全文获取类型
收费全文 | 2050篇 |
免费 | 99篇 |
国内免费 | 580篇 |
出版年
2024年 | 12篇 |
2023年 | 29篇 |
2022年 | 35篇 |
2021年 | 51篇 |
2020年 | 63篇 |
2019年 | 72篇 |
2018年 | 58篇 |
2017年 | 73篇 |
2016年 | 97篇 |
2015年 | 75篇 |
2014年 | 70篇 |
2013年 | 108篇 |
2012年 | 85篇 |
2011年 | 88篇 |
2010年 | 65篇 |
2009年 | 127篇 |
2008年 | 136篇 |
2007年 | 169篇 |
2006年 | 114篇 |
2005年 | 100篇 |
2004年 | 84篇 |
2003年 | 95篇 |
2002年 | 82篇 |
2001年 | 78篇 |
2000年 | 67篇 |
1999年 | 38篇 |
1998年 | 48篇 |
1997年 | 32篇 |
1996年 | 84篇 |
1995年 | 38篇 |
1994年 | 34篇 |
1993年 | 30篇 |
1992年 | 38篇 |
1991年 | 38篇 |
1990年 | 45篇 |
1989年 | 33篇 |
1988年 | 26篇 |
1987年 | 21篇 |
1986年 | 40篇 |
1985年 | 15篇 |
1984年 | 16篇 |
1983年 | 5篇 |
1982年 | 22篇 |
1981年 | 14篇 |
1980年 | 12篇 |
1979年 | 18篇 |
1978年 | 15篇 |
1977年 | 18篇 |
1976年 | 13篇 |
1975年 | 2篇 |
排序方式: 共有2729条查询结果,搜索用时 15 毫秒
1.
2.
M. Tobler† 《Journal of fish biology》2005,66(3):877-881
Feigning death, a hunting strategy in which a healthy individual acts as if it was dead to trick prey into its reach, is reported for a population of the Central American cichlid Parachromis friedrichsthalii . Possible mechanisms leading to the evolution of such a behaviour are discussed. 相似文献
3.
The application of culture-dependent studies to quantify Fe-metabolizing microorganisms from the environment is a necessity, as there are so far no universal functional marker genes for application in culture-independent studies. Media composition can vary between studies, therefore, we determined the effects of three different growth media on the quantification (MPNs) and identity (via cloning and sequencing of dominant DGGE bands) of nitrate-reducing Fe(II)-oxidizers and lactate- or acetate-oxidizing Fe(III)-reducers from a lacustrine sediment: low sulphate freshwater medium (FWM), sterile filtered bicarbonate-buffered lake water (BLW) and a mixture of both (MIX). We consistently found fewer cells in the BLW than in the FWM and the MIX. The DGGE banding patterns of the microbial communities enriched in different media types clustered together according to the e? donor and acceptor couples and not according to the medium used. Thus, although the medium composition significantly influenced the quantification and thereby conclusions on the abundance and potential significance of the targeted group within the ecosystem, biodiversity assessments through enrichment cultures were less influenced by the medium, but instead were affected by the type and concentration of the e? donor/acceptor. 相似文献
4.
Eva Decru Nathan Vranken Pedro H. N. Bragança Jos Snoeks Maarten Van Steenberge 《Journal of fish biology》2020,96(5):1186-1201
Based on literature, museum collections and three recent expeditions, an annotated species list of the Lake Edward, East Africa, drainage system is presented, excluding the endemic haplochromines. A total of 34 non-Haplochromis species belonging to 10 families and 21 genera are recorded from the system. Three of these are endemic and two others have been introduced in the region. Six species are new records for the Lake Edward system. A species accumulation curve indicates that we probably covered most of the non-Haplochromis species in the area sampled during the recent expeditions, but undetected species might still be present in the Congolese part of the system, which is poorly sampled. A comparison of the species list with those of neighbouring basins confirmed the placement of the Lake Edward system within the east-coast ichthyofaunal province. 相似文献
5.
W. S. Laughlin A. P. Okladnikov A. P. Derevyanko A. B. Harper I. V. Atseev 《American journal of physical anthropology》1976,45(3):651-659
Among the materials excavated by the 1975 joint USSR–USA team in Siberia are two burials from Shaman's Cape, Olkhon Island, Lake Baikal. One is a middle-aged male of the Serovo culture, 6,000 B.C. , and the other is a young male of the Glaskovo culture of 2,000 B.C. This later burial displays an unusual pathology affecting the nose and post-cranial regions of the pelvic girdle and lower limbs. Osteon analysis confirms the determination of age at death and illustrates the difference between normal and pathological bone. Numerous cultural materials were associated with these burials, including harpoon heads, knives, a compound fishhook and a pestle with the Serovo man, and nephrite ornaments with the Glaskovo man. The skulls, though far apart in time, are pronouncedly Mongoloid and alike in their low cranial vaults. A low, broad and inclined ascending ramus resembles Chukchi, Eskimos and Aleuts. These two specimens document the Mongoloid character of the early inhabitants of Lake Baikal. 相似文献
6.
7.
Stable isotope analysis was used to investigate the migratory status and to determine the relative contribution of allochthonous and autochthonous sources of carbon for the major riverine fish species ( Barbus altianalis and Labeo victorianus ) in rivers draining the Kenyan side of Lake Victoria. The two fish species derived carbon from both C4 and C3 plant sources, although L. victorianus exhibited less enriched isotopic carbon values. Fish samples from stations under direct influence of effluents from sugar factories exhibited enriched δ13 C signals. Assuming that this reflects carbon sourcing from riparian C4 plants, it suggests that carbon from terrestrial sources can be a major energy source in some rivers. This heavy carbon enrichment associated with sugar factories was spatially restricted and occurred in all seasons, implying that sub-populations of the two fish species are non-migratory. The large migratory populations of these two species, for which Lake Victoria was once famous, may be no more. 相似文献
8.
A stable carbon isotope study of dissolved inorganic carbon cycling in a softwater lake 总被引:3,自引:0,他引:3
Andrew L. Herczeg 《Biogeochemistry》1987,4(3):231-263
The dissolved inorganic carbon (DIC) cycle in a softwater lake was studied using natural variations of the stable isotopes of carbon,12C and13C. During summer stratification there was a progressive decrease in epilimnion DIC concentration with a concomitant increase in 13CDIC), due to preferential uptake of12C by phytoplankton and a change in the dominant CO2 source from inflow andin situ oxidation to invasion from the atmosphere. There was an increase in hypolimnion DIC concentration throughout summer with a concomitant general decrease in 13CDIC from oxidation of the isotopically light particulate organic carbon that sank down through the thermocline from the epilimnion.Mass balance calculations of DI12C and DI13C in the epilimnion for the summer (June 23–September 25) yield a mean rate of net conversion of DIC to organic carbon (Corg) of 430 ± 150 moles d-1 (6.5 ± 1.8 m moles m-2 d-1. Net CO2 invasion from the atmosphere was 420 ± 120 moles d-1 (6.2 ± 1.8 m moles m-2 d-1) with an exchange coefficient of 0.6 ± 0.3m d-1. These results imply that at least for the summer months the phytoplankton obtained about 90% of their carbon from atmosphere CO2. About 50% of CO2 invasion and conversion to Corg for the summer occurred during a two week interval in mid-summer.DIC concentration increased in the hypolimnion at a rate of 350 ± 70 moles DIC d-1 during summer stratification. The amount of DIC added to the hypolimnion was equivalent to 75 ± 20% of net conversion of DIC to Corg in the euphotic zone over spring and summer implying rapid degradation of POC in the hypolimnion. The 13C of DIC added to the deep water (-22.) was too heavy to have been derived from oxidation of particulate organic carbon alone. About 20% of the added DIC must have diffused from hypolimnetic sediments where relatively heavy CO2 (-7) was produced by a combination of POC oxidation and as a by-product of methanogenesis. 相似文献
9.
Presence and development of populations of the introduced brown alga Sargassum muticum in the southwest Netherlands 总被引:23,自引:10,他引:13
Of the three Brachionus species used in aquaculture, Brachionus rubens, B. calycilorus and B. plicatilis, the latter is most widely used in raising marine fish and shrimp larvae due to its tolerance to the marine environment. In freshwater aquaculture the use of B. rubens and B. calycilorus is limited, probably because inert food products are readily available as feed for freshwater larvae.The rotifer Brachionus plicatilis is used in large numbers as the first food organism in intensive cultures of marine fish and shrimp larvae. An adequate supply of these rotifers relies on mass cultures. The reproductive rate of rotifers in these cultures depends on food quality and quantity, salinity, temperature and pH of the medium. Removal of waste products from culture tanks leads to higher and more efficient production of rotifers over extended periods of time. Rotifers have to be enriched with polyunsaturated fatty acids, which are essential for proper development and survival of marine fish and shrimp larvae.The future use of preserved rotifers and their resting eggs may help to overcome unforeseen failures of live cultures and may lead to more efficient use of these organisms in raising freshwater and marine fish and shrimp larvae. 相似文献
10.
Food composition of Clarias gariepinus (= C. lazera) (Cypriniformes,Clariidae) in Lake Kinneret (Israel) 总被引:2,自引:2,他引:0
The feeding behaviour of the freshwater piscivore, Clarias gariepinus (C. lazera) (C & V 1840) was studied over two periods: 1973–1975 and 1981–1982, in Lake Kinneret (Israel). The total number of fish analysed was 264 and their sizes (SL) and weights varied between 238 and 830 mm (146 to 5728 g). More than fifty species of plants and animals from the plankton, benthos and nekton of Lake Kinneret were identified in the intestines of C. gariepinus. Preyed fish were the most abundant food component (81%) and constituted the highest biomass, with Mirogrex terraesanctae representing the majority (although other species were also found). The potential impact of piscivory in the Kinneret ecosystem is considered. 相似文献