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   469篇
  免费   39篇
  国内免费   82篇
测绘学   2篇
地球物理   54篇
地质学   35篇
海洋学   443篇
天文学   1篇
综合类   43篇
自然地理   12篇
  2024年   3篇
  2023年   7篇
  2022年   13篇
  2021年   8篇
  2020年   24篇
  2019年   21篇
  2018年   15篇
  2017年   13篇
  2016年   20篇
  2015年   16篇
  2014年   19篇
  2013年   51篇
  2012年   9篇
  2011年   17篇
  2010年   12篇
  2009年   22篇
  2008年   33篇
  2007年   31篇
  2006年   28篇
  2005年   17篇
  2004年   21篇
  2003年   11篇
  2002年   21篇
  2001年   20篇
  2000年   18篇
  1999年   13篇
  1998年   13篇
  1997年   14篇
  1996年   20篇
  1995年   6篇
  1994年   6篇
  1993年   11篇
  1992年   7篇
  1991年   3篇
  1990年   1篇
  1989年   1篇
  1988年   1篇
  1987年   2篇
  1986年   8篇
  1985年   1篇
  1984年   4篇
  1983年   2篇
  1982年   3篇
  1981年   3篇
  1980年   1篇
排序方式: 共有590条查询结果,搜索用时 15 毫秒
251.
介绍了国内外常用的导向钻机给进机构的结构形式,并对其特点进行分析总结,为给进机构的设计提供参考。  相似文献   
252.
根据2013年5月和11月在厦门周边海域60个取样站获得的大型底栖动物数据,运用功能群方法分析该海域大型底栖动物功能群的时空格局及其与环境因子的关系。共鉴定大型底栖动物9门160科411种,其中肉食者152种,浮游生物食者102种,碎屑食者103种,杂食者48种和植食者6种,菲律宾蛤仔(Ruditapes philippinarum)和丝鳃稚齿虫(Prionospio malmgreni)分别是该海域春、秋季的主要优势种。总体而言,浮游生物食者和肉食者为该海域的主要优势功能群,但各区域的优势功能群存在一定差异,围头湾的主导功能群为肉食者和碎屑食者,大嶝海域以肉食者和浮游生物食者为主,而九龙江口和同安湾则以碎屑食者和浮游生物食者占优势。典范对应分析结果表明,水深和底温是影响该海域大型底栖动物功能群时空格局的主要因素,活性磷酸盐、底盐、有机质和叶绿素a等对其分布也有一定的影响。  相似文献   
253.
福建南部沿岸水域尖头斜齿鲨的食性   总被引:9,自引:0,他引:9  
王军  丘书院 《台湾海峡》1996,15(4):400-406
对福建南部沿岩水域尖头斜齿鲨食性的分析表明:尖头斜齿鲨以硬骨鱼类,头路类,长尾类和口足类为主要食料,其营养级为3.45,年平均饱满指数为9.45‰,摄食率为76%,其雌鱼在分娩期间仍有摄食,但强度降低,雌、雄个体食物组成无明显差异,幼鱼摄食强度大于成鱼。  相似文献   
254.
Life Strategies in Two Bacterivorous Marine Nematodes: Preliminary Results   总被引:2,自引:0,他引:2  
Abstract. Temperature dependence of sex ratio and maximal densities of the estuarine, deposit-feeding nematodes Pellioditis marina (Rhabditidae ) and Diplolaimelloides meyli (Monhysreridae ) were investigated in vitro . Both species are characteristic for organically enriched habitats. Data from competition culture experiments with both species are integrated with information from respiration measurements at different temperatures, and from observations on the influence of temperature and bacterial density on uptake rates.
Sex ratio was significantly influenced by temperature in both species, with the highest relative numbers of males at the highest temperatures. Total numbers attained fluctuated only moderately in the 10 to 20 °C interval, but increased and decreased highly significantly at 25 °C for D. meyli and P. marina , respectively.
Respiration at temperatures from 5 °C to 30 °C was measured with nematodes from monospecific cultures using a modified Clark electrode procedure. Respiration was dependent upon temperature in the entire range for P. marina , but not for D. meyli , where only at 25 °C a clear respiratory acceleration was observed.
Feeding experiments with P. marina showed a dominant influence of bacterial density on uptake rates, with a lower but still significant temperature effect.
These data are discussed in relation to the overall life strategies of both species.  相似文献   
255.
南极大磷虾(Euphausia superba)为南大洋生态系统中的关键种,也是南极生态系统食物网中的重要枢纽。该种秋冬季转换期的营养信息对于理解其知之甚少的越冬机制非常重要。但关于此方面的少数研究在时空变化上仍存在着差异。为此,我们调查了南极半岛秋季(4-5月)和冬季(6月)磷虾成体δ13C和δ15N值的个体、月份及区域性差异。我们的目标旨在检验该期间磷虾的营养变化以及磷虾与其在南极海洋生态系统中的摄食环境之间的关系。结果如下:(1)磷虾δ13C值与体长之间无显著关系,但δ15N值与体长之间则存在显著相关性;(2)秋季磷虾δ13C值呈现增长趋势,但冬初季节并无显著变化,此期间δ15N值无显著不同;(3)布兰斯菲尔德与南设得兰群岛之间的δ15N平均值显著不同。我们的数据表明南极半岛秋至初冬转换期间磷虾成体营养呈现个体、季节性及区域性变化。  相似文献   
256.
The temporal patterns of feeding activity of Lipophrys pholis and Coryphoblennius galerita were studied by snorkelling dives during high-waters. The feeding activity was especially intense during the rising-tide when compared to the ebbing-tide. The relative abundances of C. galerita and small L. pholis observed on rock-walls were higher during the rising- and high-tide. However, L. pholis larger than 7–8 cm showed higher relative abundance on rock-walls during the ebbing phase, apparently because they move up and down with the tide, and many individuals were in their downward movements as the tide ebbs. It is hypothesized that the asymmetry in feeding pattern observed in both species reflects a trade off between two conflicting needs, maximizing feeding time and reaching their low-tide shelters in time, avoiding the risk of getting stranded in unsuitable locations.  相似文献   
257.
Field measurements on leaf removal by populations of sesarmid crabs at different locations in the Bangrong mangrove forest, Phuket, Thailand, indicated that crabs on average can remove 87% of the daily leaf litter fall by ingestion or burial. The removal rate is correlated positively with the number of crab burrows and negatively with tidal inundation time. The results from the field were supplemented with observations on the behavior of Neoepisesarma versicolor in laboratory microcosms and a mangrove mesocosm. N. versicolor feeds primarily at night and total time spent feeding was up to an order of magnitude higher in the artificial microcosms than under simulated in situ conditions in the mesocosm. Most of the time during both day and night was spent resting near the entrance or inside burrows. N. versicolor mainly feeds on mangrove leaves and scraps of food material from the sediment surface. This is supported by examinations of stomach content, which showed that 62% is composed of higher plant material and 38% of detritus and mineral particles from the sediment. The nutritive value of leaves and detritus is insufficient to maintain crab growth. Sesarmid crabs may instead obtain the needed nutrients by occasional consumption of nitrogen-rich animal tissues, such as carcasses of fish and crustaceans, as indicated by the presence of animal remains in the stomach and the willingness of crabs to consume fish meat. Laboratory experiments on leaf consumption and leaf preferences of N. versicolor indicate that they preferentially feed on brown leaves, if available, followed by green and yellow leaves. If all species of sesarmid crabs in the Bangrong mangrove forest consume leaves at the same rate as N. versicolor, they could potentially ingest 52% of the total litter fall.  相似文献   
258.
Understanding the feeding selectivity on phytoplankton by shellfi sh is currently a big challenge. In order to investigate the feeding behavior of bay scallop ( Argopecten irradians ) on phytoplankton, we compared its compositions of phytopigments in digestive glands with those in the surrounding seawater, and conducted fi ve consecutive investigations between July and November 2016 in a bay scallop culture area along coast of Qinghuangdao City, northwest of the Bohai Sea, China. Phytopigments in four-size fractionated phytoplankton of seawater (micro-(20-200 μm);nano(L)-[10-20 μm];nano(S)-[2.7-10 μm], and pico-[<2.7 μm]) and digestive glands of A . irradians were examined to investigate the selective feeding of A . irradians . Results show that fucoxanthin and peridinin constituted the major part of taxonomically diagnostic carotenoids (TDCs) in the micro- and nano(L)-phytoplankton in seawater. Compared with total phytoplankton biomass of seawater (TPB, sum of the four sizes), a substantial decrease of fucoxanthin proportion to total DCs in digestive glands was observed while that of peridinin, 19′-butanoyloxyfucoxanthin, alloxanthin and 19′-hexanoyloxy-fucoxanthin showed an obvious increase when those pigments were mainly confi ned to micro-sized phytoplankton (20-200 μm). However, zeaxanthin and prasinoxanthin were mainly confi ned to nano(s)- and pico-phytoplankton, of which the proportions in digestive glands were usually lower in TPB. The contribution of lutein to total DCs in digestive glands (with an average of 7.23%) increased compared with TPB of seawater (with an average of 0.63%) during all fi ve sampling times.  相似文献   
259.
海洋微塑料作为生物载体的生态效应   总被引:2,自引:0,他引:2  
随着海洋塑料垃圾的不断增多, 海洋微塑料作为一种新型的海洋污染物逐渐受到重视。目前关于微塑料来源分布及分析方法的认识较为普遍, 大部分研究注重于海洋生物误食微塑料、微塑料吸附及释放有毒物质的环境及生态效应, 而微塑料作为微生物、附生动植物的载体作用仍有待研究。文章综述了微塑料作为生物载体的三个作用: 1) 聚集作用。微塑料表面易覆盖生物膜形成微型生物群落, 为基因水平移动提供场所, 可能引发致病基因、抗生素抗性基因(antibiotic resistance genes, 简称ARGs)的转化、转导。2) 扩散作用。来源于污水的微塑料上可能附着生长着有害藻种、致病菌耐药菌或其他生物, 随水流移动到适宜区域, 可能引发赤潮或导致致病菌及耐药菌的传播扩散, 形成生物入侵。3) 首次提出微塑料的“捕食增强作用”概念, 即有机物、营养盐、生物群落聚集在微塑料颗粒上可提高捕食者的捕食效率, 从而促进海洋动物摄食微塑料, 进而加剧微塑料颗粒对海洋生物的毒理病理作用。文章着重阐述了“微塑料+生物”的生态效应。  相似文献   
260.
The diet of three Antarctic copepods, Calanus simillimus , Rhincalanus gigas and Pleuromamma robusta , was studied from samples collected during the European iron fertilization experiment (EIFEX) in austral fall 2004. The diet was investigated using microscopical gut content analysis. The food spectra of the three copepod species showed a clear overlap. The pennate diatom Fragilariopsis kerguelensis was commonly found in all guts. Large numbers of cells were fractured, but some were found complete when arranged as chains. The same was true for the genus Chaetoceros , a centric diatom, of which even their long and complete setae could be observed. In general the ingested food particles extended over a wide size range up to a few 100 μm. The copepod C. simillimus , for example, fed on large foraminifera and swallowed them whole. Compared to C. simillimus and R. gigas , P. robusta guts contained a higher percentage and larger diversity of hard-shelled protozoa including tintinnids, Radiolaria and Acantharia.  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