Este artículo se inicia describiendo el problema fundamental de la geometría de webs, dando asimismo algunos resultados clásicos de esta teoría. Finalmente, se describe la estructura del espacio de relaciones abelianas de webs planares que admiten automorfismo infinitesimal. Como resultado de esto se obtienen algunas consecuencias.

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In this work we introduce the notion of Galois foliations on P2 , defined as those folations whose Gauss applications restricted to a Zariski open subset is a Galois covering. We also present some examples and acriterium for identifying such foliations. / En el presente trabajo introducimos la nocion de foliaciones Galois sobre P2C, definidas como aquellas cuya aplicacion de Gauss restringida aun abierto Zariski es un recubrimiento Galois. Asimismo, presentamo salgunos ejemplos y un criterio para identicar este tipo de foliaciones.

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Thesis (Ph. D.)--University of Wyoming, 2005. / Title from PDF title page (viewed on March 10, 2008). Includes bibliographical references (p. 89-96).

Thesis (Ph. D.)--University of Wyoming, 2006. / Title from PDF title page (viewed on June 26, 2008). Includes bibliographical references (p. 169-177).

North American beavers (Castor canadensis) build dams in stream channels, thus creating impoundments that flood surrounding riparian areas. Due to the widely circulating global pool of mercury in the atmosphere, mercury is deposited onto the landscape both near and far from point sources, including areas occupied by beavers. The organic form of mercury, methylmercury, is a potent neurotoxin with potential to cause harm to both humans and wildlife due to its ability to biomagnify up food chains. Recently flooded areas, such as those resulting from beaver impoundments, create ideal environments for the methylation of mercury. These impoundments can release methylmercury to downstream food webs where there is potential for it to be transferred to higher trophic level organisms. Beaver impoundments can also boost productivity in aquatic systems, so increases in mercury may be accompanied by an increase in nutrients and algal and invertebrate biomass. The findings here describe increased concentrations of methylmercury in water, algae, and invertebrates downstream from in-channel beaver dams in the southern Canadian Rockies. There was, however, no significant increase in nutrients or algal and invertebrate biomass downstream from impoundments. An examination of trophic transfer of mercury in these stream systems reveals that uptake is enhanced at low concentrations. The uptake pathway from water to algae is especially important but is attenuated in higher trophic levels due to a small relative difference in trophic level between predators and prey. The overall rate of trophic transfer in these systems falls within the low end of the typical range, and low baseline concentrations mean that methylmercury is not biomagnifying to dangerous levels in these low-productivity mountain systems. Beavers can provide important ecosystem services such as improving landscape heterogeneity, creation of new habitat for invertebrates and fish and improved angling opportunities, but they also enhance mercury export. Therefore, in systems that are mercury-sensitive such as those with low pH or long-lived, slow-growing predatory fish species, beaver influence should be considered as an important source of methylmercury.

Hamilton Harbour, a semi-enclosed bay located at the western end of Lake Ontario, is listed as one of the most polluted systems in the Great Lakes. Anthropogenic influences such as four wastewater treatment plants, two steel mills and shoreline development have lead to degradation of this system. A Remedial Action Plan is in place to clean up the harbour by 2015. This study examined the food web dynamics of Hamilton Harbour including 21 species of fish, benthic invertebrates, plankton and macrophytes. Using carbon and nitrogen stable isotopes spatial and seasonal variability throughout the harbour was examined. Zooplankton and phytoplankton collected at three different sites in the harbour showed no significant difference spatially but did show seasonal trends, reaching the highest nitrogen values in early summer. Benthic invertebrates, when observed in δ13C: δ15N biplots, group together by sampling site in each season. Seasonally benthic invertebrates acquire higher nitrogen signatures in summer then decrease in fall at all sites. The fish community in the harbour do not have spatially distinct isotope signatures. Seasonally nitrogen signatures increased at all sites while carbon signatures remained between -25 ‰ and -26 ‰. Overall the plankton and benthic invertebrate nitrogen isotope signatures are higher than the fishes. This indicates that there is a recent change in nutrient source. The likely candidate for nutrient input is an anthropogenic source, such as the wastewater treatment plants discharging into the harbour. Isotope signatures show large variation in fish species collected indicating that the fishes are omnivore generalists that take advantage of available food sources throughout the harbour. Further remediation work, such as habitat modifications, can now be tailored towards generalist omnivores that move throughout the harbour.

Hamilton Harbour, a semi-enclosed bay located at the western end of Lake Ontario, is listed as one of the most polluted systems in the Great Lakes. Anthropogenic influences such as four wastewater treatment plants, two steel mills and shoreline development have lead to degradation of this system. A Remedial Action Plan is in place to clean up the harbour by 2015. This study examined the food web dynamics of Hamilton Harbour including 21 species of fish, benthic invertebrates, plankton and macrophytes. Using carbon and nitrogen stable isotopes spatial and seasonal variability throughout the harbour was examined. Zooplankton and phytoplankton collected at three different sites in the harbour showed no significant difference spatially but did show seasonal trends, reaching the highest nitrogen values in early summer. Benthic invertebrates, when observed in δ13C: δ15N biplots, group together by sampling site in each season. Seasonally benthic invertebrates acquire higher nitrogen signatures in summer then decrease in fall at all sites. The fish community in the harbour do not have spatially distinct isotope signatures. Seasonally nitrogen signatures increased at all sites while carbon signatures remained between -25 ‰ and -26 ‰. Overall the plankton and benthic invertebrate nitrogen isotope signatures are higher than the fishes. This indicates that there is a recent change in nutrient source. The likely candidate for nutrient input is an anthropogenic source, such as the wastewater treatment plants discharging into the harbour. Isotope signatures show large variation in fish species collected indicating that the fishes are omnivore generalists that take advantage of available food sources throughout the harbour. Further remediation work, such as habitat modifications, can now be tailored towards generalist omnivores that move throughout the harbour.