全文获取类型
收费全文 | 2617篇 |
免费 | 363篇 |
国内免费 | 382篇 |
专业分类
测绘学 | 95篇 |
大气科学 | 98篇 |
地球物理 | 424篇 |
地质学 | 546篇 |
海洋学 | 1547篇 |
天文学 | 7篇 |
综合类 | 163篇 |
自然地理 | 482篇 |
出版年
2024年 | 8篇 |
2023年 | 40篇 |
2022年 | 88篇 |
2021年 | 94篇 |
2020年 | 114篇 |
2019年 | 104篇 |
2018年 | 91篇 |
2017年 | 107篇 |
2016年 | 114篇 |
2015年 | 100篇 |
2014年 | 141篇 |
2013年 | 197篇 |
2012年 | 108篇 |
2011年 | 199篇 |
2010年 | 138篇 |
2009年 | 163篇 |
2008年 | 188篇 |
2007年 | 137篇 |
2006年 | 166篇 |
2005年 | 129篇 |
2004年 | 114篇 |
2003年 | 103篇 |
2002年 | 90篇 |
2001年 | 74篇 |
2000年 | 77篇 |
1999年 | 88篇 |
1998年 | 71篇 |
1997年 | 48篇 |
1996年 | 49篇 |
1995年 | 47篇 |
1994年 | 41篇 |
1993年 | 23篇 |
1992年 | 28篇 |
1991年 | 14篇 |
1990年 | 9篇 |
1989年 | 5篇 |
1988年 | 3篇 |
1987年 | 6篇 |
1986年 | 2篇 |
1985年 | 12篇 |
1984年 | 11篇 |
1983年 | 6篇 |
1982年 | 3篇 |
1981年 | 9篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1971年 | 1篇 |
排序方式: 共有3362条查询结果,搜索用时 46 毫秒
991.
Composition, structural characteristics and temporal patterns of fish assemblages in non-tidal Mediterranean lagoons: A case study 总被引:2,自引:0,他引:2
The importance of transitional water ecosystems as nursery habitats and feeding grounds for fish species is well-known. Detailed studies of colonization patterns of fish guilds in response to biotic and abiotic drivers are however unevenly distributed among ecosystem types. We address here the temporal variability of fish assemblages in small non-tidal lagoons in the Mediterranean basin. The study was carried out at the Acquatina lagoon (Lecce, Italy) where four stations, situated in two habitat types along a confinement gradient, were sampled twice per month for one year with fyke nets. Forty-five taxa ranging across 20 families were collected, with the most abundant species, Atherina boyeri, accounting for more than 95% of total abundance. Pooling all species together (excluding sand smelt), the structural features of the assemblage, relative abundance of families, and abundance of individual species all showed significant temporal patterns. Mean abundance peaked in Summer and Autumn and fell in Winter, whereas taxonomic richness and diversity were highest in Summer and lowest in Spring. Within the fish assemblage, multivariate ordination showed temporal segregation of species belonging to the same family or genus and expected to be functionally similar, suggesting that they avoid competition for space and resources by timing inward migration and peak occurrence differently. Of the environmental driving forces, which also showed temporal patterns of variation, salinity was the main factor affecting the distribution of individuals and species. The catch of young individuals of several marine species confirmed the role of this small lagoon as a nursery and feeding area, and emphasized the need for further studies. 相似文献
992.
Carbonate system parameters (pH and alkalinity) were used to estimate the coastal water CO2 fluxes off central Chile (30°S) during September 2007. Coastal waters rich in nitrate and silicate were strongly CO2 supersaturated and normally poor in chlorophyll a. MODIS satellite chlorophyll a data suggest that phytoplankton biomass remained particularly low during September 2007 although coastal waters were highly fertilized with nitrate and silicate. The phytoplankton gross primary productivity in macronutrient-rich waters was very low with the exception of shallow waters (e.g. within or near bays). Several iron-enrichment bottle experiments show that fCO2 rapidly decreases during iron-enrichment treatments compared to controls. This suggests that iron limitation of phytoplankton growth (mainly diatoms) plays a role in maintaining high-CO2 outgassing by preventing rapid interception of upwelled CO2. 相似文献
993.
Miroslav Gačić Vedrana Kovačević Simone Cosoli Andrea Mazzoldi Jeffrey D. Paduan Isaac Mancero-Mosquera Sadegh Yari 《Estuarine, Coastal and Shelf Science》2009
Surface patterns of the low-frequency current in a 20 × 30 km region in front of the Venice Lagoon were analysed from a 13-month-long HF radar data set. Surface circulation was related to prevalent wind regimes in the area and to the tidal flow through the lagoon inlet. Three different categories of wind-forcing were defined: bora (NE wind), sirocco (SE wind), and finally the category containing all other wind directions and calms (winds lower than 3 m/sec). Mean flow and vorticity spatial distributions were discussed for different wind conditions. The coastal area about 5 km wide is characterized by a flow field with maximum vorticity. Outside the coastal boundary layer the interior flow is part of the Adriatic basin-wide cyclonic circulation. Two counter-rotating vortices of the dimension of about 4–5 km were evidenced in the average flow field in front of the lagoon inlet (Malamocco inlet) in all situations except for the bora. The vortex-pair is probably associated with the tidal flow through the inlet. The bora wind induces a strong southward coastal jet detached from the coast by about 5 km homogenizing the flow and eliminating the dipole. The average coastal flow pattern in calm wind conditions was also analysed as a function of the strong inflow/outflow (currents in the inlet > 0.7 m s−1) from the lagoon inlet. In both cases the vorticity pattern is similar, with the negative vorticity to the left of the inlet and positive to the right looking seaward. 相似文献
994.
In the coastal catchments of Shandong Province the water scarcity is aggravated due to saltwater intrusion, reducing the usability of water resources available. Such a situation calls for sustainable integrated water resources management (IWRM). The idea for the objectives and implementation of the IWRM are explained in this paper. The general objective of the planned project disscussed in the present study is to bring together German traditional expertise in water resources management and newer developments in the context of the European Water Framework Directive; the research efforts aim to relieve the desperate water scarcity situation in the costal area of Shandong Province. 相似文献
995.
Because of the variable, unpredictable nature of many potential nutrient sources for coastal phytoplankton primary production needs in the north-western Gulf of Mexico, benthic regeneration was investigated as a more constant source of nutrients to this ecosystem. Water column ammonia profiles taken at several locations on the south Texas inner-shelf in the last seven years showed peak concentrations in bottom waters. Benthic chamber measurements verified the presence of ammonia fluxes at the mud-water interface in south Texas coastal waters. These fluxes were related to benthic faunal activity as measured by sediment metabolism. Laboratory experiments, designed to test the effect of benthic faunal removal on nutrient regeneration, suggested that the fauna play a role in regulating this process. How this regulation may occur is discussed. It was estimated that annual benthic regeneration rates can supply 69% of the nitrogen required to support phytoplankton primary production in these coastal waters. 相似文献
996.
Records of massive fish kills and paralytic shellfish poisoning (PSP) in Europe and North America go back to the 17th century. But, it was not until the 1940s when the relationship between PSP, red tide and toxic dinoflagellateGonyaulax was established. Recent records show that PSP and related poisons caused by toxic dinoflagellates in coastal waters and estuaries, are a world-wide problem. Diarrhetic shellfish poisoning (DSP) and neurotoxic poisoning (NSP), believed earlier as bacterial or viral infections are now shown to be caused by other toxic dinoflagellates such asDinophysis. The shellfish most often involved in the poisoning are mussels and clams. Other dinoflagellates,Gyrodinium, occasionally cause massive fish kills in vast coastal areas, resulting in fishery and economic losses.Factors promoting toxic dinoflagellate bloom development and PSP/DSP outbreaks are not fully understood. In previous studies, temperature was considered as the principal factor influencing dinoflagellate blooming. Recent studies showed that other factors such as salinity, sunlight, freshwater runoff and water stability are also important. Pollution from land drainage and sewage discharge in inshore waters were also implicated.Current knowledge indicates that although chemical and biotic factors are important forin-situ growth of dinoflagellate cells, convergence by thermal and tidal fronts is essential for cell accumulation and bloom development. Advances in physical oceanographic research, modelling and remote sensing enabled the detection of fronts and bordering eddies with high precision. There is a potential for an increased use of these technological advances in predicting and monitoring the bloom development.The present paper overviews the history and distribution of toxic dinoflagellates, and the physical factors influencing bloom development and PSP/DSP outbreaks. Future research needs to improve the predictability and control of this world-wide hazard are also discussed. 相似文献
997.
Several cores were compared to elucidate the grade of mercury retention in sediments of the Baltic Sea near Oulu, Finland. The known history of the mercury discharge to waste water from a chlor-alkali plant, the primary pollution source, was compared to that in the sedimentary record. One core was dated by lead-210 and varve counts. During the past thirty years, the rate of sedimentation was up to 10 mm per year corresponding to a dry matter accumulation rate of about 3 kg m–2a–1. Under these conditions, most of the mercury reaching the sediment was retained. Correspondingly, the mercury levels in fish were considerably lower, with a delay of some five years.The present mercury content of the upper layers is some 1/4 or 1/5 that of the late 1970's, but it is still some twentyfold compared to levels at the beginning of the century. The extent to which the present mercury level in the sediment represents the various phases of industrial emission remains unsolved due to the origin of mercury in this sea area from several other sources and due to difficulties in exact core correlations from different years. 相似文献
998.
G. A. Kellaway 《Environmental Geology》1996,28(1):34-39
The hot springs of Bristol and Bath and two geothermal wells at Southampton are located on a 155-km-long Avon–Solent Fracture
Zone extending in a NW–SE direction from the Severn Estuary to the English Channel. Initiated during the Variscan earth movements
and reactivated in Miocene times, the structure, which extends across the English Channel to France, is still active. With
this discovery, it should now be possible to throw fresh light on the origin and movement of the thermal water at Bath and
thus to protect the hot springs from derogation by limestone quarrying.
Received: 16 October 1995 · Accepted: 13 February 1996 相似文献
999.
1000.
黄河每年有8×108~10×108t泥沙进入河口地区。由于黄河的受水盆地浅,区域上又位于构造强烈沉降区,进入河口地区的泥沙约有80%淤积在陆上及滨海地区,逐步形成了现代的黄河三角洲。通过RS与GIS综合对比和地质成因分析,可揭示整个三角洲在河流动力和海洋动力综合作用下的快速淤进、蚀退,河床抬高和尾闾改道等一系列演变,其中又以海岸带的演变最为明显。近几年,受黄河来水来沙量减少和人为加固堤坝等因素影响,河口地区造陆速度减缓,其他地区海岸带蚀退变慢,河床进一步抬高,整个三角洲基本处于淤蚀平衡状态。 相似文献