Aim:To establish the dynamics of C. furcoides in horizontal and temporal scales; and to determine the main ecological factors related to its dynamics. Methods: Samples were taken in five stations between July 2002 and July 2003. Physical and chemical variables were sampled monthly. Density was evaluated by sampling carried out within the photic zone. Growth rate (r), Turnover rate (T), Generation Time (gt), Niche Width (NW), Taylor's Power Law, and the rate of population change (σ s ), were used. Canonical Correspondence Analysis (CCA) was used too. . The first three components of CCA explained 92.2% of the variation. Density was positively associated with chlorophyll a, NH 4 + , RWCS and wind direction. Light attenuation, NO 3 -, SiO 2 and O 2 were negatively associated with C. furcoides. Discussion: C. furcoides is a S strategist; it increases its density in the warmest periods under eutrophic conditions, low light penetration and high thermal stability; it is independent of the temperature but dependent of changes in rainfall and nutrients, -especially nitrogen -and not soluble phosphorus. Up Río Chico presented the best conditions for the increase of C. furcoides, since this station presented the highest levels of total nitrogen, and the highest relative stability. Conclusion: C. furcoides has a very similar ecology to that of C. hirundinella. It is an organism highly variable in temporal and spatial scales, with a wide niche and a clustered distribution. It belongs to the Morpho-funtional Group V and to L o and L M Assotiations.Keywords: Ceratium furcoides, phytoplankton, dinoflagellates, temporal and spatial dynamics, tropical reservoir.Resumo: Objetivo: determinar a dinâmica de C. furcoides em escalas horizontal e temporal, e determinar os principais fatores ecológicos relacionados com sua dinâmica. Métodos: As amostras foram coletadas em cinco estações entre julho de 2002 e julho de 2003. As variáveis físicas e químicas foram amostrados mensalmente e a densidade foi estimada a través de coletas realizadas dentro da zona fótica, a taxa de crescimento (r), taxa de rotatividade (T), Tempo de Geração (GT), Largura do nicho (NW), Power Taylor's Law, e a taxa de mudança da população (σ s ) foram utilizados.
Understanding the demographic history and genetic make-up of colonizing species is critical for inferring population sources and colonization routes. This is of main interest for designing accurate control measures in areas newly colonized by vector species of economically important pathogens. The biting midge Culicoides imicola is a major vector of orbiviruses to livestock. Historically, the distribution of this species was limited to the Afrotropical region. Entomological surveys first revealed the presence of C. imicola in the south of the Mediterranean basin by the 1970s. Following recurrent reports of massive bluetongue outbreaks since the 1990s, the presence of the species was confirmed in northern areas. In this study, we addressed the chronology and processes of C. imicola colonization in the Mediterranean basin. We characterized the genetic structure of its populations across Mediterranean and African regions using both mitochondrial and nuclear markers, and combined phylogeographical analyses with population genetics and approximate Bayesian computation. We found a west/east genetic differentiation between populations, occurring both within Africa and within the Mediterranean basin. We demonstrated that three of these groups had experienced demographic expansions in the Pleistocene, probably because of climate changes during this period. Finally, we showed that C. imicola could have colonized the Mediterranean basin in the Late Pleistocene or Early Holocene through a single event of introduction; however, we cannot exclude the hypothesis involving two routes of colonization. Thus, the recent bluetongue outbreaks are not linked to C. imicola colonization event, but rather to biological changes in the vector or the virus.
In this study we address the role of surface anisotropy on the hysteretic properties of magnetite Fe3O4 nanoparticles and the circumstances yielding both horizontal and vertical shifts in the hysteresis loops. Our analysis involves temperature dependence and particle size effects. Different particle sizes ranging from 2 up to 7 nm were considered. Our theoretical framework is based on a three-dimensional classical Heisenberg model with nearest magnetic neighbor interactions involving tetrahedral (A) and octahedral (B) irons. Cubic magnetocrystalline anisotropy for core spins, single-ion site anisotropy for surface spins, and interaction with a uniform external magnetic field were considered. Our results revealed the onset of low temperature exchange bias field, which can be positive or negative at high enough values of the surface anisotropy constant (KS). Susceptibility data, computed separately for the core and the surface, suggest differences in the hard-soft magnetic character at the core-surface interface. Such differences are KS-driven and depend on the system size. Such a hard-soft interplay, via the surface anisotropy, is the proposed mechanism for explaining the observed exchange bias phenomenology. Our results indicate also that the strongly pinned spins at high enough surface anisotropy values are responsible for both the horizontal and vertical shifts in the hysteresis loops. The dependences of the switching and exchange bias fields with the surface anisotropy and temperature are finally discussed.
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