On the existence of central configurations of <Emphasis Type="Italic">p</Emphasis> nested regular polyhedra

In this paper we prove, for all p ≥ 2, the existence of central configurations of the pn-body problem where the masses are located at the vertices of p nested regular polyhedra having the same number of vertices n and a common center. In such configurations all the masses on the same polyhedron are equal, but masses on different polyhedra could be different.     

Connections between hydrodynamics, benthic landscape and associated fauna in the Balearic Islands, western Mediterranean

Linking the abiotic and biotic traits of ecosystems is a critical step towards understanding their structure and functioning. Here we attempt to determine the connections between the hydrodynamics, benthic landscape and the associated fish communities on the coastal continental shelf off the Balearic Islands (western Mediterranean). Specifically we investigate the role the hydrodynamics play in shaping the benthic landscape, and whether the hydrodynamics affect the composition and structure of demersal fish communities. A realistic numerical model was used to establish the hydrodynamic characteristics of the area. The study area showed high hydrodynamic variability on a medium spatial scale (tens of km) in terms of mean water velocity (ū). Principal component analysis was used to determine the main gradients of macro-epibenthic variability. Redundancy analysis (RDA) was used to model the effect of the hydrodynamics on macro-epibenthic species. RDA was also used to model the effect of the hydrodynamics and macro-epibenthos on the abundance of the associated fish fauna, and on its biomass at a community level using biomass spectra classes. The results showed that the hydrodynamics had a significant influence on the distribution of both macro-epibenthic species and the associated fish species. The latter was also influenced by the macro-epibenthos. Fish size appeared to be a key attribute for the distribution of species across gradients of ū and macro-epibenthic change. Our findings can be applied in ecosystem-based fisheries management, as they show that it is necessary to take into account both the biotic and abiotic traits of the habitats when the habitat use and requirements of the associated species are defined.     

New radiometric and geomorphologic evidences of a last glacial maximum older than 18 ka in SW European mountains: the example of Redes Natural Park (Cantabrian Mountains,NW Spain)

Abstract

The first numerical age determinations from radiocarbon dating establish the chronology of glacial events in Redes Natural Park (Cantabrian Mountain, NW Spain). A core drilled in an ice-dammed deposit provided a minimum age of 28 990 ± 230 years BP for the maximum glacial expansion (phase I). Another core from a cirque bottom-fill provided organic sediment with 20 640 ± 300 years BP, a minimum age for the first glacial retreat (phase II). Radiometric dating of proglacial deposits interpreted as synchronous with the last glacial maximum phase in neighbouring Cornelia basin (Picos de Europa), yield ages of 40 480 ± 820 years BP. The chronological data presented in this work are consistent with the model of glacier evolution established in the Pyrénées, with a glacial maximum phase for the last glacial period older than 18 ka. © 2002 Éditions scientifiques et médicales Elsevier SAS. AU rights reserved.     

Dynamic hybrid terrain representation based on convexity limits identification

Hybrid digital terrain models combine terrain data with different topologies and resolutions. Cartographic digital terrain models are typically composed of regular grid data that can be locally refined by adding a Triangulated Irregular Network (TIN) that represents morphologically complex terrain parts. Direct rendering of both datasets to visualize the digital terrain model generates discontinuities, as the meshes are disconnected. The utilization of complete/partial precomputed tessellation solutions solves the problem of quality, but limits the applicability of the representation to models with a fixed relative position between datasets. In this paper, we present a new scheme for hybrid terrain representation that permits the dynamic generation of the adaptive tessellation required to join the grid and TIN models. Our proposal permits the dynamic modification of the relative position between datasets. This increases the representation capabilities for those applications where this property is interesting as, for example, urban and landscape planning applications. The algorithm we propose is based on the identification of convex areas on the TIN and the efficient generation of triangles to join the models based on this convex structure. As a result, high quality models without discontinuities are obtained, increasing the flexibility of previous solutions based on fixed precomputations.     

Long-term divergent tidal flat benthic community recovery following hypoxia-induced mortality

Macrobenthos recovery after hypoxia-induced mass mortality was assessed in an estuarine tidal mudflat during 3 years. During the first 2 years, a Pearson-Rosenberg type of community recovery took place along with the improving bottom water oxygen conditions. After 3 months, spionid polychaetes became superabundant (i.e. opportunistic peak), followed rapidly by a steep decline (i.e. ecotone point). Subsequently, a moderate increase in species richness and a steep increase in biomass, related to the growth of long-lived species occurred (i.e. transition region). Afterwards, however, the recovering community diverged again from the ambient, undisturbed, sediments due to enhanced recruitment success of long-lived species presumably resulting from the lowered interference from bioturbation during early recovery stages in the disturbed plots. Hence, despite early community recovery may be more or less deterministic, lagged divergent community reassembling may occur at the longer-term, thereby contributing to benthos patchiness in areas which are frequently subjected to disturbances.     

Geomorphological and ecological features of blowouts in a western Mediterranean coastal dune complex: a case study of the Es Comú de Muro beach-dune system on the island of Mallorca, Spain

Many of the coastal dune systems along western Mediterranean shores are in an advanced state of fragmentation and show distinct signs of erosion, largely because of blowout development along the dune front. The Es Comú de Muro beach-dune system on the island of Mallorca (Spain) is a good example of this. In order to better understand and quantify the current situation, 58 blowouts along a ca. 1.5-km-long dune front were investigated. In each case, a number of morphometric and ecological variables were analyzed as a basis for comparison and classification, in particular blowout dimensions and orientation, inner morphometry and topography, morphological types, the role of vegetation in defining the state of the foremost dune line, and the link between vegetation and blowout typology. In comparison with a recent preliminary investigation, the results of the present study provide a more comprehensive picture of the advanced state of fragmentation along the dune front. The blowouts are not evenly distributed, highest densities occurring along the southernmost part of the beach, lowest densities along the northern part. The blowouts were subdivided into two categories on the basis of their shape and general structure, trough blowouts being the most prevalent, followed by mixed trough-saucer shapes. Distinctly saucer-shaped blowouts could not be distinguished. In addition, the blowouts were subdivided into two morphological categories, i.e. simple and branched. It was also possible to link the morphological state of the dune front to certain ecological parameters, in particular vegetation which, in the present case, comprised herbaceous and woody plants. Cluster analyses of species associations (Bray-Curtis similarity indices) were carried out on the basis of the presence/absence of each species. It is shown that, on account of presence counts and the degree of similarity of species associations, some species play a more important role in stabilizing the mobile dune sands than others, a foremost candidate being Ammophila arenaria. It is concluded that active support for such plants should form part of any management strategy aiming to reduce and reverse coastal dune fragmentation processes.