Ecological risk assessment of Lake Bourget (France, Savoie) littoral sediment contaminated by heavy metals, PAHs and PCBs through chemical analyses, biomarkers, laboratory and in situ bioassays
Résumé
In the early 1980’s, the water level of Lake Bourget (France) was artificially regulated for hydroelectricity. This reduction in the amplitude of fluctuations negatively impacted the development of reed-beds. In autumn 2017, the water level will be lowered in order to stimulate the growth of the reed-beds. We assume that the modifications of the physical conditions of the shallow littoral zone (water depth, oxygen content and redox potential, temperature, …) could change the mobility and bio-availability of contaminants at the water-sediment interface. As a matter of fact, whereas the sediment is generally a sink for contaminants entering the lake, it may act as a source in such changing conditions. The aim of the present work is to assess the basal contamination and ecotoxicity of the sediment of the littoral zone of Lake Bourget in order to study further this phenomenon.
For this purpose, we applied a Sediment Quality Triad approach (Chapman, 2000) on sediments from the shoreline of Lake Bourget (France) in order to evaluate their contamination and ecotoxicity. We first characterized the total content of heavy metals (Cd, Cr, Cu, Ni, Pb and Zn), the 16 polycyclic aromatic hydrocarbons (PAHs) identified as priority pollutants by the USEPA and the 7 indicator Polychlorinated biphenyls (PCBs, congeners 28, 52, 101, 118, 138, 153 and 180) in the surface sediment of 10 sites all around the lake. We then conducted laboratory microcosm assays to assess the biological response of cultured species (Lemna minor, Daphnia magna and Chironomus riparius) exposed to these sediments in controlled conditions. Finally, we performed in situ investigations such as biomarker measurements - Acetylcholine-esterase and Glutathione S-Transferase - on the autochthonous macro-fauna (Corbicula fluminea, chironomids, gammarids and oligochaetes) and fitness tests on caged laboratory cladocerans (Daphnia magna).
The geographical repartition of the contaminants was mostly in accordance with the pollution sources identified on the watershed. The ecological risk assessment based on Quotient risk and Consensus-Based Sediment Quality Guidelines from MacDonald et al., (2000) showed that sediment from some littoral sites could have a toxic effect on the biota. Despite bio-accumulation of contaminants observed on the laboratory organisms exposed to these sediments and on the autochthonous organisms, no significant toxic effect was observed regarding survival, growth, reproduction, feeding rate endpoints. In addition, biomarker activities measured in the autochthonous benthic organisms did not significantly point out more stressful sites in particular.
Our results highlight the importance of studying the biological response of exposed organisms to assess an appropriate ecological risk of a complex mixture of pollutants. The next step will be devoted to the study of contaminants bioavailability and mobility under water level fluctuations, with the assumption that these changes lead to modifications of organisms’ exposure and contaminant bioaccumulation.