Chemical stressors influence aquatic ecosystem processes

Leaf litter decomposition is a fundamental ecosystem process for the energy provisioning in streams, mainly mediated by microbial leaf decomposers and leaf-shredding, detritivorous macroinvertebrates. Both decomposers and detritivores are under chemical stress from pesticides entering surface waters. Amongst these, fungicides may pose a particular risk, as they can negatively affect aquatic microbial decomposers but also detritivores via both waterborne exposure and by influencing the quality of their food. The overall objective of my thesis was to broaden the knowledge of fungicide effects on organisms and processes mediating leaf litter decomposition in streams as well as the interactions between decomposers and detritivores. \nFungicides affected microbial decomposers by altering fungal biomass and community composition, and by changing the microbial fatty acid profile. These structural effects subsequently resulted in effects on microbial leaf litter decomposition. However, the strength of functional responses was dependent on the exposure history of microorganisms to chemical stressors, with previously exposed organisms showing less negative or even positive responses to fungicide exposure. Such a functional adaptation of microbial decomposers to chemical stress was congruently observed on a larger biogeographical scale within Europe, despite distinct structural responses at the individual study sites. Moreover, fungicides caused indirect effects on detritivores by reducing the palatability of leaf material and affecting the food choice of detritivores. Structural alterations on the microbial level led to a reduced food quality of leaf litter. Feeding on leaf litter of lower quality ultimately affected detritivores’ food processing (consumption and excretion) and resulted in lower lipid content and growth. Similar effects, although more pronounced, were observed for detritivores directly exposed through water. Nevertheless, neither effect pathway should be ignored given their additive action. \nRisks for fungicide effects at the base of the aquatic food web under field conditions can be expected, since effects on decomposers and detritivores were observed at field-relevant fungicide concentrations during this thesis. These findings in combination with the predicted higher fungicide use in the future due to agricultural intensification are reasons for concern, given the central roles of decomposers and detritivores in aquatic ecosystem functioning.