全文获取类型
| 收费全文 | 708篇 |
| 免费 | 90篇 |
| 国内免费 | 5篇 |
专业分类
| 电工技术 | 1篇 |
| 综合类 | 22篇 |
| 化学工业 | 106篇 |
| 机械仪表 | 2篇 |
| 建筑科学 | 47篇 |
| 矿业工程 | 9篇 |
| 能源动力 | 12篇 |
| 轻工业 | 476篇 |
| 水利工程 | 91篇 |
| 石油天然气 | 5篇 |
| 一般工业技术 | 5篇 |
| 冶金工业 | 2篇 |
| 自动化技术 | 25篇 |
出版年
| 2025年 | 8篇 |
| 2024年 | 24篇 |
| 2023年 | 26篇 |
| 2022年 | 41篇 |
| 2021年 | 36篇 |
| 2020年 | 58篇 |
| 2019年 | 54篇 |
| 2018年 | 44篇 |
| 2017年 | 34篇 |
| 2016年 | 29篇 |
| 2015年 | 33篇 |
| 2014年 | 36篇 |
| 2013年 | 35篇 |
| 2012年 | 38篇 |
| 2011年 | 31篇 |
| 2010年 | 20篇 |
| 2009年 | 22篇 |
| 2008年 | 29篇 |
| 2007年 | 36篇 |
| 2006年 | 28篇 |
| 2005年 | 20篇 |
| 2004年 | 14篇 |
| 2003年 | 15篇 |
| 2002年 | 9篇 |
| 2001年 | 6篇 |
| 2000年 | 7篇 |
| 1999年 | 7篇 |
| 1998年 | 3篇 |
| 1997年 | 2篇 |
| 1996年 | 7篇 |
| 1995年 | 5篇 |
| 1994年 | 3篇 |
| 1993年 | 4篇 |
| 1992年 | 10篇 |
| 1991年 | 6篇 |
| 1990年 | 2篇 |
| 1989年 | 1篇 |
| 1988年 | 2篇 |
| 1987年 | 2篇 |
| 1986年 | 2篇 |
| 1985年 | 5篇 |
| 1984年 | 3篇 |
| 1983年 | 3篇 |
| 1982年 | 1篇 |
| 1981年 | 1篇 |
| 1980年 | 1篇 |
排序方式： 共有803条查询结果，搜索用时 15 毫秒
91.
Urban lakes are prone to eutrophication because of a number of factors, including a high water residence time (slow flushing rate), and high nutrient loads from artificial feeding of waterfowl and fish by visitors and from waterfowl faeces, falling leaves and nutrient run‐off from adjacent areas. This study focuses on the ecology of two similar‐sized urban lakes in Porto, Portugal, that exhibit some differing characteristics conducive to different trophic states and nutrient removal efficiencies. Although similar in many aspects, the two reservoirs developed quite diverse phytoplankton communities. The higher nitrogen load to Palácio de Cristal Lake could be the result of the higher concentration in its underground influent stream, while a less‐oxidized nitrogen form (ammonia) is higher in Serralves Lake. Nitrate concentrations are higher in Palácio de Cristal Lake than in Serralves as a result mainly of high nitrate loads (maximum values above 80 mg L−1) in its influent stream. Interestingly, however, its nitrate removal efficiency is greater than for Serralves Lake. Cyanobacteria are the dominal phytoplankton In both lakes throughout the year. However, only one genus of this group (Pseudoanabaena mucicola) was the dominant phytoplankton in Palácio de Cristal Lake, while five Cyanobacteria species co‐dominated in Serralves lake. Regular monitoring of Cyanobacteria and their toxins in urban recreation lakes is advised. 相似文献
92.
Anna Lourantou Jean-Pierre Thomé Anne Goffart 《Lakes & Reservoirs: Research and Management》2007,12(4):261-274
Bütgenbach Reservoir is situated in the High Ardennes plateau in eastern Belgium (50°25′N; 6°13′E). It is used principally for flood control and for production of hydroelectric energy. It has been subjected to a previous series of studies because of its eutrophication problems and their impacts on the local economy. Bütgenbach Reservoir was emptied during spring 2004 for dam restoration, being refilled in mid‐September of the same year. Selected physicochemical and biological parameters (temperature, dissolved oxygen, pH, inorganic and organic nutrients, chlorophyll‐a) were measured fortnightly during the lake's productive season (mid‐April to mid‐October 2005) at three sampling sites throughout the water column. The water quality conditions after refilling of the lake were compared to previous studies accomplished at the same sampling sites (prior to emptying the lake). The actual trophic status was mesotrophic to eutrophic, based on the combination of total phosphorous and chlorophyll‐a concentrations, as well as water transparency. Bütgenbach Reservoir generally exhibits good water quality, based on the French water quality system SEQ‐eau. A longitudinal decrease in water quality was observed from upstream to downstream, because mainly of the differences in lake bottom morphology and water residence time, and their impacts on nutrient distribution in the lake. 相似文献
93.
Tek Bahadur Gurung Ram Prasad Dhakal Jay Dev Bista 《Lakes & Reservoirs: Research and Management》2006,11(3):141-148
The seasonal patterns of phytoplankton primary production, chlorophyll‐a concentration, cell number and several other limnological variables in Lake Phewa, located in the active monsoon zone in Central Himalaya, Nepal, were studied for a year beginning in April 2001. During the study period, the gross primary production and chlorophyll‐a concentrations were relatively low during the monsoon season. The phytoplankton cell number, represented by 24 genera, also fluctuated seasonally, but tended to increase in the pre‐ and post‐monsoon period. These results suggest that the monsoon plays a crucial role in the primary production and phytoplankton dynamics for Lake Phewa. Among the phytoplankton species, Microcystis aeruginosa, a representative species for eutrophic lakes, was the dominant phytoplankton. At the same time, however, it is clear that the lake is not yet heavily eutrophic. The present study suggests that the exchange of lake water during the monsoon season contributes to maintaining the health of the lake against further degradation. Nevertheless, the silt carried in the monsoon rain run‐off from the lake's catchment area suggests increasingly serious degradation problems for this small mountainous lake. 相似文献
94.
Changes in environmental variables, as well as in planktonic algae and in crustacean zooplankton abundance, were investigated in a meso‐eutrophic reservoir during two successive years: one wet, another dry. In the wet winter, both total phosphorus and soluble reactive phosphorus reached their maxima, whereas water transparency achieved the minimum during the same period. The obtained data suggest that total phosphorus and soluble reactive phosphorus peaks were related to the increase in nutrient loading as a consequence of the intense rainfall which occurred during this period. Phytoplankton composition was dominated by Cyclotella spp., except during the dry winter when Anabaena became dominant. The dominance of this alga seemed to be related to nitrate depletion. The establishment of more stable environmental conditions (e.g. low water turbulence, longer water retention time) and the increase of irradiance during the dry period (summer 2001 and winter 2001/2002) also favoured growth of this cyanobacterium. Concomitant to Anabaena dominance, Ceriodaphnia abundance decreased while nauplii increased. Although variation between the two years of study seemed to be related to the sequence of a pluvial and a dry year, further research is needed to evaluate whether variations in precipitation intensity influence reservoir ecological processes. 相似文献
95.
Water holes characteristic of the middle sections of fragmented, lowland, semi‐arid rivers act as refuge areas for biota during low flow and drought. However, river regulation and catchment development has altered the hydrological and nutrient status of these refuges. A series of artificial ponds were used to imitate these habitats and experiments were designed to determine whether stochastically fluctuating water levels with maximum amplitude of 65 cm would counteract the influence of nutrient enrichment on the growth of three key aquatic plant species. The species (Cyperus gymnocaulos, Triglochin procerum and Typha domingensis) represented different functional groups that inhabit contrasting zones across the elevation gradient. Each species was grown at a different elevation within the experiment reflecting each species' elevation preference and supplied with the equivalent to 0, 10 or 30 g N m?2 year?1. Under a static water regime nutrient enrichment doubled the total biomass of all three species. In contrast under a fluctuating water regime, nutrient addition did not influence total biomass of either T. domingensis or T. procerum and the performance of C. gymnocaulos was reduced by ca. 30%. These data suggest that if environmental flows were managed to provide a natural stochastically fluctuating water regime, this would nullify the influence of nutrient enrichment and limit the distribution and spread of invasive species such as Typha, that exists under the current altered conditions. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
96.
The Almbach, a headwater stream in the Upper Austrian foothills of the Alps bordered by mountain pastures was studied to gain insight into the benthic macroinvertebrate community and its habitat. Two springs, one of them fenced in to prevent livestock access, and two more sites of the headwater section were investigated. A discriminant analysis (DA) based on hydrochemistry and phytobenthos revealed three functions with 86% of all samples correctly classified to the four sites. The first function clearly separated the two springs with phosphorus and oxygen as the most important discriminating factors. Spring sites and headwater sites were split up by the second function mainly due to nitrate concentration, but no differences could be detected for the two stream sites. With the exception of the fenced spring, indicator taxa could be assigned to all sites by means of an Indicator Species Analysis (ISA). Polyvalent taxa groups such as Gastropoda and Ostracoda were indicators for the unfenced spring site. This is in accordance to observed low abundances of sensitive plecopteran taxa and suggests a negative impact of livestock trampling on the invertebrate community. Redundancy Analysis explained 99% of the macroinvertebrate‐environment relation. The first axis separated springs from stream sites and was correlated with nitrate, pH, water temperature and phytobenthos eveness. Phosphorus was responsible for splitting the unfenced spring and the downstream station from the fenced spring and the upstream location along the second axis. Ephemeropteran taxa like Habrophlebia sp. were related to elevated pH, increased water temperature and low nitrate concentrations. In accordance to the ISA, nitrate played an important role for tolerant Gastropoda and Ostracoda at the unfenced spring. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
97.
With the purpose of finding out whether seasonal water level fluctuations could affect water quality in a reservoir subjected to those changes, trends in environmental variables and in phytoplankton and zooplankton assemblages were analysed. The reservoir's hydrological cycle was characterized by three regimes. The maximum level phase lasted from January to the beginning of June, the emptying phase existed between mid‐June to the beginning of September and the minimum level phase lasted from mid‐September to the beginning of the first autumn/winter rain events. The highest values of total phosphorus, soluble reactive phosphorus, nitrate, water colour and chlorophyll a were found during the minimum level phase. The phytoplankton assemblage was dominated by taxa typical of meso‐eutrophic environments during the emptying and minimum level phases. However, during the maximum level phase, taxa generally found in more oligotrophic systems were observed here also. Similar to other disturbed systems, the zooplankton assemblage was dominated by Rotifera, except in summer and autumn when the cladoceran Ceriodaphnia quadrangula and/or the copepod Tropocyclops prasinus became dominant. Although those shifts seem to be related to water level variations, further research is needed to evaluate to what extent they might also be induced by other seasonal factors acting independently of water fluctuations. Based upon the obtained data, suggestions for reservoir management are proposed. 相似文献
98.
通过 1 984～ 1 998年河南开封不同土壤耕层有机质与氮磷钾养分分析 ,论述了不同农业利用方式和农田管理措施下不同土壤耕层养分的时空变化特征和硝态氮累积特性 .由于“重氮磷 ,轻有机肥 ,忽视钾素补给” ,土壤养分变化的总趋势以下降 ,钾素耗竭性明显 .土壤养分空间变化较为复杂 ,但不同土壤间耕层养分为普通淡色潮湿雏形土 >底锈干润雏形土 >石灰淡色潮湿雏形土 >灌淤旱耕人为土 >简育干润雏形土 >干润砂质新成土 .硝态氮累积表现为由上至下递减或由上至下递增和相对积累于土体某一深度 .最后研究提出了土壤持续利用的有效技术途径 . 相似文献
99.
Xuhui Ying Jinfang Ma Qionglin Liang Yiming Wang Gang Bai Guoan Luo 《Journal of food science》2013,78(8):C1173-C1182
The main constituents in an aqueous extract of Tricholoma matsutake (Tm) were identified by high‐performance liquid chromatography coupled with diode array detection and electrospray ionization time‐of‐flight mass spectrometry (HPLC‐DAD/TOF‐MS) and ion trap mass spectrometry (HPLC‐DAD/Trap‐MSn). The main factors in the extraction process which affect the yields of nutrients were optimized by single‐factor experiments and orthogonal experiment design. In total, 12 constituents were identified from the aqueous extract of Tm, including tyrosine, cytidine, uridine, eritadenine, phenylalanine, nicotinamide, inosine, guanosine, tryptophan, adenosine, 5′‐deoxy‐5′‐methylthioadenosine and riboflavin. The optimized extraction conditions were: the ratio of water to sample was 10 : 1 (v/w), Tm was extracted by ultrasonic‐assisted extraction for 10 min, followed by water bath heating at 60 °C for 1 h. Among these extraction factors, the heating temperature is significant based on analysis of variance (ANOVA). The yields of nutrients were affected dramatically at high temperature leading to the loss of nutrients, especially for nucleosides and some amino acids. 相似文献
100.
David Julian McClements 《Journal of food science》2015,80(7):N1602-N1611
There has been a surge of interest in the development of nanoscale systems for the encapsulation, protection, and delivery of lipophilic nutrients, vitamins, and nutraceuticals. This review article highlights the challenges associated with incorporating these lipophilic bioactive components into foods, and then discusses potential nanoscale delivery systems that can be used to overcome these challenges. In particular, the desirable characteristics required for any nanoscale delivery system are presented, as well as methods of fabricating them and of characterizing them. An overview of different delivery systems is given, such as microemulsions, nanoemulsions, emulsions, microgels, and biopolymer nanoparticles, and their potential applications are discussed. Nanoscale delivery systems have considerable potential within the food industry, but they must be carefully formulated to ensure that they are safe, economically viable, and effective. 相似文献