Seasonal changes in vertical distribution of zooplankton in an oligotrophic, subarctic lake (Lake Takvatn, Norway)     

Raul Primicerio   《Limnologica》2000,30(4):301-310

Zooplankton vertical distribution was studied in Lake Takvatn (69°07′ N) by discrete sampling of the water column during the open water season. Attention was focused on predation risk and competition to understand when, where and to what degree a given species should aggregate along the water column. Temperature profiles and phytoplankton abundance and composition were recorded to assess degree of heterogeneity and food availability in the pelagic zone. Vertical segregation was evident during thermal stratification. Rotifers partitioned the water column, with species less susceptible to predation (e.g. Conochilus unicornis) in the epilimnion, where they overlapped with the predators Asplanchna priodonta and Polyphemus pediculus, and species more vulnerable to these predators (e.g. Keratella cochlearis) in the hypolimnion. Cladocerans remained in the epilimnion, in a year when predation by fish was limited. The copepodites of Eudiaptomus graciloides and Cyclops scutifer maintained different depths staying respectively near surface and in the meta-hypolimnion. A broader distribution with increasing density was evident among rotifers and C. scutifer nauplii. Density-dependent habitat selection behaviour is considered as a mechanism explaining the observed distribution of predators and competing prey and promoting species coexistence.  相似文献   

Optical and infrared photometry of new very low‐mass stars and brown dwarfs in the σ Orionis cluster     

V. J. S. Bjar  M. R. Zapatero Osorio  R. Rebolo 《Astronomische Nachrichten》2004,325(9):705-713

We present an RI photometric survey covering an area of 430 arcmin² around the multiple star σ Orionis. The observations were conducted with the 0.8 m IAC‐80 Telescope at the Teide Observatory. The survey limiting R and I magnitudes are 22.5 and 21, and completeness magnitudes 21 and 20, respectively. We have selected 53 candidates from the I vs. R–I colour‐magnitude diagram (I = 14–20) that follow the previously known photometric sequence of the cluster. Adopting an age of 2–4 Myr for the cluster, we find that these objects span a mass range from 0.35 M_⊙ to 0.015 M_⊙. We have performed J‐band photometry of 52 candidates and K_s photometry for 12 of them, with the result that 50 follow the expected infrared sequence for the cluster, thus confirming with great confidence that the majority of the candidates are bona fide members. JHK_s photometry from the Two Micron All Sky Survey (2MASS) is available for 50 of the candidates and are in good agreement with our data. Out of 48 candidates, which have photometric accuracies better than 0.1 mag in all bands, only three appear to show near‐infrared excesses. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

Seismic response of a bridge–soil–foundation system under the combined effect of vertical and horizontal ground motions     

Zhenghua Wang  Leonardo Dueñas‐Osorio  Jamie E. Padgett 《地震工程与结构动力学》2013,42(4):545-564

Different levels of model sophistication have recently emerged to support seismic risk assessment of bridges, but mostly at the expense of neglecting the influence of vertical ground motions (VGMs). In this paper, the influence of VGMs on bridge seismic response is presented and the results are compared with the case of horizontal‐only excitations. An advanced finite element model that accounts for VGMs is first developed. Then, to investigate the effect of soil–structure interaction (SSI) including liquefaction potential, the same bridge with soil‐foundation and fixed boundary conditions is also analyzed. Results show that the inclusion of the VGMs has a significant influence on the seismic response, especially for the axial force in columns, normal force of bearings, and the vertical deck bending moments. However, VGMs do not have as much influence on the seismic demand of the pile cap displacements or pile maximum axial forces. Also, the significant fluctuation of the column axial force can reduce its shear and flexural capacity, and a heightened reversal of flexural effects may induce damage in the deck. In addition, relative to the fixed base case, SSI effects tend to reduce response quantities for certain ground motions while increasing demands for others. This phenomenon is explained as a function of the frequency content of the ground motions, the shift in natural vertical periods, and the VGM spectral accelerations at higher modes. Moreover, the mechanisms of liquefaction are isolated relative to SSI effects in nonliquefiable soils, revealing the influence of liquefaction on bridge response under VGMs. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

Co-clustering geo-referenced time series: exploring spatio-temporal patterns in Dutch temperature data     

Xiaojing Wu  Raul Zurita-Milla  Menno-Jan Kraak 《International journal of geographical information science》2013,27(4):624-642

Clustering allows considering groups of similar data elements at a higher level of abstraction. This facilitates the extraction of patterns and useful information from large amounts of spatio-temporal data. Till now, most studies have focused on the extraction of patterns from a spatial or a temporal aspect. Here we use the Bregman block average co-clustering algorithm with I-divergence (BBAC_I) to enable the simultaneous analysis of spatial and temporal patterns in geo-referenced time series (time evolving values of a property observed at fixed geographical locations). In addition, we present three geovisualization techniques to fully explore the co-clustering results: heatmaps offer a straightforward overview of the results; small multiples display the spatial and temporal patterns in geographic maps; ringmaps illustrate the temporal patterns associated to cyclic timestamps. To illustrate this study, we used Dutch daily average temperature data collected at 28 weather stations from 1992 to 2011. The co-clustering algorithm was applied hierarchically to understand the spatio-temporal patterns found in the data at the yearly, monthly and daily resolutions. Results pointed out that there is a transition in temperature patterns from northeast to southwest and from ‘cold’ to ‘hot’ years/months/days with only 3 years belonging to ‘cool’ or ‘cold’ years. Because of its characteristics, this newly introduced algorithm can concurrently analyse spatial and temporal patterns by identifying location-timestamp co-clusters that contain values that are similar along both the spatial and the temporal dimensions.  相似文献   

Effect of aeration on steady-state conditions in non- and partially aerated low-loaded biofilter     

A. Albuquerque  A. González-Martínez  F. Osorio 《International Journal of Environmental Science and Technology》2012,9(3):395-408

Excessive growth of biomass and retention of solids associated with air bubbles lead to bed clogging, which affects the biofilters?? performance. Two experiments were carried out in a submerged biofilter at the flow velocity of 0.5?m?h^?1, for an organic loading rate of 51?g C m^?3?h^?1 and a nitrogen loading rate of 13?g NH₄-N?m^?3?h^?1, one with the biofilter not aerated, the other with the biofilter partially aerated. The results showed that the higher head losses occurred in the upper section of the biofilter, where there was a greater biomass development and a higher removal of organic carbon, ammonia and solids, with the maximum allowed head loss being reached in 16 and 8?days. In any case, the steady-state conditions were achieved after 2?days and were interrupted on the tenth day of experiment E1 and on the fifth day of experiment E2. This allowed defining different operating cycles that enabled an average organic removal rate of 12.7?g C m^?3?h^?1 (27?%) and an average ammonia removal rate of 1.1?g NH4-N?m^?3?h^?1 (9?%) without aeration, and of 35.8?g C m^?3?h^?1 (76?%) and 6.3?g NH4-N?m^?3?h^?1 (51?%) with aeration. Regardless of the aeration conditions, more than 90?% of TOC and NH4-N removal occurred in the upper section. After the backwashing cycle, the biofilter returned to steady-state conditions in 6?h (without aeration) and 7?h (with aeration).  相似文献   

Gaussian beam synthetic seismograms in a vertically varying medium     

Raul Madariaga 《Geophysical Journal International》1984,79(2):589-612

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High-frequency radiation from crack (stress drop) models of earthquake faulting   总被引：1，自引：0，他引：1  

Raul Madariaga 《Geophysical Journal International》1977,51(3):625-651

We present a theory for the radiation of high-frequency waves by earthquake faults. We model the fault as a planar region in which the stress drops to the kinematic friction during slip. This model is entirely equivalent to a shear crack. For two-dimensional fault models we show that the high frequencies originate from the stress and slip velocity concentrations in the vicinity of the fault's edges. These stress concentrations radiate when the crack expands with accelerated motion. The most efficient generation of high-frequency waves occurs when the rupture velocity changes abruptly. In this case, the displacement spectrum has an ω^-2 behaviour at high frequencies. The excitation is proportional to the intensity of the stress concentration near the crack tips and to the change in the focusing factor due to rupture velocity. We extend these two-dimensional results to more general three-dimensional fault models in the case when the rupture velocity changes simultaneously on the rupture front. Results are similar to those described for two-dimensional faults. We apply the theory to the case of a circular fault that grows at constant velocity and stops suddenly. The present theory is in excellent agreement with a numerical solution of the same problem.
Our results provide upper bounds to the high-frequency radiation from more realistic models in which rupture velocity does not change suddenly. The ω^-2 is the minimum possible decay at high frequencies for any crack model of the source.  相似文献   

Identifying Environmental and Human Factors Associated With Tick Bites using Volunteered Reports and Frequent Pattern Mining   总被引：1，自引：0，他引：1       下载免费PDF全文

Irene Garcia‐Martí  Raul Zurita‐Milla  Arno Swart  Kees C. van den Wijngaard  Arnold J.H. van Vliet  Sita Bennema  Margriet Harms 《Transactions in GIS》2017,21(2):277-299

Tick populations and tick‐borne diseases like Lyme borreliosis have been steadily increasing since the mid‐1990s. Realizing the threat that ticks pose to public health, two Dutch citizen science projects have collected tick bite reports since 2006. This unique volunteered geographical dataset, which currently has nearly 35,000 reports, was used to identify environmental and other circumstantial factors associated with tick bites. For this, we first enriched the tick bite reports with temperature, precipitation, vegetation and volunteered data associated with the location of the tick bite. Using this enriched dataset, we then derived a series of features to characterize the environmental and volunteer‐related conditions in which each tick bite occurred. Next, we discretized these features using the Jenks Natural Breaks algorithm and, after that, we mined frequent environmental patterns associated with tick bites using the AprioriClose algorithm. Finally, we checked that these patterns are specifically associated with the tick bites by comparing them with the frequent patterns mined from pseudo‐random locations. The frequent patterns were visualized using heat maps and ring maps and two representative patterns associated with tick bites were projected into geographic space to study their spatio‐temporal distribution. Our results show that factors linked to human activity are more relevant to model tick bites than seasonal accumulations of temperature, vegetation or precipitation. In particular, the number of warm and dry days per season are present in a significant number of patterns and the majority of tick bites are produced within a distance of half a kilometer of a forest, recreational or built‐up area. The study of patterns in the time‐series revealed that there are several persistent patterns consistently occurring each year and the validation process showed that the volunteer tick bites collection is capturing environmental conditions associated with tick bites, suggesting that these reports have a high scientific value. These results support the creation of a Dutch tick bite risk map that, in turn, will open the door to the design of public health interventions to reduce the incidence of Lyme disease.  相似文献   

Context and differentiation: Comparative analysis of two community-based fishers’ organizations     

Raul P. Lejano  Alma Ocampo-Salvador   《Marine Policy》2006,30(6):726-736

There has evolved, as of late, a strong interest in decentralized modes of governance over fisheries and other marine resources, particularly that of community-based resource management (CBRM). However, analysis of this institutional model is hindered by simplistic, typological depictions of CBRM. We use organizational theory to argue how, within the broad category of CBRM, there exists a rich variation of institutional forms. We illustrate the analysis by showing how two ostensibly similar fishers’ organizations in Batangas, Philippines, differ in important ways and, furthermore, how these differences respond to particular challenges and opportunities found in their respective contexts. We also discuss implications of this research for program evaluation.  相似文献   

Long-term tidal level distribution using a wave-by-wave approach     

Sonia Castanedo  Fernando J. Mendez  Raul Medina  Ana J. Abascal 《Advances in water resources》2007

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