Investigation of Proliferative Kidney Disease in Brown Trout and Habitat Characteristics Associated with a Swiss Wastewater Treatment Plant

Wastewater treatment plants (WWTP) play a critical role in mitigating adverse environmental impacts of urban and industrial wastewater by removing pollutants and reducing the risk of contamination. Discharges of treated effluents from WWTPs can still have significant effects on freshwater ecosystems, particularly on sensitive species like brown trout. We analyzed the effects of a modern WWTP on a freshwater ecosystem, studying fish biodiversity and biomass, the occurrence of the parasitic disease Proliferative Kidney Disease (PKD) on brown trout, river water temperatures throughout the year and fish habitat and diversity. No major differences between up- and downstream of the WWTP were observed in habitat structure and attractiveness, fish biomass and species diversity, nor in PKD prevalence or intermediate host presence. However, immediately at the WWTP’s effluent and continuing downstream, the water temperature rose by almost 1 °C. While WWTPs are crucial for reducing the environmental impact of urban and industrial wastewater, their effluents can still have important consequences for freshwater ecosystems. Brown trout are particularly susceptible to increases in water temperature, especially regarding PKD severity and consequent mortality. To ensure the long-term health and sustainability of freshwater ecosystems, it is imperative to improve measures to minimize water temperature rises and mitigate downstream cascade effects on sensitive organisms like brown trout.