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   448篇
  免费   36篇
  国内免费   74篇
测绘学   1篇
大气科学   1篇
地球物理   67篇
地质学   58篇
海洋学   393篇
综合类   24篇
自然地理   14篇
  2024年   4篇
  2022年   12篇
  2021年   9篇
  2020年   23篇
  2019年   13篇
  2018年   19篇
  2017年   18篇
  2016年   15篇
  2015年   8篇
  2014年   12篇
  2013年   44篇
  2012年   8篇
  2011年   23篇
  2010年   19篇
  2009年   38篇
  2008年   32篇
  2007年   24篇
  2006年   30篇
  2005年   25篇
  2004年   15篇
  2003年   16篇
  2002年   17篇
  2001年   13篇
  2000年   16篇
  1999年   14篇
  1998年   8篇
  1997年   8篇
  1996年   14篇
  1995年   4篇
  1994年   7篇
  1993年   6篇
  1992年   7篇
  1991年   5篇
  1990年   2篇
  1989年   3篇
  1987年   3篇
  1986年   2篇
  1985年   7篇
  1984年   3篇
  1983年   5篇
  1982年   5篇
  1981年   2篇
排序方式: 共有558条查询结果,搜索用时 15 毫秒
161.
于2011-2013年5月和10月在厦门五缘湾海域设置7个站位进行大型底栖动物调查,并结合2005年的历史数据,运用AMBI和M-AMBI指数法评价该海域海底生态环境质量的时空变化状况。2011-2013年共鉴定大型底栖动物9门106科193种,多毛类(83种)、甲壳类(53种)和软体动物(25种)是该海域的主要优势类群,平均总密度和湿质量生物量分别为(518±292)ind/m2和(54.8±88.9)g/m2。多毛类的短鳃树蛰虫(Pista brevibranchia)为该海域的绝对优势种,其平均密度可达(170±179)ind/m2,其他优势种还有似蛰虫(Amaeana trilobata)、西方似蛰虫(Amaeana occidentalis)、丝鳃稚齿虫(Prionospio malmgreni)、梳鳃虫(Terebellides stroemii)、独毛虫属一种(Tharyxsp.)、菲律宾蛤仔(Ruditapes philippinarum)、长指马尔他钩虾(Melita longidactyla)和伍氏蝼蛄虾(Austinogebia wuhsienweni)等。评价结果显示,该海域底栖生态系统受轻度干扰,生态环境质量介于一般—优良之间,与2005年的调查相比,2011-2013年该海域的生态环境质量有所好转,这可能与五缘湾及周边海域近年来进行的海堤开口改造、海洋环境整治,以及海湾纳潮量及水交换能力的改善有关。  相似文献   
162.
为研究钦州湾大型底栖动物的生态特征,探讨影响大型底栖动物分布的主要因素,于2011年5月在钦州湾进行大型底栖动物调查,并同步采集沉积物样品,进行理化性质分析。使用Surfer软件绘制各群落特征指数的平面分布图,用SPSS软件分析群落特征指数与沉积物环境因子之间的关联性。共采集到大型底栖动物种类55种。平均生物量为105.48 g/m2,平均密度为50ind/m2。香农-维纳指数(H')、种类均匀度指数(J')和丰富度指数(D)的平均值分别为1.44,0.63和0.64。群落特征指数与沉积物环境因子之间的相关性分析结果显示,密度与Cu,Pb,Zn,Cd,Cr,Hg以及有机碳呈显著负相关;种类数与所分析的10项沉积物环境因子之间均呈显著负相关;均匀度指数与Hg和有机碳呈显著负相关;丰富度指数与硫化物呈显著负相关;生物量和香农-威纳指数与沉积物各环境因子之间均没有显著的相关性。  相似文献   
163.
Pennate diatoms are monophyletic. Their principal cell wall elements, called valves, are shaped like a ship's hull. Within the pennates, the araphids are paraphyletic; they possess rimoportulae and pore fields located at the valve apices. The pore fields exude mucilage pads with which cells attach to one another to form chains. Many taxa use the pads also for attachment to substrata. Only a few genera are truly planktonic. The main question addressed in this study is whether the planktonic lifestyle is ancestral or derived. Phylogenies inferred from nuclear SSU rDNA gene sequences of diatoms indicated that the attached lifestyle is ancestral among the araphids, whereas a typically planktonic lifestyle seems to have developed at least three times and possibly four times independently. Acquisition of a planktonic lifestyle from benthic ancestry was accompanied by a reduction in the silicification of cell-wall elements, but changes in morphological characters shared by all four clades were not detected. The reason why only three or four araphid pennate clades have adopted a planktonic lifestyle may be related to constraints associated to their sexual reproduction mode. Partner cells of opposite mating type align with one another and produce isogametes. These gametes lack flagella; they move to one another in an amoeboid fashion, which functions well on surfaces, but seems a liability in a turbulent water column. The planktonic lineages must have overcome this constraint, e.g. by sinking to the bottom, or aggregating, to perform sexual reproduction. Members of the four araphid pennate lineages are now common constituents of the plankton, suggesting that they are ecologically successful.  相似文献   
164.
北黄海表层沉积物中的底栖有孔虫分布与海洋环境   总被引:1,自引:0,他引:1  
对2006年8月取自北黄海的302个表层沉积物样品中的底栖有孔虫群落进行了分析,结果表明:北黄海底栖有孔虫群落以适应低温、低盐的冷水种和广温、广盐种为主,胶结质壳底栖有孔虫含量明显高于我国其他海区。利用Q型因子分析提取了5个主因子,通过将主因子代表的底栖有孔虫组合与环境影响因子之间的对比印证,揭示了其内在联系,北黄海底栖有孔虫分布的决定性因素是表层沉积物的粒度,而温度和盐度则起到次要作用,该海区水深变化对底栖有孔虫分布的影响没有明显的规律性。  相似文献   
165.
A detailed stable isotopic study based on benthic foraminifera from 1165 samples of ODP Site 1148 (18° 50.17.3’N, 116° 33.93’E, water depth 3308.3 m), northern South China Sea, provides an excellent oxygen isotopic record with an average resolution of 30 ka. It contains the most continuous δ18O data with highest resolution for the whole Neogene sequence in the world. The δ18O curve shows a step-like increasing upwards and records 5 increases, 3 decreases and 2 stable stages during the Neogene, reflecting the general trend of global cooling. Among these events the δ18O decrease at 17.2–14.5 Ma, and two δ18O increases at 14.5-13.6 and 3.0-2.4 Ma are most marked and globally comparable. The intervals at 13.6-10.2 and 6.0-3.0 Ma with the lowest-amplitude and least fluctuation in δ18O represent the most stable bottom water periods for the South China Sea.  相似文献   
166.
167.
Larger benthic foraminifera Nummulites are common within Eocene, circum-Tethyan limestones. Despite their importance as sediment producers, contradictions in the literature constrain current understanding about the location of the Nummulites‘factory’. The El Garia Fm was deposited on a ramp with localized palaeohighs, and whilst some authors suggested that the locus of Nummulites production was in shallow water across the palaeohighs, others concluded that production was significantly reduced over these palaeohighs, and concentrated in the surrounding deeper (30–60 m) water. There are also marked dissimilarities between recent models in terms of the continuity, correlation and resolution of depositional sequences. To assess these models, we integrate studies of the architecture and geometry of the El Garia Fm with detailed taphonomic, biometric, biofabric and palaeoecological characterization of Nummulites tests. We conclude that the highest rates of sediment production occurred in euphotic water over the palaeohighs and in nearshore environments. Nummulites on the palaeohighs were transported into the surrounding deeper water by oceanic and storm currents that swept the platform top, producing a nummulitic sediment package that thickened and became increasingly fine-grained and fragmented into outer ramp environments. This transport exerted a major control on development of the ramp-like geometries often seen at outcrop. Our findings question the validity of a recent sequence stratigraphic model that identifies decimetre-scale Milankovitch cycles, even in largely allochthonous, ‘bio-retextured’, mid/outer ramp sediments. Our findings also suggest that the thin packages of El Garia Fm on the palaeohighs, which have previously been interpreted as condensed sections that can be correlated with thicker, more distal accumulations, actually represent remnants of the sediment that was produced on the highs and ‘exported’ into the basin.  相似文献   
168.
Benthic foraminiferal assemblages were analyzed from three black shale intervals in the upper Aptain to lower Albian of the Vocontian Basin, SE France based on Q-mode principal component analyses. Variations in the distribution patterns of benthic foraminifera around these events suggest differences in the origin of the black shales. Differences between faunas of bioturbated marly and laminated black shale facies have been observed in the Niveau Paquier, Oceanic Anoxic Event (OAE) 1b and Niveau Leenhardt. Here, the faunal composition and plankton/benthos ratios suggest eutrophic conditions during the deposition of organic-rich sediments leading to black shales. No major variations have been observed in black shales of the upper Aptain Niveau Jacob. Benthic assemblages and low plankton/benthos ratios indicate mesotrophic conditions. Third order sea-level changes are believed to control mainly the origin of the investigated black shale levels.  相似文献   
169.
Bottom sediment was analysed, via grab, trawl and underwater photography in 1991–1992 in the Barents, White, Kara, Norwegian and Greenland Seas, and the large scale features of macrobenthos distribution are described. The maps of alpha-diversity, biomass and trophic zone distribution on the investigated shelf are presented. Statistical assessment of the bottom community structure changes during the last 60–70 years for the Barents Sea area is presented. It is shown that on regional and transregional levels the researched communities are of natural undisturbed character and that the revealed changes may be both the result of natural processes or sampling errors. Local disturbances of benthos composition and structure, supposedly caused by man, are found to be rare not far from the Novaya Zemlya and Murman coasts.  相似文献   
170.
The three main estuaries in the French Atlantic coast – the Seine, Loire and Gironde, all with high tidal regimes – are interfaces between the continental and the coastal ecosystems. The Seine and Gironde are highly contaminated, whereas the Loire remains in a more natural state. Both the Seine and Gironde have suffered from harbour construction, and as a result, their biological units are extremely compartmentalized. Benthic species and communities have adapted to tolerate temporal physical and chemical changes ( e.g. salinity, substrata, depth, and levels of fine particles and oxygen) and human activities ( e.g. dredging, shipping traffic, and habitat reduction). Although numerous bio-indicators and indices are used to define the ecological quality status (EcoQS) of coastal waters, very few of them were developed specifically for environments with a mosaic of conditions and salinity levels, including freshwater. The main problem appears to be that all the indices for determining anthropogenic stress examine the abundances of stress-tolerant species, which may also be able to tolerate natural stressors such as those occurring in estuaries. This paper takes a look at the development status of the benthic indicators and index approaches used in the three main estuaries along the French Atlantic coast. In addition, it examines the adaptation of the different benthic indicators to the taxonomic sufficiency principle, and the adaptation of the Benthic Opportunistic Polychaetes Amphipods index (BOPA) and the Benthic Opportunistic Annelida Amphipod index (BO2A) for use in the freshwater zones of transitional waters ( i.e. up to the upper limit of the tidal range). Several perspectives are discussed in light of the diverse disturbances and the heterogeneity of such stressed zones, such as the use of multimetric and scoring approaches.  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