全文获取类型
收费全文 | 2453篇 |
免费 | 231篇 |
国内免费 | 289篇 |
专业分类
测绘学 | 89篇 |
大气科学 | 208篇 |
地球物理 | 304篇 |
地质学 | 434篇 |
海洋学 | 432篇 |
天文学 | 8篇 |
综合类 | 178篇 |
自然地理 | 1320篇 |
出版年
2024年 | 15篇 |
2023年 | 31篇 |
2022年 | 106篇 |
2021年 | 137篇 |
2020年 | 109篇 |
2019年 | 121篇 |
2018年 | 104篇 |
2017年 | 89篇 |
2016年 | 114篇 |
2015年 | 115篇 |
2014年 | 145篇 |
2013年 | 146篇 |
2012年 | 128篇 |
2011年 | 143篇 |
2010年 | 124篇 |
2009年 | 141篇 |
2008年 | 119篇 |
2007年 | 122篇 |
2006年 | 146篇 |
2005年 | 166篇 |
2004年 | 91篇 |
2003年 | 99篇 |
2002年 | 82篇 |
2001年 | 76篇 |
2000年 | 80篇 |
1999年 | 47篇 |
1998年 | 29篇 |
1997年 | 44篇 |
1996年 | 20篇 |
1995年 | 17篇 |
1994年 | 15篇 |
1993年 | 12篇 |
1992年 | 7篇 |
1991年 | 7篇 |
1990年 | 8篇 |
1988年 | 3篇 |
1987年 | 2篇 |
1986年 | 3篇 |
1985年 | 4篇 |
1984年 | 3篇 |
1983年 | 2篇 |
1982年 | 1篇 |
排序方式: 共有2973条查询结果,搜索用时 843 毫秒
641.
642.
643.
Jutarat Kittiwanich Tamiji Yamamoto Toshiya Hashimoto Keiko Tsuji Osamu Kawaguchi 《Journal of Oceanography》2006,62(4):493-509
A numerical ecosystem model expressing both phosphorus and nitrogen cyclings in the pelagic system of Hiroshima Bay, Japan,
was developed in order to investigate the seasonal variations of these elements and their annual budgets. Based on the geophysical
and chemical structures of the bay, Hiroshima Bay was divided into northern and southern boxes, themselves divided into two
layers of an upper 5 m and a lower layer according to the stratification of the water column. The model consists of equations
representing all relevant physical and biological processes. The results revealed that the internal regeneration of materials
was an important source of bioavailable nutrients for phytoplankton growth in the water column. The incorporation of phytoplankton
aggregation improved the accuracy of the outputs in comparison to the observed data, especially during the stratified summer
season. The results also indicated that phosphorus limitation of phytoplankton growth occurs in the upper layer during summer
while light limitation occurs in the lower layer. In addition, physical processes such as diffusion and advection were also
important as they ensured that most of regenerated nutrients in the lower layer were transported to the upper layer. Thus,
these processes might support the high primary production and the production of oysters that are extensively cultured in this
bay. Considering the informative results obtained, the model used in this study provides a sound basis and tool to describe
the dynamics of phosphorus and nitrogen cyclings in Hiroshima Bay. 相似文献
644.
Petihakis G Triantafyllou G Pollani A Koliou A Theodorou A 《Marine environmental research》2005,59(5):493-518
The Pagasitikos gulf ecosystem is studied through the analysis of experimental field data acquired during several monitoring projects and the application of a complex biogeochemical model. The gulf was separated into three different parts (internal, top central-external, bottom central-external) according to the patterns exhibited by the key ecosystem indicators. Unlike other semi-enclosed gulfs Pagasitikos can be characterised as meso-oligotrophic undergoing periods of P or N limitation. Although the signal of nutrient inputs is not very clear in the field data, their importance is assessed through simulation. Increased phosphate concentrations either due to mixing or due to anthropogenic activities can result in phytoplanktonic blooms with significant contribution by diatoms. The effect of hydrodynamic patterns on primary production has been demonstrated through ecosystem modeling indicating that due to long stratification periods, all nutrients released through the benthic regeneration are trapped in the deeper layers, developing a microbial food web. However when the thermocline erodes nutrients find their way up in the upper layers of the euphotic zone and the system turns into more classical type with primary producers growing significantly faster. 相似文献
645.
646.
An Ecosystem Model Including Nitrogen Isotopes: Perspectives on a Study of the Marine Nitrogen Cycle
Chisato?YoshikawaEmail author Yasuhiro?Yamanaka Takeshi?Nakatsuka 《Journal of Oceanography》2005,61(5):921-942
We have developed an ecosystem model including two nitrogen isotopes (14N and 15N), and validated this model using an actual data set. A study of nitrogen isotopic ratios (δ15N) using a marine ecosystem model is thought to be most helpful in quantitatively understanding the marine nitrogen cycle.
Moreover, the model study may indicate a new potential of δ15N as a tracer. This model has six compartments: phytoplankton, zooplankton, particulate organic nitrogen, dissolved organic
nitrogen, nitrate and ammonium in a two-box model, and has biological processes with/without isotopic fractionation. We have
applied this model to the Sea of Okhotsk and successfully reproduced the δ15N of nitrate measured in seawater and the seasonal variations in δ15N of sinking particles obtained from sediment trap experiments. Simulated δ15N of phytoplankton are determined by δ 15N of nitrate and ammonium, and the nitrogen f-ratio, defined as the ratio of nitrate assimilation by phytoplankton to total nitrogenous nutrient assimilation. Detailed
considerations of biological processes in the spring and autumn blooms have demonstrated that there is a significant difference
between simulated δ15N values of phytoplankton, which assimilates only nitrate, and only ammonium, respectively. We suggest that observations of
δ 15N values of phytoplankton, nitrate and ammonium in the spring and autumn blooms may indicate the ratios of nutrient selectivity
by phytoplankton. In winter, most of the simulated biogeochemical fluxes decrease rapidly, but nitrification flux decreases
much more slowly than the other biogeochemical fluxes. Therefore, simulated δ15N values and concentrations of ammonium reflect almost only nitrification. We suggest that the nitrification rate can be parameterized
with observations of δ15N of ammonium in winter and a sensitive study varying the parameter of nitrification rate. 相似文献
647.
Larval fish assemblages in nearshore coastal waters off central Chile: temporal and spatial patterns 总被引:1,自引:0,他引:1
E. Hernndez-Miranda A. T. Palma F. P. Ojeda 《Estuarine, Coastal and Shelf Science》2003,56(5-6):1075-1092
In this study we identified spatial and temporal patterns in the distribution and abundance of larval stages of several fish species in nearshore waters off central Chile. Larvae were sampled monthly at two close (20 km apart) but contrasting localities, El Quisco and Las Cruces. Surveys corresponded to standard plankton tows stratified according to bathymetry and distance from shore. Our results indicate that at both localities: (1) there is a seasonal reproductive pattern for most of the species studied; (2) there is a seasonal-related change in larval species composition and abundance, with austral Winter–Spring being the time of greatest diversity; (3) larval stages of several species that, as adults occupy intertidal, estuarine–riverine, subtidal, benthic-demersal, epipelagic or mesopelagic habitats, are found within these coastal environments; (4) there is a distinctive cross-shelf pattern of larval distribution, which seems to correspond, at least for the intertidal species, with the shallower (<30 m depth) portion of area surveyed; and (5) there is a coupling between the patterns of distribution and abundance of the entire ichthyoplankton assemblage with short-term physical features such as wind forcing, Ekman transport, and local currents. Our findings suggest that both the specific composition as well as the abundance of larval fish species varies spatially and temporally and that this variability may result from the interaction of physical and biological factors at different scales. 相似文献
648.
南沙渚碧礁生态系营养关系的稳定碳同位素研究 总被引:4,自引:0,他引:4
利用稳定碳同位素分析技术研究了南沙渚碧礁生态系食物网主要生物类群之间的营养关系。结果表明,生物的稳定碳同位素组成与其营养来源有密切关系。浮游植物的δ^13C为-18.3‰,与其所处海域的环境条件一致,浮游动物的δ^13C值变化较大,范围为-20.4‰~-10.9‰,表明可能存在浮游植物和碎屑两种营养来源。珊瑚和砗磲的碳同位素组成(-17‰~-15‰)与浮游动物相差较大,暗示共生虫黄藻可能在这些珊瑚的营养来源中起重要作用。底栖海参(-9.6‰)和蜘蛛螺(-12.5‰)的碳同位不比组成与它们沉积物食性的营养特征吻合。鱼类的δ^13C值变化范围较大(-17.7‰~-10.9‰),未表现出随营养级升高而增大的趋势,说明影响鱼类碳同位素组成的因素比较复杂。 相似文献
649.
Jayme M. Santangelo Adriana de M. Rocha Reinaldo L. Bozelli Luciana S. Carneiro Francisco de A. Esteves 《Estuarine, Coastal and Shelf Science》2007,71(3-4):657-668
The effects of a disturbance by sandbar opening on the zooplankton community were evaluated through a long-term study in an eutrophic and oligohaline system, Imboassica Lagoon, Rio de Janeiro, Brazil. Zooplankton samples and limnological data were collected monthly from March 2000 to February 2003. Before the sandbar was opened in February 2001, the lagoon showed eutrophic conditions, with high mean nutrient concentrations and low salinity (total nitrogen – TN = 190.28 μM, chlorophyll a content – Chl. a = 104.60 μg/L and salinity = 0.87′). During this period, the zooplankton species present, such as the rotifers Brachionus calyciflorus and Brachionus havanaensis, were typical of freshwater to oligohaline and eutrophic environments. After the sandbar opening, the lagoon changed to a lower trophic status and increased salinity (TN = 55.11 μM, Chl. a = 27.56 μg/L and salinity = 19.64′). As a result, the zooplankton community came to consist largely of the rotifer Brachionus plicatilis, marine copepods and meroplanktonic larvae, mainly Gastropoda. Salinity was the main force structuring the zooplankton community after the sandbar opening. Two years after this episode, the prior zooplankton community had not reestablished itself, indicating a low resilience to this disturbance. The conditions developed prior to a sandbar opening can be crucial to the community responses in the face of this disturbance and for the capacity of the original zooplankton community to re-establish itself. 相似文献
650.
Evaluation of 2D models for the prediction of surface depression storage using realistic reference values
下载免费PDF全文
![点击此处可从《水文研究》网站下载免费的PDF全文](/ch/ext_images/free.gif)
R. Giménez I. Mezkiritz M.A. Campo J. Álvarez‐Mozos M. González‐Audicana A. Martinez‐Agirre J. Casalí 《水文研究》2016,30(18):3197-3209
Depression storage (DS) is the maximum storage of precipitation and runoff in the soil surface at a given slope. The DS is determined by soil roughness that in agricultural soils is largely affected by tillage. The direct measurement of DS is not straightforward because of the natural permeability of the soil. Therefore, DS has generally been estimated from 2D/3D empirical relationships and numerical algorithms based on roughness indexes and height measurements of the soil surface, respectively. The objective of this work was to evaluate the performance of some 2D models for DS, using direct and reliable measurements of DS in an agricultural soil as reference values. The study was carried out in experimental microplots where DS was measured in six situations resulting from the combination of three types of tillage carried out parallel and perpendicular to the main slope. Those data were used as reference to evaluate four empirical models and a numerical method. Longitudinal altitudinal profiles of the relief were obtained by a laser profilometer. Infiltration measurements were carried out before and after tillage. The DS was largely affected by tillage and its direction. Highest values of DS are found on rougher surfaces mainly when macroforms cut off the dominant slope. The empirical models had a limited performance while the numerical method was the most effective, even so, with an important variability. In addition, a correct hydrological management should take into account that each type of soil tillage affects infiltration rate differently. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献