Fecha de publicación: 2014.
The horticultural belt of La Plata is a complex framework of lands with a marked urban influence. The unregulated use of agrochemicals is one of the most important sources of superficial and underground water contamination producing drastic changes in the biodiversity of natural communities. The shallowness of the temporary waters favors the macrophyte growth which, in turn, allows the colonization of the phytoepiphyton. Four samplings were carried out during spring 2011 and summer 2012 in four adjacent temporary waters of different agroecosystems, three of them with agricultural activity (site over 5 meters crops, CUL1 and CUL2; site within 5 meters of crops: CUL3) and one, taken as reference (REF), with livestock activity. Seven species of macrophytes were sampled, two submerged and 5 emergent. 88 phytoepiphytic species were recorded, being clorophytes and diatoms the dominant groups. The submerged macrophytes presented a higher mean abundance than the emergent ones in all the sampling sites. Oedogonium species were dominant in all temporary waters. L. peploides showed the highest total specific richness while S. californicus presented the highest mean diversity specific. The highest phytoepiphyton mean abundance was recorded in CUL1 while the least phytoepiphytic density was found in REF. Nygaard index shows that the four temporary waters are meso to eutrophic. The conductivity was the most significant factor in the distribution of diatoms species in temporary waters.
Facultad de Ciencias Naturales y Museo
Repositorio: SEDICI (UNLP). Universidad Nacional de La Plata
Fecha de publicación: 2010.
Molecular dynamics simulations have been carried out to investigate the structure and dynamics of liquid methanol confined in 3.3 nm diameter cylindrical silica pores. Three cavities differing in the characteristics of the functional groups at their walls have been examined: (i) smooth hydrophobic pores in which dispersive forces prevail, (ii) hydrophilic cavities with surfaces covered by polar silanol groups, and (iii) a much more rugged pore in which 60% of the previous interfacial hydroxyl groups were replaced by the bulkier trimethylsilyl ones. Confinement promotes a considerable structure at the vicinity of the pore walls which is enhanced in the case of hydroxylated surfaces. Moreover, in the presence of the trimethylsilyl groups, the propagation of this interface-induced spatial ordering extends down to the central region of the pore. Concerning the dynamical modes, we observed an overall slowdown in both the translational and rotational motions. An analysis of these mobilities from a local perspective shows that the largest retardations operate at the vicinity of the interfaces. The gross features of the rotational dynamics were analyzed in terms of contributions arising from bulk and surface states. Compared to the bulk dynamical behavior, the characteristic timescales associated with the rotational motions show the most dramatic increments. A dynamical analysis of hydrogen bond formation and breaking processes is also included. © 2010 American Institute of Physics.
Afiliación de los autores: Elola, M.D. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.
Afiliación de los autores: Rodriguez, J. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.
Afiliación de los autores: Laria, D. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.
Palabras claves: Breaking process; Dispersive forces; Dynamical analysis; Dynamical behaviors; Functionalized silica; Hydrogen-bond formation; Hydrophobic pore; Hydroxyl groups; Hydroxylated surfaces; Liquid methanol; Local perspective; Molecular dynamics simulations; Pore wall; Rotational dynamics; Rotational motion; Silanol groups; Silica pores; Spatial ordering; Structure and dynamics; Surface state; Time-scales; Trimethylsilyl; Trimethylsilyl groups; Dynamics; Functional groups; Hydrogen; Hydrogen bonds; Liquids; Methanol; Molecular dynamics; Rotational flow; Silica; Nanopores.
Repositorio: Biblioteca Digital (UBA-FCEN). Universidad Nacional de Buenos Aires. Facultad de Ciencias Exactas y Naturales