Leaf decomposition is an important ecosystem function and provides crucial energy for food webs, especially in headwater streams. Fungal communities dominate microbial decomposition and make leaf material palatable for shredding macroinvertebrates contributing to macroinvertebrate decomposition. Multiple stressors are known to impact fungal communities functionally (i.e., their leaf decomposition ability) as well as structurally. How the communities are impacted, however, can depend on their past stressors exposure (i.e., exposure history) and season. Project A21 aims to investigate the responses of fungal communities to multiple stressors and gain insides into underlying mechanisms. For this, in Phase II of RESIST, studies with different complexity will be conducted ranging from lab-based microcosms over outdoor mesocosms to field surveys, covering varying levels of realism and controlled conditions. Fungal communities with different exposure histories will be exposed to multiple stressors namely temperature, salinisation and factors related to drought (i.e., oxygen and flow) over several cycles of resource colonisation (WP1). After investigating the degradation caused by prolonged stressor exposure, fungal communities will be exposed to conditions similar to their origin stream assessing potentials for recovery processes. Since exposure to stressors can alter the quality of food sources, we will conduct food-choice experiments in collaboration with A08 and A23 (WP2). There gammarids with different exposure histories or infection status can select between leaf material colonised by fungal communities with varying exposure histories. In the central outdoor mesocosm studies, ExStream and field flumes, A21 will investigate the effects of temperature increase, drought and morphological degradation on microbial as well as macroinvertebrate decomposition (WP3) providing information on the functionality of stream food webs during degradation and recovery phases to all involved projects. To identify stressors which are shaping fungal communities, A21 will participate in the coordinated field study of RESIST across three catchments (WP4) and relate environmental variable gradients to structural and functional changes in fungal communities. By combining the results of the different work packages, the project aims to shed light on the underlying mechanisms of fungal community composition and identify fungal taxa tolerant and sensitive to the respective stressors. This will be an important step for reliable prediction of stress responses across whole stream food webs.