Sara Isabel Bastos Gonçalves

• Expansion of agricultural land-use and the associated application of agrochemicals can have deleterious effects on local freshwater microbial communities, with consequences for the entire ecosystem. Leaf litter decomposition is a key process in stream ecosystems being partially driven by microbial communities (particularly fungi and bacteria). Leaf-associated microbial communities are responsible for making the nutrients and energy bound in leaves available for higher trophic levels of heterotrophic food webs. Moreover, these microbial communities increase the nutritional quality of leaf litter for shredders, as they produce proteins and lipids while degrading the indigestible components of leaf litter. When exposed to anthropogenic pressures, such as the increased exposure to nutrients and fungicides associated with agricultural land-use, the structure and function of these microbial communities can be affected. In addition, the leaf species on which these microbial communities grow may act as a supplementary filter for the community structure and response to stressors. These factors and their interaction may jointly modify leaves’ nutritional quality for higher trophic level, potentially affecting activities such as shredders’ feeding and development. Despite the importance of leaf litter decomposition, little is known about the underlying mechanisms or processes driving the changes in function and structure (mainly in the aquatic hyphomycetes [AH] community) of leaf-associated microbial communities. Moreover, fungicide effects on leaf litter decomposition were investigated almost exclusively with black alder leaves due to their favourable traits to consumers (i.e., low recalcitrance and high nutrient content). Simultaneously, little is known about fungicide effects on microbial colonisation and decomposition of other leaf species, with less favourable traits or potential unknown consequences for the wider food web. The aims of this thesis are therefore to assess individually: - the effects of fungicide exposure on leaf-associated microbial communities colonising different leaf species; - the effects of combined fungicide and nutrient exposure on microbial communities with different exposure history; - the potential effects on shredders’ development resulting from feeding on different leaf species colonised by communities with different exposure history. These aims were assessed through a set of complex laboratory bioassays taking into account the environmental relevance of the tested stressors and communities. Overall, we show that microbial communities colonising leaves with less favourable traits (i.e., higher recalcitrance and lower nutrient levels such as European beech) potentially may suffer increased fungicide effects, affecting their function (i.e., leaf litter decomposition). While leaf species with more favourable traits such as black alder, enabled leaf-associated microorganisms to acquire leaf-bound energy and more easily resist potential effects induced by fungicide exposure. Moreover, our results also point towards the need to expand our mechanistic understanding on how different leaf species interact with the effects of chemical stressors on the function and structure of microbial communities. The latter is not only important due to the expected changes of leaf species input into streams but also because those can potentially translate into different food quality for shredder organisms. Secondly, leaf litter decomposition did not differ between fungicide treatments or exposure histories. While increasing levels of nutrients tended to buffer for the non-significant fungicide-induced effects on leaf decomposition. However, fungal community composition substantially changed at environmentally relevant fungicide concentrations. For example, in most communities tolerant AH species of the genus Tetracladium, known by its superior leaf decomposition efficiency, dominated at high fungicide exposure independent of exposure history. Since the changes in the fungal community composition seem decoupled from its function, our results are therefore supporting the principle of species dominance. This principle elaborates that highly efficient decomposers are responsible for maintaining leaf litter decomposition despite changes in the community structure. However, changes in the community structure can potentially affect other functions provided by fungi, such as increasing the nutritional quality of leaves for shredders. Finally, we also show that leaf species identity has a more substantial impact on gammarids’ development relative to the exposure history of the microbial community colonising the leaves. Moreover, the sex-specific feeding responses of gammarids raise questions on earlier procedures, demanding further research.