全文获取类型
收费全文 | 22901篇 |
免费 | 213篇 |
国内免费 | 188篇 |
专业分类
安全科学 | 619篇 |
废物处理 | 813篇 |
环保管理 | 3229篇 |
综合类 | 5227篇 |
基础理论 | 5389篇 |
环境理论 | 10篇 |
污染及防治 | 5820篇 |
评价与监测 | 1206篇 |
社会与环境 | 871篇 |
灾害及防治 | 118篇 |
出版年
2021年 | 154篇 |
2018年 | 256篇 |
2017年 | 246篇 |
2016年 | 395篇 |
2015年 | 315篇 |
2014年 | 431篇 |
2013年 | 1687篇 |
2012年 | 588篇 |
2011年 | 816篇 |
2010年 | 624篇 |
2009年 | 761篇 |
2008年 | 828篇 |
2007年 | 924篇 |
2006年 | 823篇 |
2005年 | 642篇 |
2004年 | 681篇 |
2003年 | 708篇 |
2002年 | 626篇 |
2001年 | 837篇 |
2000年 | 599篇 |
1999年 | 399篇 |
1998年 | 254篇 |
1997年 | 254篇 |
1996年 | 276篇 |
1995年 | 317篇 |
1994年 | 321篇 |
1993年 | 285篇 |
1992年 | 297篇 |
1991年 | 310篇 |
1990年 | 302篇 |
1989年 | 301篇 |
1988年 | 282篇 |
1987年 | 259篇 |
1986年 | 221篇 |
1985年 | 243篇 |
1984年 | 254篇 |
1983年 | 265篇 |
1982年 | 264篇 |
1981年 | 238篇 |
1980年 | 218篇 |
1979年 | 229篇 |
1978年 | 209篇 |
1977年 | 185篇 |
1976年 | 187篇 |
1975年 | 175篇 |
1974年 | 215篇 |
1973年 | 169篇 |
1972年 | 174篇 |
1970年 | 152篇 |
1967年 | 169篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
881.
The reproductive ecology of two eastern Pacific zooxanthellate coral species was examined as part of a continuing series of studies relating bleaching/mortality events caused by the El Niño–Southern Oscillation disturbance, and is described for study sites in Costa Rica, Panamá, and the Galápagos Islands (Ecuador). This study deals with the sibling agariciid species Pavona varians and Pavona sp.a over a 13?yr period (1985 to 1997). Both Pavona species are broadcast-spawners with some gonochoric, but mostly sequential hermaphroditic colonies. Minimum colony sizes (and ages) at first reproduction were 5?cm (5?yr) and 3?cm (2 to 3?yr), respectively, in P. varians and Pavona sp.a. In the Panamá and Galápagos populations, gonochoric colonies spawn eggs or sperm at least monthly. Six fecundity attributes were not significantly different in the two species, but the eggs of P. varians are white to beige and positively buoyant, and those of Pavona sp.a are dark green and neutrally to negatively buoyant. Eggs of both species lack zooxanthellae. Both species are reproductively active year-round, with maximum activity in the dry season in the nonupwelling Gulf of Chiriquí, and in the wet season in the upwelling Gulf of Panamá. Spawning is predominantly during full moon, and possibly also at new moon at most study sites. Spawning in P. varians and Pavona sp.a is 12?h out of phase, with the former species spawning ~1?h before sunrise and the latter about 1?h after sunset. The fecundity of Pavona spp. at Caño and the Galápagos Islands was much greater (19?900 to 27?900 eggs cm?2?yr?1) than at all Panamá sites (14?800 to 19?800 eggs cm?2?yr?1). Intraspecific crosses in both species resulted in swimming planula larvae after 25 to 36?h. Recruitment of P. varians was highest in Panamá, moderate in Costa Rica, and nil in the Galápagos Islands, matching, respectively, the contributions of P. varians to the pre-1982/1983 El Niño coral-population abundances in these areas. Recruitment success of P. varians at Uva Island was significantly related to maximum monthly positive sea surface-temperature (SST) anomalies that occurred in the year preceding recruitment over the period 1982 to 1996; recruitment failed when SST anomalies exceeded 1.6 to 1.9?C° during the severe ENSO events of 1982/1983 and 1997/1998. 相似文献
882.
The variability of magnesium, strontium and calcium concentrations in the two skeleton types (sclerites and axis) of the
red coral (Corallium rubrum) was assessed by using X-ray fluorescence spectroscopy and microprobe analysis as a prerequisite for their use as ecological
indicators. Axis cross sections showed light and dark circular growth bands corresponding to fast and slow growth, respectively.
Using microprobe analyses the Mg and Sr concentrations ranged from 2.8 to 3.0% and from 0.21 to 0.29%, respectively, in the
sclerites and from 2.4 to 3.0% and from 0.1 to 0.28%, respectively, in the axis. Mg/Ca and Sr/Ca ratios varied within sclerites
(average, 12% for both ratios) and within the axis of single colonies (average, 20% for Mg/Ca and 48% for Sr/Ca). Sr/Ca ratios
in the axis were lower in fast-growing branch tips than in older, more basal parts of the colonies, whereas the Mg/Ca ratio
did not differ significantly between colony regions. Mg/Ca and Sr/Ca ratios in the axis decreased significantly with depth,
and we estimated an increase of the Mg/Ca ratio of 0.004–0.006 per degree Celsius. In all depth layers, Sr/Ca ratios showed
a significant direct relationship with skeleton density in axis cross sections, whereas there was no significant relationship
for Mg/Ca. Overall, our data indicate that temperature promotes the incorporation of Mg in C. rubrum as in other calcite skeletons, whereas Sr concentrations are inversely related to growth rate. This preliminary study suggests
that Mg and Sr concentrations in the axis of the red coral have a strong potential as ecological indicator for temperature
and growth rate.
Received: 3 March 2000 / Accepted: 30 August 2000 相似文献
883.
Strontium (Sr) and calcium (Ca) contents in the otoliths of yellow and silver European eels [Anguilla anguilla (L.)] collected from coastal waters of the Baltic Sea and a freshwater lake in Sweden were examined by wavelength dispersive
X-ray spectrometry with an electron microprobe. The mean Sr/Ca ratios from elver check to otolith edge were significantly
higher for the eels from coastal waters (5.39 ± 1.09‰) than for those from the lake (0.71 ± 0.89‰). The evidence indicates
that European eels in the Baltic Sea do not necessarily migrate into freshwater streams during the growth phase.
Received: 30 September 1999 / Accepted: 6 April 2000 相似文献
884.
F. Maltagliati A. P. Peru M. Casu F. Rossi C. Lardicci M. Curini-Galletti A. Castelli 《Marine Biology》2000,136(5):871-879
The genetic relationships between morphologically indistinguishable marine and brackish populations of Syllis gracilis Grube, 1840 (Polychaeta: Syllidae) were studied by means of allozyme electrophoresis. Samples of S. gracilis from marine coastal and brackish-water habitats were examined for variation at 13 presumptive loci. In addition, a sample
of the closely related species S. prolifera (Krohn, 1852) was analysed. Five loci were multiallelic in at least one population of S. gracilis and eight loci in S. prolifera. Low to moderate levels of within-population genetic variability were found, with average expected heterozygosity values
ranging from H = 0.068 (±0.043 SE) to 0.187 (±0.069 SE) in the populations of S. gracilis; higher values were found in S. prolifera (H = 0.325 ± 0.076). The presence of various private alleles indicated a marked genetic divergence among populations of S. gracilis, with Nei's genetic distances ranging from D = 0.000 to 0.833 and a highly significant F
ST value. Furthermore, evidence for strong genetic heterogeneity between two sympatric marine populations was found. UPGMA cluster
analysis and multidimensional scaling pointed out a clear genetic divergence between brackish and marine populations. At least
two genetically divergent entities occurred in marine and brackish habitats. This could be due to local adaptation of individuals
coming from marine populations to brackish habitats, but more presumably to the occurrence of a species complex within S. gracilis.
Received: 6 June 1999 / Accepted: 7 February 2000 相似文献
885.
L. Zane L. Ostellari L. Maccatrozzo L. Bargelloni J. Cuzin-Roudy F. Buchholz T. Patarnello 《Marine Biology》2000,136(2):191-199
Meganyctiphanes norvegica (M. Sars)is a pelagic crustacean that plays a key role in marine food webs of North Atlantic Ocean and marginal seas. We
studied eight population samples collected in the European Atlantic and Mediterranean Sea. By means of single strand conformation
polymorphism analysis (SSCP) and direct sequencing, we investigated a segment of 158 base pairs of the mitochondrial gene
coding for the subunit 1 of NADH dehydrogenase. We found 12 sequence variants among the 385 individuals studied. Analysis
of molecular variance (AMOVA) showed that 14.75% of the total genetic variability was explained by differences between populations,
thus indicating absence of panmixia for these populations. Pairwise comparisons revealed three distinct genetic pools: the
first one represented by Cadiz Bay, the second one by the Ligurian Sea, and the third one included all the NE Atlantic samples.
We also investigated one population from the Alboran Sea (within the Mediterranean basin, east of the Strait of Gibraltar).
This population was found to be genetically intermediate between the NE Atlantic samples and the Ligurian sample, suggesting
that the restriction to the gene flow is not associated with the Strait of Gibraltar, but possibly with the Oran–Almeria oceanographic
front. The present work indicates that M. norvegica, although endowed with a high dispersal capacity because of its pelagic habit, can develop separate breeding units inside
the same oceanic basin (the Atlantic). Furthermore, the Ligurian sample should be considered as a distinct evolutionary entity,
separated from the Atlantic population.
Received: 2 May 1999 / Accepted: 26 November 1999 相似文献
886.
E. A. Perry G. B. Stenson S. E. Bartlett W. S. Davidson S. M. Carr 《Marine Biology》2000,137(1):53-58
Harp seals (Pagophilus groenlandicus Erxleben, 1777) comprise three populations based upon whelping areas in the Greenland Sea, White Sea, and Northwest Atlantic.
The last comprises two subpopulations, one whelping in the Gulf of St. Lawrence (“Gulf ”) and one on the pack ice of the southern
Labrador/northern Newfoundland coastal shelf (“Front”). A total of 40 female seals from the four whelping areas were collected
during the 1990 and 1992 whelping seasons. DNA sequence variation was examined in a 307 bp region of the mitochondrial cytochrome
b gene. Eleven variable nucleotide positions defined 13 genotypes: a significant fraction of the genotypic variance (F
ST=0.12, or 0.09 as measured by Weir's coancestry coefficient θ) is attributable to differentiation between Northwest and Northeast
Atlantic populations. There was no significant differentiation between the two whelping areas in the Northwest Atlantic, or
between the Greenland Sea and White Sea. These findings suggest significant reproductive isolation exists between trans-Atlantic
breeding populations.
Received: 18 January 1999 / Accepted: 22 February 2000 相似文献
887.
Relatively few insects have invaded the marine environment, and only five species of sea skaters, Halobates Eschscholtz (Hemiptera: Gerridae), have successfully colonized the surface of the open ocean. All five species occur in the
Pacific Ocean, H. germanus White also occurs in the Indian Ocean, whereas H. micans Esch- scholtz is the only species found in the Atlantic Ocean. We sequenced a 780 bp long region of the mitochondrial cytochrome
oxidase subunit I gene (COI) for a total of 66 specimens of the five oceanic Halobates species. Our purpose was to investigate the genetic variation within species and estimate the amount of gene flow between
populations. We defined 27 haplotypes for H. micans and found that haplotype lineages from each of the major oceans occupied by this species are significantly different, having
sequences containing five to seven unique base substitutions. We conclude that gene flow between populations of H. micans inhabiting the Atlantic, Pacific, and Indian Ocean is limited and hypothesize that these populations have been separated
for 1 to 3 million years. Similarly, there may be limited gene flow between H. germanus populations found in the Pacific and Indian Ocean and between H. sericeus populations inhabiting the northern and southern parts of the Pacific Ocean. Finally, we discuss our findings in relation
to recent hypotheses about the influence of oceanic diffusion on the distribution and population structure of oceanic Halobates spp.
Received: 29 July 1999 / Accepted: 23 November 1999 相似文献
888.
Western sandpiper (Calidris mauri) predation was examined by concurrent experiments and direct observations of foraging behaviour on high intertidal mudflats
of the Fraser River estuary, British Columbia. Western sandpipers foraged by either “pecking” on the surface (64% of observational
time) or probing into sediment (29%). The first experiment (probe-mark method) consisted of collecting small-volume cores
(21.2 cm3) of probed (experimental) and non-probed (control) sediment on the tidal flat, following a 22.5-min feeding period. The second
experiment (exclosure method) involved deploying exclosures immediately prior to the feeding period and subsequent collection
of cores from inside (control) and outside (experimental) the exclosures. Sediment cores were analysed for both macrofaunal
and meiofaunal size fractions. Comparisons between macro- and meiofaunal invertebrate densities in experimental and control
sediments revealed significant differences, attributed to shorebird predation, for both experiments. The probe-mark experiment
detected the removal of large infaunal polychaetes (∼ 20 mm), while the exclosure experiment showed depletion of epifaunal
harpacticoid copepods (0.063–0.5 mm). Predation on macrofaunal cumaceans was detected in both experiments. Invertebrates selected
by western sandpipers neither fell within traditional infaunal size classifications (macro- vs. meiofauna; 500 μm delineation)
nor corresponded to the highest densities of taxa. Rather, inference from experimental results and observations is that western
sandpipers forage in two modes, by: (1) surface gleaning of epibenthic copepods and cumaceans in the macro- and meiofaunal
size ranges and (2) selective probing for larger infauna, such as polychaetes. These findings were facilitated by the combination
of methodologies employed.
Received: 29 December 1999 / Accepted: 11 September 2000 相似文献
889.
负载型固体超强酸催化剂上的烷基化反应 总被引:3,自引:0,他引:3
本文利用Pt/SO^2-4/ZrO2,Pt/WO3/ZrO2负载于大比表面Al2O3固体强酸催化剂,进行异丁烷和丁烯的烷基化反应。 相似文献
890.
Maternal investment in rhesus macaques (Macaca mulatta): reproductive costs and consequences of raising sons 总被引:1,自引:0,他引:1
Maternal investment in offspring is expected to vary according to offspring sex when the reproductive success of the progeny
is a function of differential levels of parental expenditure. We conducted a longitudinal investigation of rhesus macaques
to determine whether variation in male progeny production, measured with both DNA fingerprinting and short tandem repeat marker
typing, could be traced back to patterns of maternal investment. Males weigh significantly more than females at birth, despite
an absence of sex differences in gestation length. Size dimorphism increases during infancy, with maternal rank associated
with son’s, but not daughter’s, weight at the end of the period of maternal investment. Son’s, but not daughter’s, weight
at 1 year of age is significantly correlated with adult weight, and male, but not female, weight accounts for a portion of
the variance in reproductive success. Variance in annual offspring output was three- to fourfold higher in males than in females.
We suggest that energetic costs of rearing sons could be buffered by fetal delivery of testosterone to the mother, which is
aromatized to estrogen and fosters fat accumulation during gestation. We conclude that maternal investment is only slightly
greater in sons than in daughters, with mothers endowing sons with extra resources because son, but not daughter, mass has
ramifications for offspring sirehood. However, male reproductive tactics supersede maternal investment patterns as fundamental
regulators of male fitness.
Received: 23 July 1999 / Received in revised form: 23 February 2000 / Accepted: 13 March 2000 相似文献