Abstract

The hyporheic zone at the interface between surface water and groundwater (Figure 1) plays an important ecological role for river ecosystem. The microbial communities in this zone mineralize organic matter contributing to nutrients recycling in the rivers. However, in a global change context, anthropic activities disturb the microbial communities and the hyporheic zone functioning. These physical (e.g. clogging, drying, warming) or chemical (e.g. pesticides, metals, pharmaceuticals) stressors are often multiple in rivers and their combined effects need to be studied to highlight possible interactions and prioritize management actions. In this context, the main objective of this thesis was to evaluate the effects of two frequent physical stressors in rivers (drying and clogging) on copper toxicity for microbial functions and structure in hyporheic sediment. To this end, two mesocosm experiments consisting of columns filled with sediments reproducing a hyporheic zone have been set up (Figure 2).

The first experiment combined clogging and copper-contaminated-water in order to evaluate the clogging effect on the copper distribution in the hyporheic zone and to understand the combined effects of these two stressors for the microbial communities. In the second experiment, copper-contaminated sediments were exposed to 4 weeks of drying and 4 weeks of rewetting. The aim of this experiment was to determine the copper effects on the resistance and resilience of microbial communities to drying. These experiments highlighted the significance of the hyporheic vertical dimension to understand the combined stressors effects. The exposure and effects of physical stressors (clogging and drying) on microbial communities were significant in the surface compared to deeper layers. In contrary, copper had variable effects throughout the column depending on the contaminated compartment (surface water or sediment). In combination, one dominant stressor influenced microbial responses even though antagonistic interactions were found between stressors. For example, surface bacterial diversity was lower when microbial communities were exposed to copper alone compared to copper combined with drying. These interactions can be explained by a decrease of the copper bioavailability and toxicity caused by physical stressors. Overall, the microbial communities in depth were preserved from the stressors effects, representing a potential "reservoir" of sensitive species. These results provide insight on the hyporheic functioning and help understand how the effects of physical and chemical stressors on sediment microbial communities interact with the vertical dimension. To go further, these results could be combined with modeling or complemented with field studies to better predict the effects of multiple stressors on rivers and help restoration actions.

References

• Côté, I. M., Darling, E. S., and Brown, C. J. (2016). Interactions among ecosystem stressors and their importance in conservation. Proc. R. Soc. B Biol. Sci. 283, 20152592. doi: 10.1098/rspb.2015.2592.
• Datry, T., Dole-Olivier, M. J., Marmonier, P., Claret, C., Perrin, J. F., Lafont, M., et al. (2008). La zone hyporhéique, une composante à ne pas négliger dans l’état des lieux et la restauration des cours d’eau. 17.
• Fischer, H., Kloep, F., Wilzcek, S., and Pusch, M. T. (2005). A River’s Liver – Microbial Processes within the Hyporheic Zone of a Large Lowland River. Biogeochemistry 76, 349–371. doi: 10.1007/s10533-005-6896-y.

Funding

INRAE ; Fondation Evertéa

Cite the thesis

Laura Kergoat. Réponses structurelles et fonctionnelles des communautés microbiennes à des stress combinés dans la zone hyporhéique. Biodiversité et Ecologie. Université Claude Bernard - Lyon I, 2023. Français. ⟨NNT : 2023LYO10098⟩. ⟨tel-04230065v2⟩

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