全文获取类型
收费全文 | 1633篇 |
免费 | 123篇 |
国内免费 | 451篇 |
专业分类
测绘学 | 58篇 |
大气科学 | 182篇 |
地球物理 | 770篇 |
地质学 | 694篇 |
海洋学 | 255篇 |
天文学 | 59篇 |
综合类 | 67篇 |
自然地理 | 122篇 |
出版年
2023年 | 9篇 |
2022年 | 12篇 |
2021年 | 12篇 |
2020年 | 15篇 |
2019年 | 10篇 |
2018年 | 23篇 |
2016年 | 21篇 |
2015年 | 14篇 |
2014年 | 24篇 |
2013年 | 27篇 |
2012年 | 89篇 |
2011年 | 122篇 |
2010年 | 44篇 |
2009年 | 52篇 |
2008年 | 102篇 |
2007年 | 96篇 |
2006年 | 99篇 |
2005年 | 92篇 |
2004年 | 104篇 |
2003年 | 59篇 |
2002年 | 76篇 |
2001年 | 58篇 |
2000年 | 66篇 |
1999年 | 38篇 |
1998年 | 33篇 |
1997年 | 33篇 |
1996年 | 35篇 |
1995年 | 51篇 |
1994年 | 41篇 |
1993年 | 55篇 |
1992年 | 39篇 |
1991年 | 57篇 |
1990年 | 53篇 |
1989年 | 57篇 |
1988年 | 62篇 |
1987年 | 39篇 |
1986年 | 32篇 |
1985年 | 39篇 |
1984年 | 24篇 |
1983年 | 24篇 |
1982年 | 17篇 |
1981年 | 13篇 |
1980年 | 17篇 |
1964年 | 14篇 |
1943年 | 8篇 |
1942年 | 9篇 |
1937年 | 14篇 |
1935年 | 9篇 |
1934年 | 10篇 |
1923年 | 10篇 |
排序方式: 共有2207条查询结果,搜索用时 15 毫秒
991.
992.
Copper is currently the most common biocide in antifouling paints. Levels of this metal were measured in the water, particulate matter and sediments from a shellfish farming area in the Sacca di Goro (Northern Adriatic Sea) over one year. With respect to the 1980s, copper environmental level increased twofold. The release of copper from shellfish farmers' boats was also estimated to be > 250 kg Cu y(-1). Clams Tapes philippinarum were collected in the same area and seasonally exposed to a sublethal (10 microg Cu l(-1)) concentration of copper. Physiological traits were significantly affected by copper exposure (scope for growth declined as a result of reduced clearance rate, increased oxygen consumption and a generally lower absorption efficiency). The results of this study are cause for concern for shellfish farming activities at least in Northern Adriatic, where shellfish farming is a monoculture of T. philippinarum. A strict interpretation of the precautionary principle might suggest that more rigorous regulatory action to control copper inputs in the field would be justified. 相似文献
993.
Neuroendocrine disruption in Mya arenaria clams during gametogenesis at sites under pollution stress 总被引:1,自引:0,他引:1
This study examined the neuroendocrine status of clams on intertidal mud flats in the St. Lawrence Estuary and Saguenay Fjord areas during late gametogenesis. The impact of pollution was determined by a test battery of early stress markers (metallothioneins, heme levels, glutathione S-transferase activity), tissue damage (lipid peroxidation and DNA damage) and morphologic characteristics (age, soft-tissue weight ratio and growth index). Neuroendocrinal status was examined by tracking serotonin and dopamine metabolism, monoamine adenylate cyclase activity in synaptosomes, monoamine oxidase and arachidonate cyclooxygenase activities in relation to gametogenetic activity: pyrimidine synthesis, (aspartate transcarbamoylase activity or ATC), vitellogenin-like proteins and gonado-somatic index. The results show that clam soft tissue weights were reduced at sites close to harbours and higher at sites near domestic wastewater outfalls. The age-to-length ratio of clams was generally higher at impacted sites, suggesting reduced growth. The biomarkers of stress or damage confirmed that oxidative stress, DNA damage, metallothioneins and glutathione S-transferase activity were significantly increased at varying degrees, at the polluted sites. Vitellogenin-like proteins and gametogenetic activity were significantly increased at sites influenced by domestic wastewaters. Furthermore, the clams were still in active gametogenesis and not ready for spawning, as indicated by the concordance of the serotonin/dopamine ratio with vitellogenin-like proteins and pyrimidine synthesis. However, gonadal cyclooxygenase activity was increased at polluted sites and significantly correlated with most of the stress biomarkers, suggesting that the clams were in a state of inflammation rather than at the spawning stage. Finally, a multivariate analysis revealed that the sites were readily discriminated with high efficiency (>71%) and that both neuroendocrine physiological markers and stress responses were identified as the major components, thus explaining the global physiological response of the clams. We conclude that the effects of pollution compromise the clams' health status and that the initiation of gametogenesis in environments contaminated by municipal wastewaters or harbours contributes to the toxic effects of pollution. 相似文献
994.
Copper complexation by fulvic acid affects copper toxicity to the larvae of the polychaete Hydroides elegans 总被引:1,自引:0,他引:1
Copper toxicity is influenced by a variety of environmental factors including dissolved organic matter (DOM). We examined the complexation of copper by fulvic acid (FA), one of the major components of DOM, by measuring the decline in labile copper by anodic stripping voltammetrically (ASV). The data were described using a one-site ligand binding model, with a ligand concentration of 0.19micromol site mg(-1) C, and a logK' of 6.2. The model was used to predict labile copper concentration in a bioassay designed to quantify the extent to which Cu-FA complexation affected copper toxicity to the larvae of marine polychaete Hydroides elegans. The toxicity data, when expressed as labile copper concentration causing abnormal development, were independent of FA concentration and could be modeled as a logistic function, with a 48-h EC(50) of 58.9microgl(-1). However, when the data were expressed as a function of total copper concentration, the toxicity was dependent on FA concentration, with a 48-h EC(50) ranging from 55.6microgl(-1) in the no-FA control to 137.4microgl(-1) in the 20mgl(-1) FA treatment. Thus, FA was protective against copper toxicity to the larvae, and such an effect was caused by the reduction in labile copper due to Cu-FA complexation. Our results demonstrate the potential of ASV as a useful tool for predicting metal toxicity to the larvae in coastal environment where DOM plays an important role in complexing metal ions. 相似文献
995.
996.
997.
Sediment quality of the lower St. Johns River (LSJR) estuary, Florida was evaluated using synoptic data on benthic community structure, levels of potential stressors (chemical contaminants, ammonia and sulfide), and other basic habitat characteristics (depth, dissolved oxygen, salinity, temperature) collected at seven stations, three times a year from July 2000-July 2002. Un-ionized ammonia and hydrogen sulfide were detected at toxic levels on at least one sampling occasion at four stations. Chemical contamination of sediment at probable bioeffect levels also was observed at four stations. Concentrations of pesticides or other chemical substances typically associated with human activities (e.g., PCBs) were detectable at all stations, though not always present at concentrations likely of causing significant bioeffects. A total of 251 taxa and 9783 individuals were identified and enumerated from the benthic infaunal samples. Polychaete worms and molluscs dominated the benthic fauna at all seven stations. The opportunistic and pollution-tolerant polychaete Streblospio benedicti was the most abundant species overall (from all samples combined), appearing as a dominant at five of the seven stations. Overall, the sites sampled as part of this study indicate a highly stressed benthos resulting from multiple anthropogenic impacts. 相似文献
998.
Estrogen receptor (ER) agonists and androgen receptor (AR) antagonists in effluents from Norwegian North Sea oil production platforms 总被引:6,自引:0,他引:6
The in vitro estrogen receptor (ER) agonist and androgen receptor (AR) antagonist potencies of offshore produced water effluents collected from the Norwegian Sector were determined using recombinant yeast estrogen and androgen screens. Solid phase extraction (SPE) concentrates of the effluents showed E2 agonist activities similar to those previously reported for the United Kingdom (UK) Continental Shelf (<0.1-4 ng E2 L(-1)). No activity was detected in the filtered oil droplets suggesting that produced water ER activity is primarily associated with the dissolved phase. Targeted analysis for methyl- to nonyl-substituted alkylphenol isomers show the occurrence of known ER agonists in the analysed samples. For the first time, AR antagonists were detected in both the dissolved and oil associated phase at concentrations of between 20 and 8000 microg of flutamide equivalents L(-1). The identity of the AR antagonists is unknown, however this represents a significant input into the marine environment of unknown compounds that exert a known biological effect. It is recommended that further analysis using techniques such as bioassay-directed analysis is performed to identify the compounds/groups of compounds that are responsible in order to improve the assessment of the risk posed by produced water discharges to the marine environment. 相似文献
999.
C.M.HEANLEY 《地质学报》1928,(Z1)
INTRODUCTION This article has been written as an explanatory note to an exhibit of stone artifacts sent to the meeting of the Society. It is also hoped that for people in South China it will be an introduction to the prehistoric archeology 相似文献
1000.
主要介绍了大样本恒星演化所面临的困难,为解决困难所做的努力及对未来的展望。 相似文献