Assessing the Impact of Combined Sewer Overflow on the Habitat of Lampetra planeri: A Case Study in Flanders, Belgium

anglais. Lampetra planeri or Brook Lamprey is an aquatic fish-like species, characterized by its lack of jaws and long eel-like morphology. The species is usually found in small streams with gravel or sandy riverbeds and has seen a strong population decline during the last few decades. The primary goal of this thesis was to investigate if combined sewer overflows prevent the further spread of the Lampetra planeri in Flanders through habitat deterioration. The secondary goal was the development of a GIS methodology which could be used for similar ecological issues in the future. Eight watercourses in Belgium were selected for the analysis. Segments from each watercourse were enriched with a modelled water pollution score and the presence or absence of Lampetra planeri, sourced from existing fish survey data. The pollution score was calculated based on the presence and location of combined sewer overflows in the watercourse while taking into account the effects of dilution over distance and river flow. A Chi-square test of independence and Fisher's exact test were performed and showed a statistically significant relationship between the presence of combined sewer overflows and the presence of Lampetra planeri. However, several considerations must be addressed when interpreting these findings. Variations in the composition of combined sewer overflow discharges as well as the frequency of overflow events were not taken into account. Additional factors that could potentially influence the presence/absence of Lampetra planeri, such as the occurrence of migration barriers, morphological river quality and landuse on the riverbanks, were not included in the analysis. However, the methodology has a modular structure and allows for other factors to be included in future research.

anglais. Combined Sewer Overflows (CSOs) are components of a sewer system, designed to prevent flooding by releasing untreated wastewater into the environment during heavy rainfall. However, these discharges contain pollutants that may harm aquatic life, including species such as the eel-like Brook Lamprey. During the last few decades, Brook Lamprey populations have declined significantly. While water quality has already been improved, a number of factors are likely to still restrict the species’ habitat expansion. This study explores whether CSOs have a negative effect on the habitat of Brook Lamprey in Flanders, Belgium. Eight watercourses were selected for analysis. At multiple points along these watercourses, two variables were determined. The first variable contains the habitat situation: the presence or absence of Brook Lamprey, based on historical fish survey data. The second variable contains an estimation of the pollution originating from upstream CSOs, based on their number and distance. A statistical analysis was conducted to explore the relationship between these two variables. The results indicated that a relationship exists between the presence of CSOs and the presence of Brook Lamprey. In other words: the proximity of an upstream CSO reduces the likelihood of Brook Lamprey presence. Although the study relies on a simplified representation of real-word conditions, the results provide valuable insights needed to allow for continuous improvements to the water quality and biodiversity of our rivers. Additionally, the study developed a method which can easily be adapted and used to investigate similar habitat-environment relationships.