全文获取类型
收费全文 | 1124篇 |
免费 | 123篇 |
国内免费 | 278篇 |
专业分类
1525篇 |
出版年
2024年 | 11篇 |
2023年 | 30篇 |
2022年 | 32篇 |
2021年 | 62篇 |
2020年 | 74篇 |
2019年 | 68篇 |
2018年 | 49篇 |
2017年 | 77篇 |
2016年 | 58篇 |
2015年 | 51篇 |
2014年 | 63篇 |
2013年 | 102篇 |
2012年 | 56篇 |
2011年 | 33篇 |
2010年 | 49篇 |
2009年 | 58篇 |
2008年 | 50篇 |
2007年 | 67篇 |
2006年 | 62篇 |
2005年 | 48篇 |
2004年 | 36篇 |
2003年 | 22篇 |
2002年 | 46篇 |
2001年 | 33篇 |
2000年 | 34篇 |
1999年 | 20篇 |
1998年 | 15篇 |
1997年 | 24篇 |
1996年 | 23篇 |
1995年 | 19篇 |
1994年 | 22篇 |
1993年 | 18篇 |
1992年 | 20篇 |
1991年 | 14篇 |
1990年 | 11篇 |
1989年 | 7篇 |
1988年 | 11篇 |
1987年 | 9篇 |
1986年 | 8篇 |
1985年 | 8篇 |
1984年 | 3篇 |
1983年 | 3篇 |
1982年 | 3篇 |
1981年 | 5篇 |
1980年 | 3篇 |
1977年 | 2篇 |
1976年 | 2篇 |
1975年 | 1篇 |
1973年 | 1篇 |
1958年 | 1篇 |
排序方式: 共有1525条查询结果,搜索用时 15 毫秒
141.
142.
【目的】滨海湿地生态系统位于淡水与海水交互地带,含有高浓度Fe2+的地下水渗透到沉积物表层形成的湿地径流和周期性潮汐淹水形成的含氧-缺氧界面有利于铁氧化细菌介导的Fe2+的生物氧化过程发生。然而,目前缺乏对滨海湿地生态系统中铁氧化细菌类群的全面评估。【方法】以上海崇明西沙湿地公园及浙江舟山市朱家尖岛东沙沙滩两地共5处滨海湿地沉积物为研究对象,分析沉积物的氧气穿透深度等环境参数,并基于16S rRNA基因扩增子测序技术,全面解析不同滨海湿地生态系统中细菌与铁氧化细菌的群落组成与分布特征。【结果】与崇明西沙湿地相比,朱家尖岛东沙沙滩有更深的氧气穿透深度,达到10 mm以上。非度量多尺度分析(non-metric multidimensional scaling, NMDS)统计结果表明,细菌群落结构主要受到区域位置不同导致的环境条件差异的影响,而铁氧化细菌的群落结构则受到采样的区域位置和沉积物氧气穿透深度的共同影响。崇明西沙湿地和朱家尖岛东沙沙滩的优势细菌为蓝细菌门(Cyanobacteria)、γ-变形菌纲(Gammaproteobacteria)、拟杆菌门(Bacteroidetes)、α-变形菌纲(Alphaproteobacteria)和放线菌门(Actinobacteria);优势铁氧化细菌为嘉利翁氏菌属(Gallionella)、红细菌属(Rhodobacter)、Lepthothrix和Sideroxydans。【结论】通过对崇明西沙湿地和朱家尖岛东沙沙滩沉积物中栖息的铁氧化菌的调查发现,铁氧化细菌的群落组成与湿地沉积物类型导致的氧气穿透深度差异具有密切联系。 相似文献
143.
Being inspired in the process of teaching and studying on tourism geography and heritage landscapes, the author attempts to broaden the scope of traditional studies in landscape ecology. Upon reviewing the progress, limitations and boundaries of landscape ecology, he criticizes the negligence of waterscapes by academics. The author further examines some waterscape conservation policy programs practiced by different nations. These include 10 National Seashores, 4 National Lakeshores, 4 National Rivers, 13 National Marine Sanctuaries and 1 Marine National Monument in USA, 13 Marine National Parks, 11 Marine Sanctuaries and 4 multiple use Marine Reserves in the State of Victoria, Australia, 3 Marine Nature Reserves and 43 Heritage Coasts in UK, and 272 National Water Parks in China. Based on these explorations, the author proposes a few hypotheses on waterscape ecology. Finally, he concludes that as one among many sub-disciplines of applied ecology, waterscape ecology can only achieve its full-fledged growth through concerted efforts among academics, and the supports from both governments and non-government organizations. Meanwhile, the site-specific policy practices with the rapid increasing social needs will likely facilitate the development of this infant school. Hopefully, the birth and development of waterscape ecology will contribute to the prosperity of global academics and the maintenance of earth level environmental health. 相似文献
144.
145.
The seasonal variation and the vertical distribution of the phytoplanktonic population of the lagoon of Cullera, an elongated coastal lagoon with estuarine circulation of water, has been studied in three sampling stations: mouth, centre and source. Seasonal variation is determined by a marine-freshwater interaction. In winter, the sea influence is important, a marine water wedge of anoxic water arrives at the sampling station located at the source and marine and brackish water species dominate the phytoplankton. Also marine species of zooplankton and fish enter the system, which may then be considered as exploited by the sea. In spring the marine wedge retreats from the source but remains in the centre and mouth, salinity diminishes, vertical mixing persists and phytoplankton is dominated by Cyclotella species. From late spring to autumn the freshwater influence prevails and a sharp stratification of the water is produced in the stations at the mouth and the centre, by means of a steep halocline coincident with an oxycline. The phytoplankton in this period follows a typical succession like those described in freshwater eutrophic lakes. Vertical distribution of phytoplankton is determined by the presence of the oxycline, originated by the marine water wedge, whose depth varies seasonally but which is always present in the mouth and centre of the lagoon; only few species of algae can be found below its level. 相似文献
146.
Human activities and climate variability drive fast‐paced change across the world's estuarine–coastal ecosystems 下载免费PDF全文
James E. Cloern Paulo C. Abreu Jacob Carstensen Laurent Chauvaud Ragnar Elmgren Jacques Grall Holly Greening John Olov Roger Johansson Mati Kahru Edward T. Sherwood Jie Xu Kedong Yin 《Global Change Biology》2016,22(2):513-529
Time series of environmental measurements are essential for detecting, measuring and understanding changes in the Earth system and its biological communities. Observational series have accumulated over the past 2–5 decades from measurements across the world's estuaries, bays, lagoons, inland seas and shelf waters influenced by runoff. We synthesize information contained in these time series to develop a global view of changes occurring in marine systems influenced by connectivity to land. Our review is organized around four themes: (i) human activities as drivers of change; (ii) variability of the climate system as a driver of change; (iii) successes, disappointments and challenges of managing change at the sea‐land interface; and (iv) discoveries made from observations over time. Multidecadal time series reveal that many of the world's estuarine–coastal ecosystems are in a continuing state of change, and the pace of change is faster than we could have imagined a decade ago. Some have been transformed into novel ecosystems with habitats, biogeochemistry and biological communities outside the natural range of variability. Change takes many forms including linear and nonlinear trends, abrupt state changes and oscillations. The challenge of managing change is daunting in the coastal zone where diverse human pressures are concentrated and intersect with different responses to climate variability over land and over ocean basins. The pace of change in estuarine–coastal ecosystems will likely accelerate as the human population and economies continue to grow and as global climate change accelerates. Wise stewardship of the resources upon which we depend is critically dependent upon a continuing flow of information from observations to measure, understand and anticipate future changes along the world's coastlines. 相似文献
147.
Class, ethnic, and gender differences among fisherfolk powerfully influence how the benefits and costs of coastal resource management programs are perceived and experienced in Philippine fishing communities. These and other social differences also limit the efficacy of community participation in these programs and hence the role that local communities can be expected to play in fisheries co-management regimes. Greater attention to institutional changes is needed if the management potential of such regimes is to be realized. 相似文献
148.
149.
In this work we assessed spatial and temporal variation of leaf pigment content of Clusia hilariana, a dominant CAM tree on the sandy coastal plains of SE Brazil. Chlorophyll a + b concentration, chlorophyll a/b ratio, and total carotenoid concentration were measured three times during the wet season. Chlorophyll a + b and total carotenoids decreased while the chlorophyll a/b ratio increased after a series of dry spells. We discuss our results in the context of Clusia literature, and argue that leaf-level pigmentation changes are an important source of phenotypic flexibility during acclimation
to fluctuating light levels and groundwater availability. 相似文献
150.
Aims: This study sought to evaluate the distribution of the enterococcal surface protein ( esp ) gene in Enterococcus faecium in the Pacific coast environment as well as the distribution and diversity of the gene in Northern California animal hosts.
Methods and Results: Over 150 environmental samples from the Pacific coast environment (sand, surf zone, fresh/estuarine, groundwater, and storm drain) were screened for the esp gene marker in E. faecium , and the marker was found in 37% of the environmental samples. We examined the host specificity of the gene by screening various avian and mammalian faecal samples, and found the esp gene to be widespread in nonhuman animal faeces. DNA sequence analysis performed on esp polymerase chain reaction amplicons revealed that esp gene sequences were not divergent between hosts.
Conclusions: Our data confirm recent findings that the E. faecium variant of the esp gene is not human-specific.
Significance and Impact of the Study: Our results suggest that the use of the esp gene for microbial source tracking applications may not be appropriate at all recreational beaches. 相似文献
Methods and Results: Over 150 environmental samples from the Pacific coast environment (sand, surf zone, fresh/estuarine, groundwater, and storm drain) were screened for the esp gene marker in E. faecium , and the marker was found in 37% of the environmental samples. We examined the host specificity of the gene by screening various avian and mammalian faecal samples, and found the esp gene to be widespread in nonhuman animal faeces. DNA sequence analysis performed on esp polymerase chain reaction amplicons revealed that esp gene sequences were not divergent between hosts.
Conclusions: Our data confirm recent findings that the E. faecium variant of the esp gene is not human-specific.
Significance and Impact of the Study: Our results suggest that the use of the esp gene for microbial source tracking applications may not be appropriate at all recreational beaches. 相似文献