数学概念的学习   总被引：4，自引：0，他引：4  

韩振芳  张青  杨小姝 《河北北方学院学报(自然科学版)》2006,22(2):74-77,82

数学的一个显著特点就是它的抽象性.抽象的主要表现是:定义了一系列新的概念.谈了数学概念学习的步骤,数学概念理解程度的体现,以及学习概念应抓住的几个环节.  相似文献   

Isoëtes sabatina (Isoëtaceae,Lycopodiopsida): Taxonomic distinctness and preliminary ecological insights     

Rossano Bolpagni  Sara Magrini  Andrea Coppi  Angelo Troìa  Janne Alahuhta  Marit Mjelde  Mattia M. Azzella 《水产资源保护:海洋与淡水生态系统》2021,31(10):2690-2696

1. Isoëtes sabatina is the rarest aquatic quillwort in Europe. Although recently found (2013) in Lake Bracciano (central Italy), the species is just one step away from extinction with an estimated population not exceeding 400 individuals and a spatial range of a few hundred square metres.
2. Lake Bracciano is a deep, oligo-mesotrophic Mediterranean volcanic lake that has been subjected to human activities. From January to October 2017, the lake experienced a dramatic water level decrease (up to −1.50 m), which significantly affected the littoral zone and the habitat of I. sabatina.
3. To improve the chances of survival of I. sabatina, the first eco-taxonomic investigation on this species was carried out to describe its genetic distinctness, physical and chemical requirements and companion species.
4. The phylogenetic position of I. sabatina was investigated by applying standard DNA barcoding methods. Simultaneously, during summer 2019, the physical and chemical features of water and sediments of the I. sabatina population and five small Alpine lakes colonized by Isoëtes echinospora – a supposed close relative – were characterized. These data were then compared with the available data on the trophic requirements of the target obligate aquatic Isoëtes, together with Isoëtes lacustris and Isoëtes malinverniana.
5. The present survey confirmed the taxonomic and ecological distinctness of I. sabatina – providing the first evidence of genetic differentiation from I. echinospora. Isoëtes sabatina grows in waters with temperature, conductivity and total alkalinity up to 30°C, 561 μS cm⁻¹ and 3.45 meq L⁻¹, respectively.
6. The edaphic requirements of I. sabatina confirm its outstanding conservation value, and this study offers a basic understanding of how to prevent its extinction. Now, all possible actions must be taken immediately to save this species.

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Big impacts from small abstractions: The effects of surface water abstraction on freshwater fish assemblages     

Nixie C. Boddy  Kevin M. Fraley  Helen J. Warburton  Phillip G. Jellyman  Doug J. Booker  Dave Kelly  Angus R. McIntosh 《水产资源保护:海洋与淡水生态系统》2020,30(1):159-172

1. Abstractions and diversions are prevalent in river networks worldwide; however, specific mechanisms and measures reflecting changes in functional characteristics of aquatic assemblages in response to flow abstraction have not been well established. In particular, the influence of small takes on fish assemblages is poorly understood.
2. Field surveys and stable‐isotope analyses were used to evaluate the impact of differing levels of flow abstraction on fish assemblage structure, and native–non‐native patterns of coexistence, associated with small surface water abstractions in four streams in New Zealand. Study design accounted for longitudinal processes (spatial autocorrelation) to isolate the effects of abstractions on fish assemblages.
3. Reaches with reduced flows downstream of abstraction points had significantly lower fish abundances per metre of stream length, probably because of decreased habitat size, altered interspecific interactions and barriers to movement. The loss of larger fish in reaches with high abstraction resulted in shallower mass–abundance slopes and shorter stable isotope‐derived food‐chain lengths, likely to have been caused by fewer trophic links in the food web. The large fish absent from these reaches were flow‐sensitive introduced salmonids, resulting in higher relative abundances of small‐bodied native fish, probably as a result of predatory and competitive release.
4. Quantification of metrics designed to characterize ecosystem functioning as well as abundance and species composition indicated that small water abstractions can alter both the structure and composition of stream fish assemblages and modify the outcomes of native–non‐native species interactions. Thus, a better understanding of the effects of small abstractions could be used to improve the strategic management of fish in invaded riverscapes.

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Floodplains of a regulated southern alpine river (Brenno,Switzerland): ecological assessment and conservation options     

M. Brunke 《水产资源保护:海洋与淡水生态系统》2002,12(6):583-599

• 1. The fifth‐order southern alpine Brenno River and its floodplains are severely impaired by embankments, water abstractions and the construction of dams for hydropower generation. The river's annual mean discharge is reduced to 27% of the natural flow and the number of small (50–100 m³ s^?1) and medium‐sized (101–150 m³ s^?1) floods is reduced significantly.
• 2. Lateral hydrological connectivity has decreased strongly as a result of the flow regulation, whereas the significance of vertical connectivity has increased. The remnants of the middle and lower floodplains still contain springbrooks, ponds, and intermittently connected channels, which are all sustained by emerging groundwater.
• 3. Aquatic floodplain habitats can be classified partially by faunal composition. The lotic/lentic gradient appears to be a dominant compound factor structuring invertebrate assemblages when considering all aquatic habitats. The exfiltration of groundwater in conjunction with a reduced lateral connectivity enables the occurrence of many stenotopic species.
• 4. A comparison of present floodplains with those shown on historical maps shows that the fluvial landscape has changed markedly. The proportions of functional floodplain units characterized by autogenic succession and trends to late successional stages (i.e. senescence) have increased.
• 5. Floodplains of the Brenno river are still considered to represent important ecological areas and they are protected by law. Whilst the legislative protection of floodplains is the basis for conservation, this does not include the restoration of driving hydrological processes. An annual release of small‐ and medium‐sized floods could generate a diversity of disturbances, thereby promoting rejuvenation and counteracting senescence.
• 6. Alternative conservation options that circumvent ongoing contracts of water use, though generally promising, are limited because of the operation of hydropower schemes. At present the exfiltrations of groundwater mitigate the impacts of flow regulation on the ecological integrity of the floodplains.

Copyright © 2002 John Wiley & Sons, Ltd.  相似文献   

树脂法提取冠毒素的研究   总被引：2，自引：0，他引：2  

吴晓玉  丰娟  周静  储炬 《江西农业大学学报》2007,29(4):660-664

通过分析比较几种大孔吸附树脂和离子交换树脂对发酵液中冠毒素的分离纯化效果,结果显示:1400大孔吸附树脂对发酵液冠毒素的吸附量最大,用700 g/L乙醇洗脱,洗脱率为79%,回收率为48%。该类大孔吸附树脂操作简便、快速,比较适用于工业上的提取工艺。  相似文献   

基于遥感图像处理的图谱单元提取与分析     

陈燕  齐清文  杨桂山 《水土保持学报》2006,20(2):193-196

以遥感图像为基本信息源，对黄土高原沟间地地貌信息图谱的基本单元墚、峁进行提取与分析，主要内容包括图像预处理、图像识别、模式图操作、数据库管理和属性库管理5个部分。通过提取地物个体单元的边界得到每种图谱类型的标准参数和模式图，对未知类型的图谱个体单元提取参数后与模式图比较分析，归为相应类型的数据库中。最后对下一步的工作进行了展望。  相似文献   

西南岩溶山区水资源可持续利用评价指标选取及权重确定   总被引：4，自引：1，他引：3  

楚文海  高乃云  鄢贵权  苏维词 《水土保持通报》2008,28(1):59-64

降雨径流是引起土壤侵蚀的主要动力,因此准确预测不同下垫面条件下的径流量是进行土壤侵蚀预报和水土流失治理的关键.SCS-CN方法是目前国际上预测无径流观测资料地区降水产流的主要模型.利用黄土高原地区6个试验小区的153场径流试验资料,选取3种具有代表性的土地利用方式:谷子、苜蓿和高粱,对SCS-CN方法在黄土高原的适应性进行了初步检验,优化出了适应黄土高原地区3种土地利用方式的模型参数.结果表明,优化后的模型参数对降雨径流的预测精度明显优于标准SCS-CN法;优化后的λ值为0.01,优化的谷子、苜蓿和高粱地CN2 值分别是74,74,72.  相似文献   

芸豆植物凝集素的提取及血凝效果研究     

郭春生  于蕾妍  葛蔚  张丽  唐超  王清吉 《中国兽药杂志》2008,42(1):49-50

采用蒸馏水、生理盐水和磷酸缓冲液（pH7．8）,对芸豆进行吸涨与浸提。用硫酸铵沉淀法得到芸豆植物凝集素。结果显示生理盐水和磷酸缓冲液提取组所得植物凝集素对红细胞的凝集效价高于蒸馏水提取组。凝血效果检测表明：磷酸缓冲液浸提组对鸡红细胞的凝集效果最好,生理盐水组次之,蒸馏水组最差。而对小鼠红细胞的凝集效果,生理盐水组与磷酸缓冲液组也优于蒸馏水组。三组提取物对羊红细胞的凝集效果都不明显。  相似文献   

亚麻种子色素提取及理化性质的研究     

金凤  王秀英  马庆 《内蒙古农业大学学报(自然科学版)》1999,(3)

本文将亚麻种子水溶性和酯溶性色素的提取及理化性质的研究公布如下：亚麻（ Ｌｉｎｕ ｍ ｕｓｉｔａｔｉｓｓｉｍ ｕ ｍ Ｖ）（ 包括胡麻） 种子的色素主要存在于亚麻种子的种皮色素层内。亚麻种子的酯溶性色素,只溶于酸化乙醇, Ｎａ Ｈ Ｃ Ｏ３ ,５０ ％ 乙酸中。呈酱红色,色素结晶呈多棱状,熔点为１６３ ℃,最大吸收光谱为（ Ｕ Ｖ）３７０ｎｍ ,并为左旋型,折光率为０ ．０３ ,酸碱稳定。亚麻种子的水溶性色素,只溶于水, Ｎａ Ｏ Ｈ、 Ｎａ Ｈ Ｃ Ｏ３ 、 Ｋ Ｏ Ｈ、 Ｈ４ Ｎ Ｏ Ｈ,溶液呈酱红色（ 碱中呈橙红色） ,晶体为雪花状无色晶体,熔点为２３７ ℃,最大吸收光谱（ Ｕ Ｖ） 为３５０ｎｍ ,左旋,折光率为０ ．０４ ,碱性条件下稳定。  相似文献   

Conservation implications of establishment success of the Critically Endangered Twee River redfin ‘Pseudobarbus’ erubescens (Skelton, ) in an artificial impoundment in South Africa          下载免费PDF全文

Martine S. Jordaan  Johannes A. van der Walt  Zanné Brink  Sonja Erasmus  Olaf L. F. Weyl 《水产资源保护:海洋与淡水生态系统》2017,27(4):886-889