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Exploration of bacterial communities and search for bacterial indicators of river quality in two contrasting watersheds: the Meuse and the Zenne

par Barberoux, Valentin
Président du jury Gypens, Nathalie
Promoteur George, Isabelle
Publication Non publié, 2025-05-19

Thèse de doctorat

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Résumé :

Riverine ecosystems are essential for biodiversity and play a fundamental role in sustaining human activities. Yet, they face growing threats from anthropogenic activities, including agricultural runoff and wastewater discharges. These stressors contribute to elevated levels of fecal indicator bacteria (FIBs), which serve as key markers of fecal contamination and microbiological water quality. This thesis investigates the impact of human activities on E. coli and other FIB contamination, while also examining bacterial community composition (BCC) to identify additional indicators of the physico-chemical quality of riverine water. The study focuses on two contrasting European watersheds: the Meuse river, with mixed land use, and the Zenne river, a heavily urbanized watershed.The Meuse river watershed exhibited high E. coli concentrations in agricultural and urban areas. In these headwaters, E. coli peaks were most pronounced in agricultural area and during summer, driven by increased microbial activity and reduced dilution. Conversely, the main river axis saw peaks in spring, likely due to the resuspension of sediments containing E. coli during higher discharge events. Urban areas also showed peaks in both seasons, attributed to untreated domestic and industrial wastewater releases.BCC analysis in the Meuse revealed taxa such as Flavobacterium and Limnohabitans, correlated positively with dissolved oxygen and negatively with nutrients and were therefore considered as indicators of good water physico-chemical quality. In contrast, Microcystis dominated in areas of poor water physico-chemical quality near the river mouth, during summer. Additionally, agricultural streams exhibited higher levels of bacterial indicators of poor river physico-chemical quality than forest streams, such as Aurantimicrobium and Pseudarcobacter.The Zenne river, heavily influenced by urbanization, showed poor water microbiological quality upstream of Brussels, likely due to agricultural runoff and untreated wastewater, presenting high levels of FIB, including Enterobacteriaceae and Enterococcaceae. Although the wastewater treatment plants (WWTPs) discharge within the Brussels section of the river caused a slight change in the microbial diversity of the Zenne downstream in spring, this effect was brief and minimal. Surprisingly, the WWTP outlets showed lower levels of FIB than the river itself, indicating relatively cleaner effluent.In conclusion, this study highlights the significant role of land use and wastewater infrastructure in determining FIB contamination in river ecosystems. It also identifies additional bacterial indicators of river physico-chemical status that could be used to assess river physico-chemical quality. Finally, the findings emphasize the need to develop effective management strategies to reduce contamination and safeguard public health.