L'influence humaine en relation avec les changements environnementaux : évolution des lacs en Nouvelle-Angleterre reconstituée par l'analyse des diatomées
|Abstract:||The human and climatic impact on four New England lakes was reconstructed by analysis of fossil diatoms preserved in lake-bottom sediments. Different diatom-based methods for the reconstruction of environmental variables (pH, total phosphorus, alkalinity) were tested. Weighted averaging and Gaussian logit regression were the most appropriate methods for the reconstruction of variables representing the principal gradient, whereas artificial neural networks and Gaussian logit regression performed better for secondary gradient variables. It was shown that different data transformations may be useful for different reconstructions. Fossil diatom assemblages of the sites recorded a regional pattern of deforestation and agricultural activity following European settlement. Despite the re-establishment of forests in the lake watersheds since 1900 AD, three lakes did not return to pre-disturbance conditions. This is due to local site characteristics, such as natural lake evolution, local natural disturbance patterns (hurricanes), and peatland development. It is therefore important to consider the natural dynamics of lakes when establishing the theoretical « natural » state for restoration purposes. At Walden Pond, Massachusetts, an accelerated nutrient enrichment was inferred from 1950 AD onwards, related to intensive recreational use of the lake. Since 1975 AD, the rate of eutrophication has diminished after management measures were implemented. The natural state of the lake reconstructed by this study represents a useful benchmark for future management decisions. In Levi Pond, a trend of increasing humidity during the past ca. 2000 years has been inferred by diatoms, corresponding to the period of Neoglacial cooling. This result indicates that diatoms may be a useful tool for future paleohydrological studies in temperate regions. The seasonal study of diatoms in Bates Pond, Connecticut, indicated that diatom assemblages are strongly influenced by stratification, which helped to identify a period of prolonged full-circulation in the past based on the fossil assemblages of the same lake. The maximum diatom productivity in autumn indicated that this is the most appropriate season for taking water samples for diatom inference model development. This thesis has provided new knowledge of the evolution of New England lakes before and after European settlement and on two pertinent methodological aspects of paleolimnologic study.|
|Document Type:||Thèse de doctorat|
|Open Access Date:||11 April 2018|
|Collection:||Thèses et mémoires|
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