It is well known that parasites are integral parts of every ecosystem and play important roles in ecosystem functioning by e.g. regulating host population dynamics, alter predator - prey interactions, change food web dynamics and energy flow. With a few exceptions this was mainly inferred from studies of metazoan parasites. Nonetheless, parasitic protists are extremely abundant infecting a multitude of organisms. Apart from their apparent biodiversity their impact on their hosts is not yet understood for all species. It gets even more complicated when stressors are considered. How will communities of parasitic protists change and how does the degradation and/or recovery change the parasitic protist - host relationship in riverine systems? Looking at this from a different angle, understanding how parasitic protists change their hosts’ response to multiple stressors during phases of degradation and recovery will help us to understand their role in the ARC and their impact on host tolerance, dispersal, and biotic interactions. It is also recognised that the gut microbiome is important for the overall health of an organism and its ability to cope with any stressors it is exposed to. The question arising here is if and how parasitic protists, or other parasites, or stressors themselves alter the gut microbiome of their hosts and how that might affect their stress response? We hypothesize that macroinvertebrates are more severely affected by stressors when infected with parasitic protists and that their stress response will deviate from the one of uninfected individuals. We also expect that multiple stressors will influence the community composition and abundance of parasitic protists as well as their host’s microbiome. In Phase II of RESIST project A23 aims to understand potential differences in stress responses of macroinvertebrate hosts due to infections with parasitic protists, as well as differences in parasitic protist communities under multiple stressors (including drought) that will have an impact on the ARC in the following recovery phases. We will do this on different scales from small-scale lab (beaker) to large-scale field flume experiments to address stress response of single species and community response under controlled conditions at different levels. Additional field samples will help to set baselines for different stressor scenarios, as well as providing an insight into how parasitic protist communities change under different stressor exposures. Project A23 will address Main Hypothesis 1 (degradation and recovery) and Main Hypothesis 2 (micro- and macro-organisms) by i) investigating the host’s response to stressors and stressor combinations (salinity and temperature), when infected and uninfected with parasitic protists and ii) identifying differences between parasitic protist communities from differently stressed/recovering environments including a comparison between environmental and macroinvertebrate samples.