首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   928篇
  免费   82篇
  国内免费   47篇
  2023年   20篇
  2022年   5篇
  2021年   15篇
  2020年   35篇
  2019年   28篇
  2018年   24篇
  2017年   40篇
  2016年   29篇
  2015年   26篇
  2014年   46篇
  2013年   64篇
  2012年   37篇
  2011年   27篇
  2010年   27篇
  2009年   26篇
  2008年   29篇
  2007年   31篇
  2006年   57篇
  2005年   37篇
  2004年   26篇
  2003年   42篇
  2002年   42篇
  2001年   32篇
  2000年   30篇
  1999年   19篇
  1998年   18篇
  1997年   16篇
  1996年   19篇
  1995年   18篇
  1994年   23篇
  1993年   16篇
  1992年   15篇
  1991年   14篇
  1990年   7篇
  1989年   11篇
  1988年   12篇
  1987年   9篇
  1986年   12篇
  1985年   13篇
  1984年   14篇
  1983年   3篇
  1982年   6篇
  1981年   16篇
  1980年   4篇
  1979年   3篇
  1978年   7篇
  1977年   5篇
  1976年   2篇
排序方式: 共有1057条查询结果,搜索用时 15 毫秒
1.
Much of the Baltic Sea is currently classified as ‘affected by eutrophication’. The causes for this are twofold. First, current levels of nutrient inputs (nitrogen and phosphorus) from human activities exceed the natural processing capacity with an accumulation of nutrients in the Baltic Sea over the last 50–100 years. Secondly, the Baltic Sea is naturally susceptible to nutrient enrichment due to a combination of long retention times and stratification restricting ventilation of deep waters. Here, based on a unique data set collated from research activities and long‐term monitoring programs, we report on the temporal and spatial trends of eutrophication status for the open Baltic Sea over a 112‐year period using the HELCOM Eutrophication Assessment Tool (HEAT 3.0). Further, we analyse variation in the confidence of the eutrophication status assessment based on a systematic quantitative approach using coefficients of variation in the observations. The classifications in our assessment indicate that the first signs of eutrophication emerged in the mid‐1950s and the central parts of the Baltic Sea changed from being unaffected by eutrophication to being affected. We document improvements in eutrophication status that are direct consequences of long‐term efforts to reduce the inputs of nutrients. The reductions in both nitrogen and phosphorus loads have led to large‐scale alleviation of eutrophication and to a healthier Baltic Sea. Reduced confidence in our assessment is seen more recently due to reductions in the scope of monitoring programs. Our study sets a baseline for implementation of the ecosystem‐based management strategies and policies currently in place including the EU Marine Strategy Framework Directives and the HELCOM Baltic Sea Action Plan.  相似文献   
2.
Determining the genetic bases of adaptations and their roles in speciation is a prominent issue in evolutionary biology. Cichlid fish species flocks are a prime example of recent rapid radiations, often associated with adaptive phenotypic divergence from a common ancestor within a short period of time. In several radiations of freshwater fishes, divergence in ecomorphological traits — including body shape, colour, lips and jaws — is thought to underlie their ecological differentiation, specialization and, ultimately, speciation. The Midas cichlid species complex (Amphilophus spp.) of Nicaragua provides one of the few known examples of sympatric speciation where species have rapidly evolved different but parallel morphologies in young crater lakes. This study identified significant QTL for body shape using SNPs generated via ddRAD sequencing and geometric morphometric analyses of a cross between two ecologically and morphologically divergent, sympatric cichlid species endemic to crater Lake Apoyo: an elongated limnetic species (Amphilophus zaliosus) and a high‐bodied benthic species (Amphilophus astorquii). A total of 453 genome‐wide informative SNPs were identified in 240 F2 hybrids. These markers were used to construct a genetic map in which 25 linkage groups were resolved. Seventy‐two segregating SNPs were linked to 11 QTL. By annotating the two most highly supported QTL‐linked genomic regions, genes that might contribute to divergence in body shape along the benthic–limnetic axis in Midas cichlid sympatric adaptive radiations were identified. These results suggest that few genomic regions of large effect contribute to early stage divergence in Midas cichlids.  相似文献   
3.
Tooth microwear feature densities were significantly increased in a population of laboratory‐reared three‐spined stickleback Gasterosteus aculeatus in four days, after they were transferred from a limnetic feeding regime to a benthic feeding regime. These results show that even in aquatic vertebrates with non‐occluding teeth, changes in feeding can cause changes in tooth microwear in just a few days, as in mammals.  相似文献   
4.
In alpine regions worldwide, climate change is dramatically altering ecosystems and affecting biodiversity in many ways. For streams, receding alpine glaciers and snowfields, paired with altered precipitation regimes, are driving shifts in hydrology, species distributions, basal resources, and threatening the very existence of some habitats and biota. Alpine streams harbour substantial species and genetic diversity due to significant habitat insularity and environmental heterogeneity. Climate change is expected to affect alpine stream biodiversity across many levels of biological resolution from micro‐ to macroscopic organisms and genes to communities. Herein, we describe the current state of alpine stream biology from an organism‐focused perspective. We begin by reviewing seven standard and emerging approaches that combine to form the current state of the discipline. We follow with a call for increased synthesis across existing approaches to improve understanding of how these imperiled ecosystems are responding to rapid environmental change. We then take a forward‐looking viewpoint on how alpine stream biologists can make better use of existing data sets through temporal comparisons, integrate remote sensing and geographic information system (GIS) technologies, and apply genomic tools to refine knowledge of underlying evolutionary processes. We conclude with comments about the future of biodiversity conservation in alpine streams to confront the daunting challenge of mitigating the effects of rapid environmental change in these sentinel ecosystems.  相似文献   
5.
A continuous culture system for a benthic food diatom Nitzschia sp. wasestablished by using properties of high nutrient and clean of deep seawater(DSW). DSW collected from 320 m depth in Muroto City, Japan, was introducedinto a glass-pipe bioreactor (14 cm length, 3 cm diam.) containing glassbeads of 0.5 cm diam. as substrata for the alga, and it was incubated at18°C · 80Em–2sec–1 · L:D=14:10. The chlorophyll a yield of benthicdiatoms in a reactor as a unit of surface area of the substratum was only0.001–0.003 g cm–2 when the flow rate of DSW was 0(batch culture conditions). However, when DSW was supplied continuously to areactor, the yield increased to 1.4 g-chl.a cm–2 alongwith the increase in flow rate of DSW. Moreover, amounts of chl.a washed outof the system were negligible, 0.0014 to 0.0045%, even though theflow rate of DSW was as much as 25 times h–1, suggesting thatsloughing of benthic diatoms from the substratum was minimized. Although theyield of diatoms fluctuated significantly at the time that the DSW wascollected, the variation could be minimized by increasing the flow rate ofDSW. These results indicate that the continuous culturing system with DSWsupports the stable and effective mass culture of benthic food diatom.  相似文献   
6.
Comparisons of benthic invertebrates between riffles and pools   总被引:2,自引:2,他引:0  
Benthic species assemblages in upstream and downstream ends of riffles and in pools were investigated seasonally in the first five orders of an alluvial gravel stream with distinct pool and riffle channel form. Riffles comprised < 10% of stream area and were separated by pools with extensive bedrock substrate (ca. 15–85% of total surface area) which was scoured during periodic high flow. Virtually all taxa were more abundant in riffles than in pools, except chironomids which were more equally distributed. Inconsistent results were obtained for upstream-downstream comparisons within riffles. Intermittent headwater reaches (orders 1 & 2) which supported half as many taxa retained this pattern during periods of flow, although riffles at these sites were dry from mid-June to mid-November. Pools which contained more gravel, indicating less disturbance during high flow, had a richer assemblage of benthic species than other pools. Many invertebrates in pools may have been there as a result of drift from their preferred riffle habitats, but the presence of gravel in the pools indicates less intense flow disturbance during floods, provides protection from the mild scouring that does occur during floods, and provides refugia from predators.  相似文献   
7.
Large bag-type (75 m3) and tube-type (105 m3) enclosures were set up in the shallow eutrophic Lake Suwa and were each stocked with exotic planktivorous whitefish (Coregonus lavaretus maraena). The release of whitefish caused the increase in nutrient concentration in the tube-type enclosure whereas no such increase was observed in the bag-type enclosure. Bottom sediment seemed to be an important source of chironomid food for whitefish. The proportion of phytoplankton measuring<10μm and 20–40μm, which respectively corresponded toOchromonas spp. andCryptomonas sp., were lower in the fish enclosures than in the control, which might have been caused by high grazing pressure by rotifers. The predation by whitefish might have affected the species composition of phytoplankton through reducing copepod predation on rotifers, not through reducing the densities of cladocerans which directly feed on phytoplankton as many investigators have reported. The phytoplankton biomass was not affected much by the release of fish. Possible reasons are that the increase in density of rotifers reduced the biomass of available phytoplankton and also that inedible Cyanophyceae were in the decreasing phase of their seasonal succession and could not increase successfully in spite of elevated nutrient levels.  相似文献   
8.
The River Nile is one of the world’s major rivers. Its’ catchment in Egypt has a population of 75,000,000. River flow is highly regulated and there are known discharges of pollutants. 1035 km of the river downstream of the Aswan high dam was studied to test the hypothesis that representative qualitative samples can be used to estimate macroinvertebrate biodiversity. Benthic macroinvertebrates are difficult to sample in large rivers and a reliable sampling strategy is required to evaluate their ecological status. Three methods for sampling have been investigated. Ekman Grab, macrophyte sweep netting and Artificial Substrate Samplers (ASS) were used to sample 15 sites from Aswan to Cairo between September 2001 and June 2002. Organisms were identified to species level where possible. Taxon accretion curves indicated that the all taxa present at a site should be collected using either 15 grab samples, 10 macrophyte samples or 5 ASS. The best time to sample was May–June. The biodiversity of macroinvertebrates in the Nile was recorded as 50 taxa with values of 7–31 at individual bank-side sites. Mid-stream biodiversity was much lower (0–19). Lowest biodiversity occurred at sites with known pollution inputs whilst highest occurred at sites with high levels of sedimentation. A regular programme for biomonitoring is recommended which will allow current status to be confirmed and future changes detected.  相似文献   
9.
Among the 19 non-native species of marine invertebrates which have invaded the Venice Lagoon and have established populations, Ruditapes philippinarum, deliberately introduced in 1983, is surely the most successful species. According to the hypothesis that alien species invasion could be favoured by an altered ecological, chemical or physical state of the system induced by anthropogenic disturbance, R. philippinarum turned out to be ‘the right species at the right moment’. By comparing historical data (1968, 1985, 1990) with 1999 data, changes in macrobenthic community, in particular bivalve molluscs, of the lagoon induced by R. philippinarum introduction and subsequent clam exploiting activity were assessed. It has been possible to describe a sharp reduction, both in terms of distribution area and density, of all other filter feeder bivalves. Moreover, by using the clearance rate of the most abundant bivalve species in 1990 and 1999 (Cerastoderma glaucum and R. philippinarum, respectively), it was possible to estimate that the filtration capacity, expressed as l h−1 m−2, has more than doubled. This has altered the functioning of the ecosystem, resulting in a stronger benthic–pelagic coupling. In this context, R. philippinarum attains control of the system. Considering all this, it is possible to state that the Venice Lagoon ecosystem has entered into a new state, probably more resistant but less resilient, with implications for future management choices.  相似文献   
10.
Here we summarize results of the EU funded research project STAR concerning the suitability of different organism groups (fish, benthic invertebrates, macrophytes, diatoms) for monitoring European rivers. In a general way, the suitability of the organism groups is classified by monitoring type, stress type, river type, temporal scale and taxonomic resolution. For example, although all organism groups are affected by acidification, the relatively low species richness of fish and macrophytes in small mountain streams makes these two groups less suitable, and, hence, we argue that benthic diatoms and/or invertebrates may be considered as more robust indicators. Similar, lines of reasoning are given for a number of stressor and stream types.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号