Freshwater ecosystems are increasingly impacted by the pervasive spread of agrochemicals, particularly herbicides. These can negatively impact macroinvertebrates, many of which are also infected by parasites. Parasites and pollutants may interact, leading to unpredictable impacts on affected organisms and consequently shape ecological dynamics in freshwater ecosystems. Therefore, understanding how these stressors interact in real-world settings is essential for environmentally realistic risk assessment and effective management practices. This thesis addresses these challenges by investigating the combined effects of the chloroacetamide herbicide metazachlor and natural parasitic infections in keystone freshwater invertebrates, bridging the gap between controlled laboratory studies and complex natural environments.

The first chapter investigates the ecotoxicological effects of the herbicide metazachlor (MTZ) on the amphipod Gammarus fossarum, considering the influence of natural parasitic infections. Acute tests revealed moderate toxicity of MTZ, with slightly reduced susceptibility in individuals infected by the acanthocephalan Polymorphus minutus. At environmentally relevant, low concentrations, MTZ significantly altered biochemical markers such as glutathione S-transferase, phenoloxidase, and glycogen, while locomotor activity remained unaffected by the herbicide but was significantly increased in P. minutus-infected hosts. Microsporidian infections had no significant effect on either behavior or physiology at the studied concentrations. These findings demonstrate that ignoring natural parasite infections in ecotoxicological tests can mask or distort contaminant effects, emphasizing the need to incorporate biotic stressors into environmental risk assessment frameworks.

The second chapter builds on earlier findings by assessing how severe, high-concentration MTZ exposures interact with parasite infections in G. fossarum. Results showed that while toxicity increased at high concentrations, amphipods infected with P. minutus consistently demonstrated partial protection against MTZ-induced mortality, while uninfected and co-infected individuals (with microsporidians and P. minutus) revealed higher mortality. However, a significant interaction was observed between the parasites and MTZ in terms of biochemical responses under acute concentrations of MTZ. This chapter reveals that parasite-mediated modifications of host physiology and stress tolerance are concentration-dependent and shaped by the complexity of co-infection.

The third chapter explores the effects of MTZ on larvae of the mayfly Ephemera danica, naturally infected with microsporidians, focusing on survival and sublethal biochemical responses during the sensitive pre-emergence stage. The data show that even at environmentally realistic concentrations, MTZ caused increased larval mortality, and altered biochemical responses. Notably, year-to-year fluctuations in natural microsporidian infection prevalence and hydrological conditions influenced both parasite dynamics and organismal sensitivity, underscoring the importance of environmental variability in mediating joint stressor effects.

Together, these studies demonstrate that biotic interactions, environmental variability, and organismal traits profoundly modulate the consequences of herbicide exposure in aquatic ecosystems. The thesis highlights the need for ecotoxicological risk assessments to move beyond oversimplified models towards approaches that integrate natural infection status, multi-stressor exposures, and environmental realism. This integrative perspective is essential to safeguarding freshwater biodiversity and function in the face of escalating chemical and ecological pressures.