Tertiary treatment methods, like ozonation, are currently under discussion to improve removal efficiencies of micropollutants by municipal wastewater treatment plants (WWTPs). In order to assess the effects of a full-scale wastewater ozonation at WWTP Wüeri, Switzerland, on the receiving stream, a total of seven in situ bioassays with Gammarus fossarum that lasted 7-days were conducted during an overall period of 33 months. Caged gammarids were exposed between 150 m up- and 400 m downstream of a WWTP effluent before, during and following the operation of the full-scale wastewater ozonation. During the release of nonozone treated wastewater, gammarid feeding was significantly reduced by up to 90% 50 and 150 m downstream of the WWTP effluent. In contrast, during the operation period of the ozonation, no significant alterations in feeding were observed downstream. The results of these bioassays were incorporated into a mathematical simulation of the release of nonozone treated wastewater in a Central European region, suggesting a 40% reduction in leaf litter breakdown and hence in energy provision for the remaining aquatic food web downstream of WWTP effluents, while the release of ozone treated wastewater did not affect this important ecosystem function.