全文获取类型
收费全文 | 30268篇 |
免费 | 481篇 |
国内免费 | 247篇 |
专业分类
安全科学 | 1079篇 |
废物处理 | 1510篇 |
环保管理 | 4164篇 |
综合类 | 4469篇 |
基础理论 | 7974篇 |
环境理论 | 8篇 |
污染及防治 | 7387篇 |
评价与监测 | 2129篇 |
社会与环境 | 2043篇 |
灾害及防治 | 233篇 |
出版年
2023年 | 140篇 |
2022年 | 262篇 |
2021年 | 323篇 |
2020年 | 201篇 |
2019年 | 274篇 |
2018年 | 480篇 |
2017年 | 485篇 |
2016年 | 763篇 |
2015年 | 568篇 |
2014年 | 893篇 |
2013年 | 2612篇 |
2012年 | 1037篇 |
2011年 | 1376篇 |
2010年 | 1141篇 |
2009年 | 1165篇 |
2008年 | 1436篇 |
2007年 | 1349篇 |
2006年 | 1229篇 |
2005年 | 1091篇 |
2004年 | 1051篇 |
2003年 | 1009篇 |
2002年 | 950篇 |
2001年 | 1110篇 |
2000年 | 787篇 |
1999年 | 493篇 |
1998年 | 363篇 |
1997年 | 384篇 |
1996年 | 397篇 |
1995年 | 465篇 |
1994年 | 398篇 |
1993年 | 343篇 |
1992年 | 377篇 |
1991年 | 357篇 |
1990年 | 327篇 |
1989年 | 322篇 |
1988年 | 296篇 |
1987年 | 241篇 |
1986年 | 247篇 |
1985年 | 247篇 |
1984年 | 267篇 |
1983年 | 256篇 |
1982年 | 268篇 |
1981年 | 221篇 |
1980年 | 165篇 |
1979年 | 181篇 |
1978年 | 162篇 |
1977年 | 133篇 |
1975年 | 137篇 |
1973年 | 167篇 |
1972年 | 142篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
931.
J.-I.?MiyazakiEmail author M.?Shintaku A.?Kyuno Y.?Fujiwara J.?Hashimoto H.?Iwasaki 《Marine Biology》2004,144(3):527-535
We examined phylogenetic relationships among three Bathymodiolus species in Japanese waters and Bathymodiolus spp. from the Manus Basin by two different approaches. Two-dimensional gel electrophoresis allowed us to compare 263–407 (average=318) proteins, giving comprehensive information on genetic distances among the species. The neighbor-joining tree presented two clusters: (1) B. japonicus and B. platifrons and (2) B. septemdierum and B. sp. Members of the first cluster contain methanotrophic endosymbiotic bacteria and members of the second cluster contain thioautotrophic endosymbionts. DNA sequencing of a fragment (415 bp) of mitochondrial cytochrome c oxidase subunit I (COI) provided a neighbor-joining tree with the same topology as that derived from protein analysis. Inspection of intraspecific variation in COI in B. japonicus and B. platifrons revealed no genetic differentiation between mussel populations of either species from cold-water seeps versus hydrothermal vents, suggesting high adaptability of these Bathymodiolus species to deep-sea chemosynthetic environments. Our results indicated genetic exchanges between mussels from distant localities, suggesting that a limited dispersal capability of the larvae is not the likely factor leading to speciation events in these Bathymodiolus species.Communicated by T. Ikeda, Hakodate 相似文献
932.
933.
934.
935.
Managing Individuals' Contributions to Maximize the Allelic Diversity Maintained in Small, Conserved Populations 总被引:2,自引:0,他引:2
Abstract: The maintenance of diversity is, from a genetic perspective, one of the key aims in a conservation program. Because the most widely used measure of genetic diversity is the expected heterozygosity of the population, or gene diversity (GD), most research has been devoted to finding optimal strategies for maximizing this parameter. Little attention has been paid, however, to the development of strategies to manage allelic diversity (AD), the number of alleles maintained in the population. Using computer simulations, we show that the strategies that maximize GD, by managing contributions from parents, keep levels of AD as high as strategies maximizing AD itself, for a wide range of situations including different numbers of molecular markers used and the possibility of evaluating a number of offspring per parent to make decisions. Because maximization of GD also minimizes levels of inbreeding, this should be the strategy of choice in any conservationprogram. 相似文献
936.
The distribution and genetic structure of many marine invertebrates in the North Atlantic have been influenced by the Pleistocene glaciation, which caused local extinctions followed by recolonization in warmer periods. Mitochondrial DNA markers are typically used to reconstruct species histories. Here, two mitochondrial markers [16S rDNA and cytochrome c oxidase I (COI)] were used to study the evolution of the widely distributed hydrozoan Obelia geniculata (Linnaeus, 1758) from the North Atlantic and the Pacific and, more specifically, in the context of North Atlantic phylogeography. Samples were collected from six geographic localities between 1998 and 2002. Hydroids from the North Atlantic, North Pacific (Japan), and South Pacific (New Zealand) are reciprocally monophyletic and may represent cryptic species. Using portions of the 16S rDNA and COI genes and the date of the last trans-Arctic interchange (3.1–4.1 million years ago), the first calibrated rate of nucleotide substitutions in hydrozoans is presented. Whereas extremely low substitution rates have been reported in other cnidarians, mainly based on anthozoans, substitution rates in O. geniculata are comparable to other invertebrates. Despite a life history that ostensibly permits substantial dispersal, there is apparently considerable genetic differentiation in O. geniculata. Divergence estimates and the presence of unique haplotypes provide evidence for glacial refugia in Iceland and New Brunswick, Canada. A population in Massachusetts, USA, appears to represent a relatively recent colonization event.Communicated by J.P. Grassle, New Brunswick 相似文献
937.
Reproductive period and size at maturity of the brown shrimp Farfantepenaeus californiensis were analyzed for correspondence to sea surface temperature (SST) in three zones of the Gulf of California. Mature females from fishing areas in the north (Puerto Peñasco), center (Guaymas), and south (Mazatlán), and monthly SST were examined. Average SST for 1983–2000 decreased from Mazatlán (26.2±0.2°C) to Puerto Peñasco (22.6±0.3°C). The seasonal variation in SST between coldest and warmest months was 7.8°C in Mazatlán and 11.4°C in Puerto Peñasco. The size of shrimps at maturity was inversely correlated to SST, increasing from Mazatlán (121 mm total length) to Puerto Peñasco (154 mm total length). The reproductive period near Mazatlán is year-round. Guaymas and Puerto Peñasco have one period of high intensity. We conclude that warmer water and low seasonal variability allow brown shrimp to reproduce more frequently, but reach smaller size at maturity.Communicated by P.W. Sammarco, Chauvin 相似文献
938.
Molecular genetic variation in tarpon (<Emphasis Type="Italic">Megalops atlanticus</Emphasis> Valenciennes) in the northern Atlantic Ocean 总被引:2,自引:0,他引:2
A.?L.?McMillen-JacksonEmail author T.?M.?Bert H.?Cruz-Lopez S.?Seyoum T.?Orsoy R.?E.?Crabtree 《Marine Biology》2005,146(2):253-261
The tarpon (Megalops atlanticus) is a highly valued game fish and occasional food fish in the eastern and western Atlantic Ocean. Tarpon have a high capacity for dispersal, but some regional biological differences have been reported. In this study we used two molecular genetic techniques—protein electrophoresis of nuclear DNA loci, and restriction fragment length polymorphism analysis of the mitochondrial DNA (mtDNA)—to assess this species population genetic structure in the eastern (coastal waters off Gabon and Sierra Leone, Africa) and western (coastal waters off Florida, Caribbean Sea) Atlantic Ocean north of the equator. Genetic differentiation was observed between tarpon from Africa and tarpon from the western Atlantic Ocean. A unique allele and haplotype, significant differences in allozyme allele and mtDNA haplotype frequencies between the African and western Atlantic samples, and significant FST analyses suggest that levels of gene flow between tarpon from these two regions is low. Among the western Atlantic Ocean collections, genetic diversity values and allele and haplotype frequencies were similar. AMOVA analyses also showed a degree of genetic relatedness among most of the western Atlantic Ocean collections: however, some significant population structuring was detected in the allozyme data. A regional jackknifed FST analysis indicated the distinction of the Costa Rica population from the other western Atlantic populations and, in pairwise analyses, FST values tended to be higher (i.e., genetic relatedness was lower) when the Costa Rican sample was paired with any of the other western Atlantic samples. These data suggest that Costa Rican tarpon could be partially isolated from other western Atlantic tarpon populations. Ultimately, international cooperation will be essential in the management of this species in both the eastern and western Atlantic Ocean.Communicated by P.W. Sammarco, Chauvin 相似文献
939.
This study delineated the mathematical forms for the reactions involved in the mass balance of As in cropland soils. Even
mathematically simplified, many model parameters are required to define the reactive processes involved. Example simulations
were conducted based on the range of parameter values and initial conditions derived from published literature. The outcomes
showed that the As inputs due to fertilizers and irrigation water caused total As content of the root zone to gradually increase
over time. The plant uptake and leaching were equally important as pathways for removal of the added As. In turn, the dissolution
kinetics of the mineral phase and the distribution coefficient of the adsorbed phase affected the availability of the As for
plant uptake and leaching. Parameters based on laboratory-derived data on the dissolution of As mineral phase, mineralization
and oxidation of As(III), and the As plant uptake however appeared to overestimate the As transformations in soils. While
the development of mathematical model was a straightforward process, its application to realistic situations was hindered
by difficulties of defining model parameter values with confidence. Current knowledge on the processes and reactions of As
transformation in the soil–plant system is inadequate to calibrate or validate the model. Studies are needed to understand
the kinetics of soil As mineral dissolution and precipitation and the dynamics of root growth and As uptake by plant in soils. 相似文献
940.
K.?T.?C.?A.?PeijnenburgEmail author E.?K.?van?Haastrecht C.?Fauvelot 《Marine Biology》2005,147(6):1279-1289
Sagitta elegans and S. setosa are the two dominant chaetognaths in the North-East (NE) Atlantic. They are closely related and have a similar ecology and
life history, but differ in distributional ranges. Sagitta setosa is a typical neritic species occurring exclusively above shelf regions, whereas S. elegans is a more oceanic species with a widespread distribution. We hypothesised that neritic species, because of smaller and more
fragmented populations, would have been more vulnerable to population bottlenecks resulting from range contractions during
Pleistocene glaciations than oceanic species. To test this hypothesis we compared mitochondrial Cytochrome Oxidase II DNA
sequences of S. elegans and S. setosa from sampling locations across the NE Atlantic. Both species displayed very high levels of genetic diversity with unique
haplotypes for every sequenced individual and an approximately three times higher level of nucleotide diversity in S. elegans (0.061) compared to S. setosa (0.021). Sagitta setosa mitochondrial DNA (mtDNA) haplotypes produced a star-like phylogeny and a uni-modal mismatch distribution indicative of a
bottleneck followed by population expansion. In contrast, S. elegans had a deeper mtDNA phylogeny and a multi-modal mismatch distribution as would be expected from a more stable population.
Neutrality tests indicated that assumptions of the standard neutral model were violated for both species and results from
the McDonald-Kreitman test suggested that selection played a role in the evolution of their mitochondrial DNA. Congruent with
these results, both species had much smaller effective population sizes estimated from genetic data when compared to census
population sizes estimated from abundance data, with a factor of ~108–109 difference. Assuming that selective effects are comparable for the two species, we conclude that the difference in genetic
signature can only be explained by contrasting demographic histories. Our data are consistent with the hypothesis that in
the NE Atlantic, the neritic S. setosa has been more severely affected by population bottlenecks resulting from Pleistocene range shifts than the more oceanic S. elegans. 相似文献