全文获取类型
收费全文 | 1827篇 |
免费 | 347篇 |
国内免费 | 351篇 |
专业分类
测绘学 | 46篇 |
大气科学 | 352篇 |
地球物理 | 396篇 |
地质学 | 801篇 |
海洋学 | 373篇 |
天文学 | 32篇 |
综合类 | 83篇 |
自然地理 | 442篇 |
出版年
2024年 | 11篇 |
2023年 | 37篇 |
2022年 | 61篇 |
2021年 | 85篇 |
2020年 | 84篇 |
2019年 | 121篇 |
2018年 | 89篇 |
2017年 | 84篇 |
2016年 | 93篇 |
2015年 | 88篇 |
2014年 | 155篇 |
2013年 | 172篇 |
2012年 | 133篇 |
2011年 | 109篇 |
2010年 | 108篇 |
2009年 | 141篇 |
2008年 | 122篇 |
2007年 | 110篇 |
2006年 | 100篇 |
2005年 | 80篇 |
2004年 | 101篇 |
2003年 | 62篇 |
2002年 | 65篇 |
2001年 | 50篇 |
2000年 | 50篇 |
1999年 | 36篇 |
1998年 | 34篇 |
1997年 | 30篇 |
1996年 | 35篇 |
1995年 | 10篇 |
1994年 | 16篇 |
1993年 | 7篇 |
1992年 | 12篇 |
1991年 | 4篇 |
1990年 | 2篇 |
1989年 | 3篇 |
1988年 | 2篇 |
1987年 | 5篇 |
1986年 | 4篇 |
1985年 | 5篇 |
1984年 | 3篇 |
1983年 | 4篇 |
1982年 | 1篇 |
1980年 | 1篇 |
排序方式: 共有2525条查询结果,搜索用时 17 毫秒
101.
102.
Massimo Ponti Andrea PasterisRoberta Guerra Marco Abbiati 《Estuarine, Coastal and Shelf Science》2009
Coastal lagoons are ephemeral habitats whose conservation requires human intervention, such as maintenance dredging of inner channels. Dredging can reduce the abundance of benthic species due to the removal of individuals with the sediment, modify sediment properties, and resuspend fine sediment, nutrients and pollutants, which can lead to eutrophication, hypoxic events and increasing toxicity. Both direct effects in the dredged channel and possible indirect effects in surrounding shallow areas could be expected. This study assesses the effects of the channel maintenance dredging, performed between October 2004 and August 2005, on the invertebrate assemblages both in channels and adjacent ponds in the northern Adriatic coastal lagoon of Pialassa Baiona. The lagoon is affected by eutrophication, chemical and thermal pollution from wastewater treatment and power plants. Three impacted sites were located in the dredged channel and three in the adjacent interconnected shallow water ponds, while three non-impacted sites were located in a channel and in a pond far from the dredged area. Replicate samples were collected from each site one time before and one time after the dredging operations. Despite the extent of the intervention, effects of the dredging on macrobenthic assemblages were detected only within the dredged channel, while in the surrounding ponds no clear and unequivocal effects were found. In particular the dredging could have promoted the increase of the abundance of the polychaete Streblospio shrubsolii in the southern and central parts of the dredged channel and the increase in abundance of the amphipod Corophium insidiosum in the northern side, compared to the controls. Instead, species diversity was reduced in the central and northern parts of the dredged channel. These effects on the macrobenthic invertebrate assemblages could be related to the observed changes of sediment characteristics, contamination and toxicity. Overall, direct effects on benthic assemblages in the dredged channels were more detectable than the possible secondary effects in the surrounding shallow ponds, where the higher spatial heterogeneity can mask any relevant effects. 相似文献
103.
As a gridless particle method, the MPS (Moving Particle Semi-implicit) method has proven useful in a wide variety of engineering applications including free-surface hydrodynamic flows. Despite its wide range of applicability, the MPS method suffers from some shortcomings such as non-conservation of momentum and spurious pressure fluctuation. By introducing new formulations for the pressure gradient and a new formulation of the source term of the Poisson Pressure Equation (PPE), and by allowing a slight compressibility, we have proposed modified MPS methods for the prediction of wave impact pressure on a coastal structure. The improved performance of the modified methods is shown through the simulation of numerous wave impact problems (including the impacts by a dam break flow, a flip-through and two cases of slightly-breaking waves) in comparison with the experimental data. 相似文献
104.
Marianne Holmer Ole Pedersen Dorte Krause-Jensen Birgit Olesen Malene Hedegrd Petersen Stephanie Schopmeyer Marguerite Koch Bente Aa. Lomstein Henning S. Jensen 《Estuarine, Coastal and Shelf Science》2009,85(2):319-326
Sulfur and oxygen dynamics in the seagrasses Thalassia testudinum and Syringodium filiforme and their sediments were studied in the US Virgin Islands (USVI) in order to explore sulfide intrusion into tropical seagrasses. Four study sites were selected based on the iron concentration in sediments and on proximity to anthropogenic nutrient sources. Meadow characteristics (shoot density, above- and below-ground biomass, nutrient content) were sampled along with sediment biogeochemistry. Sulfide intrusion was high in T. testudinum, as up to 96% of total sulfur in the plant was derived from sediment-derived sulfides. The sulfide intrusion was negatively correlated to the turnover of sulfides in the sediments regulated by both plant parameters and sediment sulfur pools. Sediment iron content played an indirect role by affecting sulfide turnover rates. Leaf production was negatively correlated with sulfide intrusion suggesting that active growth reduced sulfide intrusion. Sulfide intrusion was lower in S. filiforme (up to 44%) compared to T. testudinum consistent with a higher internal nighttime oxygen concentrations found for S. filiforme. When S. filiforme can take advantage of its ability to maintain high internal oxygen concentrations, as was the case on the USVI, it could increase its success in colonizing unvegetated disturbed sediments with potentially high sulfide concentrations. 相似文献
105.
Validation of the AZTI's Fish Index (AFI), proposed for the Basque Country (northern Spain), in assessing fish quality within the Water Framework Directive (WFD), is undertaken. The response to anthropogenic pressure is investigated, in setting the boundaries between the different quality status classes. Hence, 12 estuaries were sampled, at different frequencies, between 1989 and 2007, by means of a beam trawl. Significant (p < 0.0001) correlations were found between the AFI and oxygen saturation and ammonia. Oxygen quality standards are used to set boundaries between quality classes. Then, the AFIs obtained are compared with different anthropogenic pressures, including urban and industrial discharges, engineering works and dredging. The effects of the removal of some of these pressures are also studied. The total number of pressures within an estuary shows significant (p < 0.009) negative correlation with AFI, explaining between 51 and 62% of the variability in fish quality. The impact of pressures upon fish and demersal assemblages is detected as required by the WFD. Nonetheless, further investigation and intercalibration of the methods used, are necessary. 相似文献
106.
福建兴化湾围填海湿地景观生态影响研究 总被引:2,自引:0,他引:2
基于遥感与GIS技术,运用景观生态学的原理与方法,选取景观特征指数,通过历史回顾和类比分析,从景观自然性和景观破碎化两个方面,分析了兴化湾围填海工程对湿地景观生态造成的影响及其生态效应.历史上兴化湾围填海工程造成滩涂湿地景观萎缩、景观自然性急剧下降,斑块面积缩小、斑块隔离程度增大、景观破碎化程度加剧;兴化湾围填海规划的实施,将使得兴化湾湿地景观生态人工化与破碎化的趋势进一步加剧,其最终后果将可能导致兴化湾生物多样性下降、生态服务功能严重衰退. 相似文献
107.
108.
109.
110.