全文获取类型
收费全文 | 3372篇 |
免费 | 290篇 |
国内免费 | 243篇 |
专业分类
林业 | 249篇 |
农学 | 99篇 |
基础科学 | 302篇 |
1148篇 | |
综合类 | 1089篇 |
农作物 | 36篇 |
水产渔业 | 542篇 |
畜牧兽医 | 117篇 |
园艺 | 25篇 |
植物保护 | 298篇 |
出版年
2024年 | 24篇 |
2023年 | 81篇 |
2022年 | 92篇 |
2021年 | 151篇 |
2020年 | 116篇 |
2019年 | 144篇 |
2018年 | 101篇 |
2017年 | 166篇 |
2016年 | 171篇 |
2015年 | 146篇 |
2014年 | 189篇 |
2013年 | 210篇 |
2012年 | 346篇 |
2011年 | 293篇 |
2010年 | 199篇 |
2009年 | 193篇 |
2008年 | 184篇 |
2007年 | 206篇 |
2006年 | 157篇 |
2005年 | 131篇 |
2004年 | 115篇 |
2003年 | 81篇 |
2002年 | 62篇 |
2001年 | 59篇 |
2000年 | 58篇 |
1999年 | 44篇 |
1998年 | 41篇 |
1997年 | 28篇 |
1996年 | 21篇 |
1995年 | 24篇 |
1994年 | 23篇 |
1993年 | 17篇 |
1992年 | 8篇 |
1991年 | 6篇 |
1990年 | 4篇 |
1989年 | 5篇 |
1988年 | 5篇 |
1987年 | 3篇 |
1986年 | 1篇 |
排序方式: 共有3905条查询结果,搜索用时 203 毫秒
11.
Polymorphic mountain whitefish (Prosopium williamsoni) in a coastal riverscape: size class assemblages,distribution, and habitat associations
下载免费PDF全文
![点击此处可从《Ecology of Freshwater Fish》网站下载免费的PDF全文](/ch/ext_images/free.gif)
We compared the assemblage structure, spatial distributions, and habitat associations of mountain whitefish (Prosopium williamsoni) morphotypes and size classes. We hypothesised that morphotypes would have different spatial distributions and would be associated with different habitat features based on feeding behaviour and diet. Spatially continuous sampling was conducted over a broad extent (29 km) in the Calawah River, WA (USA). Whitefish were enumerated via snorkelling in three size classes: small (10–29 cm), medium (30–49 cm), and large (≥50 cm). We identified morphotypes based on head and snout morphology: a pinocchio form that had an elongated snout and a normal form with a blunted snout. Large size classes of both morphotypes were distributed downstream of small and medium size classes, and normal whitefish were distributed downstream of pinocchio whitefish. Ordination of whitefish assemblages with nonmetric multidimensional scaling revealed that normal whitefish size classes were associated with higher gradient and depth, whereas pinocchio whitefish size classes were positively associated with pool area, distance upstream, and depth. Reach‐scale generalised additive models indicated that normal whitefish relative density was associated with larger substrate size in downstream reaches (R2 = 0.64), and pinocchio whitefish were associated with greater stream depth in the reaches farther upstream (R2 = 0.87). These results suggest broad‐scale spatial segregation (1–10 km), particularly between larger and more phenotypically extreme individuals. These results provide the first perspective on spatial distributions and habitat relationships of polymorphic mountain whitefish. 相似文献
12.
Flow alteration and thermal pollution depress modelled growth rates of an iconic riverine fish,the Murray cod Maccullochella peelii
下载免费PDF全文
![点击此处可从《Ecology of Freshwater Fish》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Nick S. Whiterod Shaun N. Meredith Paul Humphries Bradford S. Sherman John D. Koehn Robyn J. Watts Brett A. Ingram Tom Ryan 《Ecology of Freshwater Fish》2018,27(3):686-698
The serial discontinuity concept (SDC) proposes that hypolimnetic‐releasing impoundments cause major disruptions to the naturally occurring physical, chemical and biological gradients of rivers but that this impact diminishes with distance downstream. Such a gradient in discharge, flow velocity and temperature regime occurs below a large hypolimnetic‐releasing impoundment, the Hume Dam, on the River Murray in south‐eastern Australia. To examine the effects of this disturbance gradient on a warm‐water large‐bodied freshwater fish, the Murray cod (Maccullochella peelii), a bioenergetics model was developed and calibrated to explore energy expended under differing water velocities and temperature regimes. Model simulations predicted negative growth of juveniles directly downstream of the impoundment, due largely to the energetic costs associated with active and, to a lesser extent, standard metabolism outweighing the achievable energetic gains through food consumption. As flow velocity and temperature regimes became more favourable downstream, so did the simulated growth of the species. It was not until +239 km downstream of the impoundment that the model predicted that flow velocity and temperature regimes were suitable for greater weight gains. The modelled growth responses of juvenile Murray cod are consistent with the predictions of the SDC, emphasising that changes in the bioenergetics of individuals are likely to be reflected in reduced growth rates under the changed flow velocity and temperature regimes imposed by disturbance gradients. This research represents a valuable step in the biological understanding of Murray cod within variable riverine environments and emphasises the urgency required to mitigate impacts associated with hypolimnetic impoundments. 相似文献
13.
以黄河三角洲地区台田间渗水池塘为研究对象,分析了渗水池塘的水质因子变动规律。结果表明:不同池塘间水质因子差异较大,但均表现出水体盐度较高、氮磷等营养物质含量较低、K+离子含量相对较低、Ca2+离子和Na+离子含量相对较高及K+/Na+显著低于正常海水等特点。结果说明,黄河三角洲地区台田间渗水池塘的水质条件不适合直接进行水产养殖,应对水质调整后才可做养殖之用。 相似文献
14.
环境管制政策的实施效果评价——以九龙江流域"限猪令"为例 总被引:1,自引:0,他引:1
环境管制一直是我国环境污染治理所采取的主要政策之一,对我国的环境保护起到了一定的积极作用,同时也存在一些弊端,致使其成效不甚显著甚至背离初始目标.在此情况下,如何评价环境管制政策绩效具有重要的理论和实际意义.本文以九龙江流域"限猪令"政策为例,选取政策目标实现度、主要环保指标实现、环保投入效益、公众参与及满意度四个方面... 相似文献
15.
利用渭河流域25个气象站点1980−2018年月值气象数据集,基于Penman-Monteith蒸散模型计算多个时间尺度的标准化降水蒸散发指数(SPEI),分析渭河流域气象干旱的演变、趋势、影响范围、发生频率和持续时间等时空变化特征,以期为渭河流域防灾减灾管理提供科学依据。结果表明:(1)近39a来渭河流域有明显的干湿周期变化,但整体上呈变干的趋势,干旱时段主要集中在1995−2009年,其中以2000−2009年的干旱站次比最大,平均达到36%,且干旱持续时间最长,约3.6个月,1980−1989年干旱持续时间最短,约1.6个月;(2)渭河流域秋季总体呈湿润变化趋势,而春季和夏季干旱在不断加剧,是区域年际干旱的主要驱动力;(3)渭河流域干旱以危害性较小的轻中旱为主,但2000年前后出现严重及极端干旱的站次相对较多,其中1997年研究区内发生的干旱程度较高,影响范围较广;(4)不同时间尺度各等级干旱发生频率的变化规律表现一致,均呈现出干旱等级越高发生频率越低的态势,且极端干旱在年际尺度内发生次数较为频繁,从空间上看渭河流域东北部是干旱多发区。总之,近39a来渭河流域总体干旱较为严重的时段为2000−2009年,且研究区内干旱呈北重南轻特征,因此北部地区仍需加强防灾管理。 相似文献
16.
文章主要是对我国的珠江三角洲地区的生鲜农产品冷链的物流方式进行探究,结合现在我国的生鲜农产品冷链运输技术的现状和发展前景进行大胆的预测,并提出我国的珠江三角洲地区的生鲜农产品冷链的运输技术的网络化的建议。 相似文献
17.
18.
19.
ZHOU Hua-jun 《保鲜与加工》2001,(5):1
Hydrodynamic models have been widely applied to river and coastal engineering projects. Understanding the hydrodynamic process is a prerequisite to modeling the pollution and sediment transport process. Hydrodynamic models of varying degrees of complexity should be selected appropriately according to the object of study, physical properties of the flow domain and hydrodynamic features. The essential procedure of hydrodynamic simulation is described in this paper. The main problems such as selection of model dimension in physical space, choice of variables, boundary conditions, initialization, design of computational grid, model resolution, parameterization of smaller scale fluctuations and sub-grid scale processes, model calibration and verification are investigated. 相似文献
20.
The thesis introduces the origin of demonstrate item on the city water environment quality improvement-tao-hua river water environment competitive administer, analyzes the current situation and cause of taohua river valley pollute and ecological destroy, and summarizes the gain and loss in projects which have taken place in the domestic city water environment competitive administer. On the base, a collectivity idea is put forward that it should stress recovering and rebuilding ecosystem on the item. Due to the water environment in taohua river is representative, if the item can be favorably carried out, it not only improves GuiLin water environment, makes people life advantage and expands city tour, but also produces important affects to the other domestic cities in the condition of using marketable economy means to improve city water environment quality. 相似文献