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Role of benthic macro-invertebrate bioturbation on uranium biogeochemistry in freshwater sediments


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Titre du congrès :International Conference on Radioecology and Environmental Radioactivity Ville du congrès :Bergen Date du congrès :15/06/2008

Type de document > *Congrès/colloque

Mots clés > bioturbation, écosystème aquatique, flux, invertébré, transfert, uranium

Unité de recherche > IRSN/DEI/SECRE/LRE

Auteurs > BONZOM Jean-Marc, LAGAUZERE Sandra, STORA Georges

Date de publication > 20/06/2008

Résumé

In aquatic ecosystems, sediments constitute the most important storage reservoir for metallic pollutants like uranium. Metals can be transformed into more or less toxic or reactive species due to changes in the physico-chemical conditions and microbial activity. These conditions are themselves controlled by benthic organism activity via bioturbation processes. The link between bioturbation, sediment biogeochemistry and uranium transfers, bioavailability and toxicity has been poorly studied. In this context, two objectives were drawn: (i) evaluate the effects of uranium on benthic macro-invertebrates, particularly on their bioturbation activity; and inversely, (ii) estimate the influence of bioturbation on sediment biogeochemistry and then on uranium distribution and transfers within benthic ecosystem. To aim these objectives, several laboratory studies were performed under controlled conditions. Microcosms were filled with natural sediment and water, sediment was artificially contaminated with uranium and two ubiquist and abundant freshwater invertebrate species were added (Chironomids larvae: Chironomus riparius, and Oligochaeata worms: Tubifex tubifex). Concerning the first study axis, ecotoxic tests achieved on each species show significative effects for high concentrations of uranium on survival, morphology (deformities, autotomy), growth, bioaccumulation and behaviour. Bioturbation activity was studied in more details by following sediment particle movement thanks to inert fluorescent tracers. Main results indicate that only Tubifex worms involve a strong mixing of sediment and that uranium has a negative effect on bioturbation for high concentrations. Moreover, these organisms, even exposed to high contamination levels, induce important release of uranium from the sediment to the water column. To highlight the consequences of uranium release induced by Tubifex worms, an ecotoxic test was performed on a pelagic organism, the Xenopus tadpole: a genotoxic test was carried out on blood cells and molecular effects on stress proteins were estimated. Results reveal that uranium has a negative effect on these parameters but moderated in comparison with others pollutants usually tested. The second part of the study was devoted to estimate the influence of bioturbation on biogeochemical processes occurring in sediment. Thanks to DET (Diffusive Equilibrium in Thin films) gel probes inserted in sediment, high-resolution vertical profiles of main chemical species concentrations in porewater were obtained and uranium fluxes at the sediment-water interface were then calculated. It appears clearly that bioturbation modifies the diagenetic sequence into the sediment, enhances uranium fluxes and its release to the water column, and then influences its distribution within benthic ecosystem. In addition, the dissolved oxygen 2D distribution in the sediment was analyzed by using oxygen optods. From images obtained, oxic layer depth and oxygen fluxes measurements show that bioturbation increases oxygen fluxes at the interface and then its penetration into the sediment. This effect is more pronounced when sediment is spiked with uranium. It could be explained by enhanced oxygen