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The cultural eutrophication of Lac la Biche, Alberta, Canada: a paleoecological study

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Abstract:

A multiproxy paleoecological investigation of Lac la Biche, a large boreal lake in northeastern Alberta, Canada, revealed that the lake was eutrophic before European settlement but has undergone additional cultural eutrophication in the past 30 to 50years. Annual fluxes to sediments of phosphorus, nitrogen, carbon, and inorganic sediments have increased with time. A declining N–P ratio has increasingly favored nitrogen-fixing cyanobacteria. Increased deposition of microbial pigments and diatom frustules and a recent shift in diatom species also indicate increasing eutrophication. Biogenic silica increased with time, but there is no evidence of a near-surface decline that would indicate silica limitation. Stable isotopes suggest that an increasing proportion of carbon deposited in sediments is of in-lake origin, indicating increased productivity. In the basin nearest the town of Lac La Biche, an increase in 15N followed the construction of the sewage treatment plant, but more recently, decreased 15N in both basins suggests that nitrogen fixation has become a more important source of nitrogen. Despite documented damage to the fishery of the lake, zooplankton fossils do not show evidence of a strong trophic cascade. The study illustrates the power of a multiproxy approach in obtaining reliable paleolimnological conclusions.

Une étude paléoécologique avec plusieurs variables de substitution au lac La Biche, un grand lac boréal du nord-est de l’Alberta, Canada, montre que le lac était eutrophe avant l’arrivée des européens, mais qu’il a subi une eutrophisation culturelle supplémentaire au cours des 30 à 50 dernières années. Les apports annuels aux sédiments de phosphore, d’azote, de carbone et de sédiments inorganiques se sont accrus au cours des années. Un rapport N–P en déclin a de plus en plus favorisé les cyanobactéries fixatrices d’azote. Les dépôts croissants de pigments microbiens et de frustules de diatomées, de même qu’un changement récent dans les espèces de diatomées, indiquent une intensification de l’eutrophisation. Il y a une augmentation dans le temps de la silice biogène, mais aucune indication d’un déclin près de la surface qui pourrait signaler une limitation de la silice. Les analyses d’isotopes stables laissent croire qu’une proportion croissante du carbone déposé dans les sédiments provient du lac lui-même, ce qui indique une productivité accrue. Dans le bassin versant le plus près de la ville de Lac-La-Biche, une augmentation de 15N a suivi la construction d’une usine de traitement des eaux usées; plus récemment, une réduction de 15N dans les deux bassins fait penser que la fixation d’azote est devenue une source plus importante d’azote. Malgré les données sur les dommages causés aux pêches dans le lac, les fossiles du zooplancton n’appuient pas l’existence d’une importante cascade trophique. Notre étude est un exemple du potentiel des méthodes à nombreuses variables de substitution pour l’obtention de conclusions paléolimnologiques fiables.

Document Type: Research Article

Publication date: October 1, 2008

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  • Published continuously since 1901 (under various titles), this monthly journal is the primary publishing vehicle for the multidisciplinary field of aquatic sciences. It publishes perspectives (syntheses, critiques, and re-evaluations), discussions (comments and replies), articles, and rapid communications, relating to current research on cells, organisms, populations, ecosystems, or processes that affect aquatic systems. The journal seeks to amplify, modify, question, or redirect accumulated knowledge in the field of fisheries and aquatic science. Occasional supplements are dedicated to single topics or to proceedings of international symposia.
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