elevations are used to calculate net rates of incision by streams in the middle reaches of the Arkansas River basin in the south central US. The mean of the 23 measurements of rate of stream incision in the study area is 4.2±2.7 cm/ky with a range of 0.6 to 9.7 cm/ky. Major influences on rate of stream incision in the study area include the arid to semi-arid climate of the region, the type of material being incised by streams, stream captures, and salt dissolution in the bedrock that underlies the region. Rates of incision exceeded rates of basin filling but significant deposits of unconsolidated late Cenozoic sediments occur in the study area. Basins of streams that have incised the slowest since the late Tertiary contain the thickest and most extensive amounts of unconsolidated Quaternary sediments. Rates of incision by streams in the study area are similar to or slower than rates reported for streams elsewhere in the US and the world. Streams in mountainous regions and areas affected by rapid uplift have incised at rates orders of magnitude faster than streams in the study area. 相似文献
999.
Hydrological exchange and subsurface water chemistry in streams varying in salinity in south-western Australia 总被引:1,自引:0,他引:1
Many streams in Westrn Australia are naturally saline. In others, especially in the south-western corner, land-clearance and
other human activities in the catchment have accelerated rates of salinisation of surface and groundwater. Trends in surface
water salinity are well-documented but the extent of penetration of saline stream water into the sediments has been little
studied. As many of these streams have porous sandy beds and their flows may derive from groundwater, hydrologic exchange
patterns between surface water and subsurface hyporheic water were hypothesised to govern the water chemistry of such rivers.
We predicted high rates of hydraulic conductivity, leading to a close relationship between surface and subsurface (to a depth
of 50 cm) salinity, and to a lesser extent, pH and dissolved oxygen. Where surface and hyporheic water differed in salinity,
other chemical differences were hypothesised to be similarly marked, perhaps resulting from disjunct shallow subsurface aquifers.
Triplicate wells were sampled from upwelling and downwelling zones of thirteen streams ranging in salinity from ca. 0.2 to
18 g L−1. Despite the seemingly-porous sandy beds at many sites, subsurface water chemistry only 20–40 cm below the bed sometimes
differed markedly from surface water. For example, hyporheic water was only one-fifth the salinity of surface water at some
saline sites (e.g., the Tone River) or 20 per cent more saline in streams with fresh surface water (e.g., the Weld River).
At some sites of intermediate salinity (e.g., the Warren River), subsurface water was up to three times fresher than surface
or downwelling water. Percentage saturation of dissolved oxygen in the hyporheic water was consistently low (<40%) whereas
pH was more acidic than surface water, presumably due to microbial activity. Vertical hydraulic conductivity may be limited
by layers of fine sediments and clays, implying that the meso-scale (1–100 cm) hydrological dynamics within the hyporheic
zones of these rivers are more complex than their sandy beds would indicate. Assumptions of ecosystem dynamics in saline streams
must be tempered by an understanding of hyporheic salinities as subsurface fresher water may support microbial and faunal
assemblages excluded from the surface benthos by high salinity. In saline streams, as in fresh ones, the hyporheic zone is
an important component of the stream ecosystem and equally prone to disruption by human activities. 相似文献
1000.