全文获取类型
收费全文 | 1245篇 |
免费 | 70篇 |
国内免费 | 28篇 |
专业分类
1343篇 |
出版年
2024年 | 2篇 |
2023年 | 13篇 |
2022年 | 6篇 |
2021年 | 7篇 |
2020年 | 19篇 |
2019年 | 24篇 |
2018年 | 19篇 |
2017年 | 33篇 |
2016年 | 31篇 |
2015年 | 30篇 |
2014年 | 29篇 |
2013年 | 44篇 |
2012年 | 19篇 |
2011年 | 22篇 |
2010年 | 25篇 |
2009年 | 51篇 |
2008年 | 60篇 |
2007年 | 78篇 |
2006年 | 62篇 |
2005年 | 58篇 |
2004年 | 39篇 |
2003年 | 30篇 |
2002年 | 48篇 |
2001年 | 62篇 |
2000年 | 53篇 |
1999年 | 18篇 |
1998年 | 25篇 |
1997年 | 21篇 |
1996年 | 35篇 |
1995年 | 25篇 |
1994年 | 37篇 |
1993年 | 31篇 |
1992年 | 34篇 |
1991年 | 29篇 |
1990年 | 41篇 |
1989年 | 19篇 |
1988年 | 30篇 |
1987年 | 9篇 |
1986年 | 27篇 |
1985年 | 16篇 |
1984年 | 16篇 |
1983年 | 11篇 |
1982年 | 17篇 |
1981年 | 9篇 |
1980年 | 14篇 |
1978年 | 6篇 |
1977年 | 3篇 |
1976年 | 6篇 |
排序方式: 共有1343条查询结果,搜索用时 15 毫秒
91.
Lee F. G. Gutowsky Henrique C. Giacomini Derrick T. de Kerckhove Rob Mackereth Darren McCormick Cindy Chu 《Global Change Biology》2019,25(3):1049-1062
The expanding human global footprint and growing demand for freshwater have placed tremendous stress on inland aquatic ecosystems. Aichi Target 10 of the Convention on Biological Diversity aims to minimize anthropogenic pressures affecting vulnerable ecosystems, and pressure interactions are increasingly being incorporated into environmental management and climate change adaptation strategies. In this study, we explore how climate change, overfishing, forest disturbance, and invasive species pressures interact to affect inland lake walleye (Sander vitreus) populations. Walleye support subsistence, recreational, and commercial fisheries and are one of most sought‐after freshwater fish species in North America. Using data from 444 lakes situated across an area of 475 000 km2 in Ontario, Canada, we apply a novel statistical tool, R‐INLA, to determine how walleye biomass deficit (carrying capacity—observed biomass) is impacted by multiple pressures. Individually, angling activity and the presence of invasive zebra mussels (Dreissena polymorpha) were positively related to biomass deficits. In combination, zebra mussel presence interacted negatively and antagonistically with angling activity and percentage decrease in watershed mature forest cover. Velocity of climate change in growing degree days above 5°C and decrease in mature forest cover interacted to negatively affect walleye populations. Our study demonstrates how multiple pressure evaluations can be conducted for hundreds of populations to identify influential pressures and vulnerable ecosystems. Understanding pressure interactions is necessary to guide management and climate change adaptation strategies, and achieve global biodiversity targets. 相似文献
92.
Gregory M. Chorak Carl R. Ruetz Ryan A. Thum Charlyn G. Partridge David J. Janetski Tomas O. Hk David F. Clapp 《Ecology and evolution》2019,9(16):8922-8932
Maintenance of genetic and phenotypic diversity is widely recognized as an important conservation priority, yet managers often lack basic information about spatial patterns of population structure and its relationship with habitat heterogeneity and species movement within it. To address this knowledge gap, we focused on the economically and ecologically prominent yellow perch (Perca flavescens). In the Lake Michigan basin, yellow perch reside in nearshore Lake Michigan, including drowned river mouths (DRMs)—protected, lake‐like habitats that link tributaries to Lake Michigan. The goal of this study was to examine the extent that population structure is associated with Great Lakes connected habitats (i.e., DRMs) in a mobile fish species using yellow perch as a model. Specifically, we tested whether DRMs and eastern Lake Michigan constitute distinct genetic stocks of yellow perch, and if so, whether those stocks migrate between the two connected habitats throughout the year. To do so, we genotyped yellow perch at 14 microsatellite loci collected from 10 DRMs in both deep and littoral habitats during spring, summer, and autumn and two nearshore sites in Lake Michigan (spring and autumn) during 2015–2016 and supplemented our sampling with fish collected in 2013. We found that yellow perch from littoral‐DRM habitats were genetically distinct from fish captured in nearshore Lake Michigan. Our data also suggested that Lake Michigan yellow perch likely use deep‐DRM habitats during autumn. Further, we found genetic structuring among DRMs. These patterns support hypotheses of fishery managers that yellow perch seasonally migrate to and from Lake Michigan, yet, interestingly, these fish do not appear to interbreed with littoral fish despite occupying the same DRM. We recommend that fisheries managers account for this complex population structure and movement when setting fishing regulations and assessing the effects of harvest in Lake Michigan. 相似文献
93.
M.J. Siegert A. Behar M. Bentley D. Blake S. Bowden P. Christoffersen C. Cockell H. Corr D. C. Cullen H. Edwards A. Ellery C. Ellis-Evans G. Griffiths R. Hindmarsh D. A. Hodgson E. King H. Lamb L. Lane K. Makinson M. Mowlem J. Parnell D. A. Pearce J. Priscu A. Rivera M. A. Sephton M. R. Sims A . M. Smith M. Tranter J. L. Wadham G. Wilson J. Woodward 《Reviews in Environmental Science and Biotechnology》2007,6(1-3):161-179
Antarctic subglacial lakes have, over the past few years, been hypothesised to house unique forms of life and hold detailed
sedimentary records of past climate change. Testing this hypothesis requires in situ examinations. The direct measurement
of subglacial lakes has been considered ever since the largest and best-known lake, named Lake Vostok, was identified as having
a deep water-column. The Subglacial Antarctic Lake Environments (SALE) programme, set up by the Scientific Committee on Antarctic
Research (SCAR) to oversee subglacial lakes research, state that prior exploration of smaller lakes would be a “prudent way
forward”. Over 145 subglacial lakes are known to exist in Antarctica, but one lake in West Antarctica, officially named Ellsworth
Subglacial Lake (referred to hereafter as Lake Ellsworth), stands out as a candidate for early exploration. A consortium of
over 20 scientists from seven countries and 14 institutions has been assembled to plan the exploration of Lake Ellsworth.
An eight-year programme is envisaged: 3 years for a geophysical survey, 2 years for equipment development and testing, 1 year
for field planning and operation, and 2 years for sample analysis and data interpretation. The science experiment is simple
in concept but complex in execution. Lake Ellsworth will be accessed using hot water drilling. Once lake access is achieved,
a probe will be lowered down the borehole and into the lake. The probe will contain a series of instruments to measure biological,
chemical and physical characteristics of the lake water and sediments, and will utilise a tether to the ice surface through
which power, communication and data will be transmitted. The probe will pass through the water column to the lake floor. The
probe will then be pulled up and out of the lake, measuring its environment continually as this is done. Once at the ice surface,
any water samples collected will be taken from the probe for laboratory analysis (to take place over subsequent years). The
duration of the science mission, from deployment of the probe to its retrieval, is likely to take between 24 and 36 h. Measurements
to be taken by the probe will provide data about the following: depth, pressure, conductivity and temperature; pH levels;
biomolecules (using life marker chips); anions (using a chemical analyzer); visualisation of the environment (using cameras
and light sources); dissolved gases (using chromatography); and morphology of the lake floor and sediment structures (using
sonar). After the probe has been retrieved, a sediment corer may be dropped into the lake to recover material from the lake
floor. Finally, if time permits, a thermistor string may be left in the lake water to take time-dependent measurements of
the lake’s water column over subsequent years. Given that the comprehensive geophysical survey of the lake will take place
in two seasons during 2007–2009, a two-year instrument and logistic development phase from 2008 (after the lake’s bathymetry
has been assessed) makes it possible that the exploration of Lake Ellsworth could take place at the beginning of the next
decade. 相似文献
94.
95.
U. Theodore Hammer 《International Review of Hydrobiology》1981,66(5):701-743
The purpose of the study was to compare the primary plankton productivities of lakes of different salinities and to determine the causative factors involved in their production rates. Four lakes (specific conductivity —mS cm−1 at 18°C) were initially chosen: Coragulac (9), Red Rock (25), Corangamite (38), Pink (250). Sampling and production measurements were made every two to three weeks. Three lakes were dominated by specific phytoplankton blooms: Red Rock (Anabaena spiroides), Corangamite (Nodularia spumigena). Pink (Dunaliella salina). Coragulac Lake had more diverse populations. Red Rock Tarn had some of the highest production values ever recorded. Extremely high soluble phosphate and inorganic carbon concentrations were the most important causative factors. Pink Lake had very low production rates. High salinity and low nutrient concentrations were limiting factors. The other lakes were intermediate in production and nutrient levels. Zooplankton populations were also determined. 相似文献
96.
For each of the three species of leeches and four species of triclads inhabiting the stony littoral of 100 British lakes of diverse trophic status, numbers, biomass and mean weight were correlated against a wide range of physical, chemical and other variables. Very few significant correlations were found between the various biological variables and the non-chemical variables. Significant, positive correlations were obtained for all leech and triclad variables against all of the chemicals, with the exception of Erpobdella octoculata numbers, Helobdella stagnalis mean weight and all of the Polycelis nigra variables which did not correlate with any. Relationships between biological variables and calcium content are given special consideration. Over the range of lakes investigated, total triclad numbers exceeded leech numbers but the reverse was usually true for total biomass. Correlations between the various leech and triclad variables are examined. 相似文献
97.
98.
Jens Peder Jensen Erik Jeppesen Peter Kristensen Peter Bondo Christensen Martin Sndergaard 《International Review of Hydrobiology》1992,77(1):29-42
A detailed mass balance on nitrogen was carried out in shallow and hypertrophic Lake Søbygård during 4.5 years before through 2.5 years after a 36 % reduction in nitrogen loading. Annual mean loss rate of nitrogen was 159–229 mg N m−2 d−1 before the loading reduction and 125 mg N m−2 d−1 after. In spite of a short hydraulic retention time (18–27 days) the proportion of nitrogen loading lost in the lake was high (38–53 %) and not affected by changes in loading. Calculated denitrification accounted for 86–93% of the loss rate, while 7–14% was permanently buried. Marked seasonal variations in the loss percentage were found during the season, ranging from 23 % in first quarter to 65 % in third quarter. The seasonal variation in the loss percentage of nitrogen showed a hysteresis like relationship to temperature, with a high percentage in fourth quarter. This suggests that the amount of available substrate, which mainly consists of sedimentated phytoplankton, accumulated during summer, is an important regulating factor. The ability of various published input-output models to predict the observed changes in in-lake nitrogen concentration in Lake Søbygård was tested. This study has further confirmed that small lakes with short retention and high nitrogen loading may significantly reduce the nitrogen loading of downstream aquatic environments. 相似文献
99.
For arid-region lakes, management conflicts are likely to occur between quantity and quality of water supplied: increasing quantity of water supply can lead to water quality deterioration. Such conflicts can best be resolved within an effective management program based on awareness and cooperation at all levels of water management from policy makers to experts. We propose a general framework for designing effective water resource management programs for lakes based on concrete definitions of management criteria such as water quality. The proposed system requires close interaction between policy makers, water resource managers, water suppliers and users, hydrological engineers and limnologists. The significance of mathematical modeling as a self-organizing tool of the management program is emphasized, especially with regards to designing limnological investigations directed toward lake management. We illustrate the application of this approach to water resource management in arid-region lakes (Lake Kinneret, Israel and Lake Sevan, Armenia), where artificial variability of lake morphometry due to water use is a forcing function affecting water quality. 相似文献
100.
Bazyli Czeczuga Krystyna Brzozowska Lucyna Woronowicz 《International Review of Hydrobiology》1990,75(2):245-255
The authors investigated the mycoflora and the environmental factors in the River Czarna Hańcza (10 stations) and its tributary River Marycha (1 station) as well as Lakes Hańcza (2 stations) and Wigry (2 stations) on the occurrence of various aquatic fungi. At the stations investigated the presence of 45 aquatic fungi species was noted. The following fungi, up to now unknown in Poland, were found: Monoblepharis hypogyna, Rozellopsis inflata, Cladolegnia eccentrica, Apostemidium guernisaci, Anguillospora gigantea, Geniculospora grandis, Clavariopsis aquatica, Lemonniera aquatica and Lemonniera terrestris. 相似文献