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L'influence de la chaîne trophique microbienne sur le cycle des métaux traces dans les lacs: accent sur la zone pélagique du lac Erié / The Influence of the microbial food web on trace metal cycling in lakes : an emphasis on the pelagic zone.

Twiss, Michael R. (1996). L'influence de la chaîne trophique microbienne sur le cycle des métaux traces dans les lacs: accent sur la zone pélagique du lac Erié / The Influence of the microbial food web on trace metal cycling in lakes : an emphasis on the pelagic zone. Thèse. Québec, Université du Québec, Institut national de la recherche scientifique, Doctorat en sciences de l'eau, 157 p.

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Résumé

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The low concentrations of dissolved trace metals observed in the surface waters of the lower Laurentian Great Lakes of North America during summer months are generally attributed to the sedimentary loss of biogenic particles from the epilimnion, in accordance with established scavenging models based solely upon the sorptive loss of solutes to particle surfaces. The majority of particles in the pelagic systems of these lakes are biotic-among the most productive are the autotrophic picoplankton and bacteria (0.2-2 µm) that have a high potential to scavenge trace metals. The ecological fate of picoplankton in the microbial food web is predominately consumption by microzooplankton (mixotrophic and heterotrophic organisrns, 2-200 µm). The hypothesis that microzooplankton can regenerate significant amounts of trace metal into the dissolved phase through the incomplete assimilation of trace metals from their prey was tested in the laboratory and in the field. Radionuclides were used to follow the fate of trace metals ingested in a particulate form by microzooplankton. Rapid regeneration of trace metals from the particulate to the dissolved phase (<0.2 µm) was observed in a laboratory model of a simplified Great Lakes microbial food web, composed of a mixotrophic nanoflagellate (Ochromonas) grazing [137 Cs, 109Cd,65Zn,153Gd]_ radiolabeled picocyanobacteria (Synechococcus). Most of the trace metal consumed as prey was regenerated by Ochromonas; regenerated 153Gd, 65Zn, and 109Cd had reduced bioavailability, in comparison with inorganic forms of the same elements. Trace metal regeneration was also observed in the natural plankton community sampled from the pelagic region of Lake Erie during thermal stratification. [109Cd,65Zn]-radiolabeled Synechococcus were used to label the picoplankton community in lake water and to trace the effect of natural grazing activity on the size fractionation of these metals. Trophic transfer of radionuclides (Zn > Cd) from prey (0.2-2 µm) to predators in the nanoplankton (2-20 µm) and microplankton (20-210 µm), was observed as was the recycling of regenerated metals back into the various plankton size fractions (Cd> Zn). Most regenerated trace metal radionuclides remained in the dissolved phase. A dynamic model of trace metal fate in the microbial food web under steady-state conditions was constructed using observed characteristics of the various plankton size fractions in the microbial food web (ability to scavenge 137Cs, 109Cd, 65Zn and 153Gd from the dissolved phase; potential to regenerate these metals back into the dissolved phase; population dynamics). Trace metal residence times of Cs (514 d), Cd (29 d), Zn (32 d), and Gd (66 d) predicted by the model were 46%,62%,58% and 84% greater, respectively, than residence times predicted if microzooplankton grazing activity was eliminated from the model simulations. These results are the first unequivocal demonstration of trace metal regeneration by microzooplankton grazing activity, and illustrate the importance of the microbial food web in determining the geochemical fates of particle-reactive trace metals in the pelagic surface waters of large lakes during thermal stratification.

Type de document: Thèse
Directeur de mémoire/thèse: Campbell, Peter G.C.
Informations complémentaires: Résumé avec symboles
Mots-clés libres: milieu aquatique; biogéochimie; chaine trophique; eau; géochimique; influence; lac; microbe; organisme; phytoplancton; plancton; surface; système; métaux trace
Centre: Centre Eau Terre Environnement
Date de dépôt: 17 févr. 2014 21:33
Dernière modification: 08 sept. 2016 15:16
URI: http://espace.inrs.ca/id/eprint/2005

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