Offre thèse : Beneficial effects of treatment wetlands for combined sewer overflow on water quality and macroinvertebrate communities

The aim of the thesis will be to study the effect of CSO on surface water quality and biodiversity, especially on benthic macroinvertebrates, and the mitigation effects of CSO-TWs. Benthic macroinvertebrates are sedentary and long-living organisms that play an essential role in the aquatic food chain by decomposing organic matter and providing an abundant food resource for other organisms. They do not migrate much, which keeps them exposed to pollutants present in their environment as well as to natural variations of environmental conditions (Hellmann et al. 2015). They are thus considered very good passive biomonitoring indicators for assessing water quality (Gabriels et al. 2010).

The thesis will contain both laboratory experiment and field investigations. The improvement of water quality and benthic macroinvertebrate health will be investigated for four sites (3 in Flanders, Belgium and 1 near Lyon, France). The installation of the CSO-TWs in Flanders is planned for 2026/2027, the one near Lyon is already operational. Field prospecting will be carried out before (initial state) and after installation of the CSO-TWs, at different time stages, for post-operational monitoring. The area of interest will be characterized by morphological and physico-chemical measurements, followed by sediment, water and macro-invertebrate collections, for each prospecting session. From the water and sediment samples, microplastics and their co-pollutants (e.g. heavy metals and PFAS) will be quantified and identified in order to assess the beneficial impact of planted filters on reducing the release of these types of pollutants. The collection of macro-invertebrates will initially enable a faunal inventory to be carried out to study the improvement in the site's specific richness following the installation of the wastewater treatment system. Secondly, macroinvertebrates representative of different taxa, with different diets that influence the intensity of exposure to pollutants, will be kept alive for morphological (size and body mass) and physiological measurements (enzymatic activities and oxidative stress) in the laboratory.

Laboratory results will complement those obtained in the field and provide a mechanistic understanding of potentially beneficial impacts of CSO-TW on gammarids. They will be performed on gammarids, in control conditions, placed in aquaria with a water pump system simulating river flow and exposed to CSO or effluent of a CSO-TW. Life-history-traits will be measured (growth, body mass, mortality), as well as some physiological parameters (e.g. metabolism, mitochondrial bioenergetics, oxidative stress, enzymatic activities). Mitochondria play an important role supporting energy supply in organism (Rolfe and Brown, 1997). However, their functioning leads inevitably to reactive oxygen species production (ROS), highly reactive molecules that may interact with cellular components such as lipids, proteins and DNA, causing irreversible damage (Finkel and Holbrook 2000). Alteration of their functioning could thus influence adaptive capacities of organisms in response to environmental stressors. Consequently, mitochondria are now considered as one of the main cellular components involved in individual performances and life-history-traits of organisms (Salin et al. 2019; Hood et al. 2018; Heine and Hood 2020). It becomes thus essential to focus on the mitochondrial functioning for understanding the effects of environmental stressors on benthic macroinvertebrates. In addition, behavior, influenced by internal and external stimuli, is highly involved in the interaction between the organism and its environment. Behavioral biomarkers are therefore considered early in response to several environmental stresses, including contaminants (Hellou 2011). Consequently, in addition of physiological measurements, the behavior will be studied.