Dynamic stressor regimes drive shifts in biofilm-associated parasites.

Parasites have gained increased recognition for their ecological relevance in riverine systems, which are highly vulnerable to biodiversity losses and degradation resulting from climate change and anthropogenic pressures. In recent decades, restoration efforts have been launched in many degraded watercourses to mitigate the influence of stressors. Yet, the ecological consequences of multiple stressor exposure and release remain complex, with the return to near-natural biotic communities representing only one possible trajectory. Parasites are still poorly understood in this context. To address this gap, we used the streamside mesocosm system ExStream to investigate the effects of multiple potential stressors and their release on biofilm-associated micro-eukaryotic parasites at a recently restored site of the Boye River. The Boye River system, a tributary of the Emscher River located in the Ruhr metropolis in western Germany, suffered from long-term hydromorphological degradation and pollution in the past until its restoration. In the ExStream experiment, we used a full-factorial design to manipulate flow velocity, salinity, and temperature during a stressor phase. The system entered a recovery phase afterwards, where the treatments were set to control conditions in all recovering mesocosms. Assessing metabarcoding data from biofilms that grew on the mesocosm walls revealed shifts in parasite assemblages between stressor and recovery phases, and reduced flow velocity was associated with altered community compositions and diversity decline. Our findings highlight that biofilm-associated parasite communities can provide important insights into the ecological dynamics of degraded and recovering river systems, and demonstrate that stressor legacies may persist beyond the period of exposure.