全文获取类型
收费全文 | 1626篇 |
免费 | 160篇 |
国内免费 | 361篇 |
专业分类
测绘学 | 54篇 |
大气科学 | 23篇 |
地球物理 | 265篇 |
地质学 | 392篇 |
海洋学 | 907篇 |
天文学 | 11篇 |
综合类 | 161篇 |
自然地理 | 334篇 |
出版年
2024年 | 9篇 |
2023年 | 33篇 |
2022年 | 45篇 |
2021年 | 54篇 |
2020年 | 69篇 |
2019年 | 59篇 |
2018年 | 70篇 |
2017年 | 87篇 |
2016年 | 100篇 |
2015年 | 73篇 |
2014年 | 101篇 |
2013年 | 150篇 |
2012年 | 82篇 |
2011年 | 115篇 |
2010年 | 94篇 |
2009年 | 97篇 |
2008年 | 97篇 |
2007年 | 100篇 |
2006年 | 83篇 |
2005年 | 91篇 |
2004年 | 84篇 |
2003年 | 55篇 |
2002年 | 50篇 |
2001年 | 35篇 |
2000年 | 50篇 |
1999年 | 34篇 |
1998年 | 34篇 |
1997年 | 32篇 |
1996年 | 25篇 |
1995年 | 26篇 |
1994年 | 23篇 |
1993年 | 16篇 |
1992年 | 24篇 |
1991年 | 13篇 |
1990年 | 5篇 |
1989年 | 5篇 |
1988年 | 6篇 |
1987年 | 5篇 |
1986年 | 3篇 |
1985年 | 4篇 |
1984年 | 3篇 |
1983年 | 3篇 |
1982年 | 1篇 |
1981年 | 2篇 |
排序方式: 共有2147条查询结果,搜索用时 15 毫秒
91.
92.
Yimnang Golbuu Katharina Fabricius Steven Victor Robert H. Richmond 《Estuarine, Coastal and Shelf Science》2008
This study analyzed how coral communities change along a gradient of increasing exposure to a mud-discharging river in the Enipein Catchment, Pohnpei, Micronesia. Using video transects, we quantified benthic communities at five sites along a gradient moving away from the river mouth towards the barrier reef. The most river-impacted site was characterized by a high accumulation of mud, low coral cover and low coral diversity. Although coral cover leveled off at ∼400 m from the river mouth to values found at the outer-most sites, coral diversity continued to increase with increasing distance, suggesting that the most distant site was still impacted by the river discharges. Fungiidae, Pavona, Acropora, Pachyseris and Porites rus all significantly increased in cover with distance from the river, while Turbinaria decreased. The combined presence and abundance of these six species groups, together with coral species richness, may help to indicate the effects of terrestrial runoff in similar runoff-exposed settings around Micronesia, whereas coral cover is not a sensitive indicator for river impact. Coral reefs are important resources for the people of Pohnpei. To prevent further degradation of this important resource, an integrated watershed approach is needed to control terrestrial activities. 相似文献
93.
The larval fish assemblage was investigated in the shallow, nearshore region of a proposed marine protected area in eastern Algoa Bay, temperate South Africa, prior to proclamation. Sampling was conducted at six sites along two different depth contours at ∼5 m and ∼15 m to assess shore association. Larvae were collected by means of stepped oblique bongo net tows deployed off a ski-boat, twice per season for 2 years between 2005 and 2007. In total, 6045 larval fishes were collected representing 32 families and 78 species. The Gobiidae, Cynoglossidae, Clupeidae, Engraulidae and Sparidae were the dominant fish families. Catches varied significantly among seasons peaking in spring with a mean of ∼200 larvae/100 m3. Mean overall larval density was higher along the deeper contour, at ∼15 m (40 larvae/100 m3). The preflexion stage of development dominated catches at the ∼5 m (80%) and ∼15 m (73%) depth contours. Body lengths of Argyrosomus thorpei, Caffrogobius gilchristi, Diplodus capensis, Heteromycteris capensis and Solea turbynei, all estuary associated species, were larger at the shallow sites nearer to shore. Larvae of coastal species that produce benthic eggs dominated catches (75%) in the shallow sites (∼5 m) but were less abundant (32%) farther from shore at the deeper (∼15 m) sites. All developmental stages of D. capensis, Engraulis capensis, H. capensis, Sardinops sagax and two Pomadasys species were found in the study area. It appears that some species use the shallow nearshore as a nursery area. 相似文献
94.
Pasquale Crea Concetta De Stefano Demetrio Milea Silvio Sammartano 《Marine Chemistry》2008,112(3-4):142-148
The results of a potentiometric investigation (by ISE-H+, glass electrode) on the speciation of phytate ion (Phy12−) in an ionic medium simulating the major components (Na+, K+, Ca2+, Mg2+, Cl− and SO42−) of natural seawater, at different salinities and t = 25 °C, are reported. The work was particularly aimed at determining the possible formation of mixed Ca2+–Mg2+–phytate ion pairs, and to establish how including the formation of these mixed species would affect the speciation modeling in seawater media. After testing various speciation models, that considering the formation of the MgCaH3Phy5−, MgCaH4Phy4−, Mg2CaH3Phy3− and Mg2CaH4Phy2− species was accepted, and corresponding stability constants were determined at two salinities (S = 5, 10). A discussion is reported both on the choice of the experimental conditions and on the possibility to extend these results to those typical of real seawater. A detailed procedure is also described to demonstrate that the stability of these species is higher than that statistically predicted. As reported in literature, a parameter, namely log X, has been determined in order to quantify this extra stability for the formation of each mixed species at various salinities. For example, at S = 10, log X113 = 2.67 and log X114 = 1.37 for MgCaH3Phy5− and MgCaH4Phy4− (statistical value is log Xstat = 0.60), and log X213 = 6.11 and log X214 = 2.15 for Mg2CaH3Phy3− and Mg2CaH4Phy2− (log Xstat = 1.43), respectively. Results obtained also showed that the formation of these species may occur even in conditions of low salinity (i.e. low concentration of alkaline earth cations) and low pH (i.e., more protonated ligand). 相似文献
95.
Biomass distribution and trophodynamics in the oceanic ecosystem in the Oyashio region are presented and analyzed, combining
the seasonal data for plankton and micronekton collected at Site H since 1996 with data for nekton and other animals at higher
trophic levels from various sources. The total biomass of biological components including bacteria, phytoplankton, microzooplankton,
mesozooplankton, micronekton, fishes/squids and marine birds/mammals was 23 g C m−2, among which the most dominant component was mesozooplankton (34% of the total), followed by phytoplankton (28%), bacteria
(15%) and microzooplankton (protozoans) (14%). The remainder (9%) was largely composed of micronekton and fish/squid. Marine
mammals/birds are only a small fraction (0.14%) of the total biomass. Large/medium grazing copepods (Neocalaus spp., Eucalanus bungii and Metridia spp.) accounted for 77% of the mesozooplankton biomass. Based on information about diet composition, predators were assigned
broadly into mean trophic level 3–4, and carbon flow through the grazing food chain was established based on the estimated
annual production/food consumption balance of each trophic level. From the food chain scheme, ecological efficiencies as high
as 24% were calculated for the primary/secondary production and 21% for the secondary/tertiary production. Biomass and production
of bacteria were estimated as 1/10 of the respective values for phytoplankton at Site H, but the role of the microbial food
chain remains unresolved in the present analysis. As keystone species in the oceanic Oyashio region, Neocalanus spp. are suggested as a vital link between primary production and production of pelagic fishes, mammals and birds. 相似文献
96.
利用荧光标记扩增片段长度多态性(fAFLP)技术对文蛤(Meretrix meretrix)、青蛤(Cyclina sinensis)、菲律宾蛤仔(Ruditapes philippinarum)和硬壳蛤(Mercenaria mercenaria)4种帘蛤科贝类的群体遗传多样性和种间关系进行了研究。选择EcoRⅠ/MseⅠ进行酶切,使用6个E 3/M 3引物组合进行扩增,共获得1 096个位点,多态位点比率95.1%,片段长度50~456 bp。其中,文蛤、青蛤、菲律宾蛤仔和硬壳蛤分别得到681,715,702和694个位点,相应的多态位点比率为76.8%,81.7%,83.0%和75.1%,得到17个种特异性位点,可作为4物种特征标记。分析了群体遗传相似系数和遗传多样性指数以及种间遗传相似系数。结果表明,硬壳蛤群体遗传相似系数最高(0.670 9),遗传多样性指数最低(0.236 0);菲律宾蛤仔群体遗传相似系数最低(0.592 5),遗传多样性指数最高(0.261 8);根据遗传相似系数采用UPGMA法构建了4物种32个体的聚类图,表明文蛤与菲律宾蛤仔遗传关系最近,青蛤与其他3物种遗传关系较远。 相似文献
97.
杭州湾上海石化沿岸潮间带生态环境分析 总被引:1,自引:0,他引:1
通过对1993~2002年杭州湾上海石化沿岸潮间带底栖动物群落结构的分析,得出上海石化处理达标后排放的工业废水对此段潮间带生态环境影响的范围和程度。自2002年到现在为止,本潮间带的底栖动物群落结构呈现一种适应性广、耐污染的结构组成.其生物多样性指数值在2.00~3.50之间。 相似文献
98.
99.
Collection of marine invertebrates for use as fishing bait is a substantial activity in many parts of the world, often with unknown ecological consequences. As new fisheries develop, it is critical for environmental managers to have high quality ecological information regarding the potential impacts, in order to develop sound management strategies. Crab-tiling is a largely unregulated and un-researched fishery, which operates commercially in the south-west UK. The target species is the green crab Carcinus maenas. Those crabs which are pre-ecdysis and have a carapace width greater than 40 mm are collected to be sold to recreational anglers as bait. Collection involves laying artificial structures on intertidal sandflats and mudflats in estuaries. Crabs use these structures as refugia and are collected during low tide. However, the effect that this fishery has on populations of C. maenas is not known. The impact of crab-tiling on C. maenas population structure was determined by sampling crabs from tiled estuaries and non-tiled estuaries using baited drop-nets. A spatially and temporarily replicated, balanced design was used to compare crab abundance, sizes and sex ratios between estuaries. Typically, fisheries are associated with a reduction in the abundance of the target species. Crab-tiling, however, significantly increased C. maenas abundance. This was thought to be a result of the extra habitat in tiled estuaries, which probably provides protection from natural predators, such as birds and fish. Although crabs were more abundant in tiled estuaries than non-tiled estuaries, the overall percentage of reproductively active crabs in non-tiled estuaries was greater than in tiled estuaries. As with most exploited fisheries stocks, crabs in exploited (tiled) estuaries tended to be smaller, with a modal carapace width of 20–29 mm rather than 30–39 mm in non-tiled estuaries. The sex ratio of crabs however; was not significantly different between tiled and non-tiled estuaries. These results illustrate the potential to manage fished populations using habitat provision to mitigate the effects of fishing pressure. 相似文献
100.
Boron resources are abundant in Da Qaidam salt lake of Qaidamu Basin. It has been given great attention for the polyborate species present in brine. In this study, the Raman spectroscopy was applied to investigate the existing-form of boron in brine during evaporation. The prepared solutions of MgO·2B2O3-H2O, MgO·2B2O3-MgCl2 -H2O, and MgO·2B2O3-MgSO4-H2O was also evaporated and recorded to study the influence of boron concentration, pH, and electrolytes on the borate speciation in brine. The mononborates of B(OH)3 and B(OH)4- were found to be the only forms present in the original salt lake brine. Brine evaporation promotes the formation of polyborate anions B3O3(OH)4-, B5O6(OH)4-, and B6O7(OH)62- and also disappearance of the B(OH)4- ion in brine with boron concentration of more than 11 g/L in B2O3. The pentaborate ion of B5O6(OH)4- was sensitive to the solution pH and found to be appeared under the pH value of 8.0. While the hexaborate ion of B6O7(OH)62- was observed more dependent on the electrolyte of magnesium chloride due to its special properties, such as promoting boron accumulation, lowering solution pH, and also the strong af?nity for water molecules, which is beneficial to the polymerization of borate ions in brine. The interaction mechanisms among polyborate anions during evaporation had also been proposed. 相似文献