Climate change alongside existing chemical pollution and physical habitat disturbances affects inland rivers through a combination of multiple stressors. Microalgal communities of freshwater ecosystems have been found to respond to them, based on scarce field observations. However, natural rivers are not suitable for full-scale experimental trials, thus, the use of mesocosms can be used to understand how climate and anthropogenic stressors affect microalgae within the whole trophic network context. In this thesis I have straddled both scales, looking into four main subjects:

1) the effect of a salinity gradient and flow variation on the microphytobenthic community of an ExStream mesocosm within the urban Boye catchment,

2) the responses of benthic diatom communities (using digital microscopy and DNA metabarcoding) alongside photosynthesis-related traits in microphytobenthic communities in response to stressors increase and release (e.g. flow velocity, salinity, and temperature) in a large-scale ExStream mesocosm,

3) the responses of microalgae communities and a photosynthetic biomass proxy to multiple environmental stressors in the less impacted Kinzig River catchment and

4) how the restoration affects the ecosystems in general and the diatoms in particular in the urban Boye catchment.

In the mesocosm studies, results presented here illustrate how sensitive microalgae react to even minor changes in these environmental variables, particularly flow velocity and temperature. In the less degraded Kinzig catchment, results highlighted the complex relationship between various environmental variables and microalgae biomass and community composition. In contrast the freshly restored catchment of the Boye, characterized by natural to recently restored rivers, is urban and as such affected by anthropogenic pressures, showing increased degrees of degradation both in the water and the microphytobenthic community. Furthermore, this study showed no significant differences in composition according to time since restoration, which could relate to the quick generation time of the microphytobenthic community. In conclusion, the degree of degradation of the catchment will determine the responses of microalgae of the microphytobenthos to multiple stressors through all scales of study.