Parasites, their hosts, and multiple stressors exhibit a complex web of interrelations. Parasites can modulate their hosts’ responses to various stressors while concurrently being subjected to direct effects of these stressors. This dynamic interaction can alter the host’s reaction to both parasitic infections and additional stressors. Although these relationships are increasingly documented and comprehended, the impact of the host-parasite-stressor nexus on ecosystem recovery, particularly in case of riverine food webs, remains unclear. Project A09 seeks to deeply understand host-parasite interactions in rivers facing multiple stressors and to test the ARC hypothesis, by examining how parasite infections relate to the host’s tolerance, dispersal, and biotic interactions. In Phase I, we characterised the parasite fauna in our target catchments, the Boye and the Kinzig, using molecular and morphological methods. We discovered a high diversity of microsporidian species in amphipods, identifying several new host-parasite associations, and described a potentially novel parasitic ciliate from amphipods in the Boye. Our study of freshwater snails as intermediate hosts for trematodes revealed 23 species in the Boye, while trematode diversity was low in the Kinzig due to the limited abundance of host snails. We did not find a direct correlation between recovery status (time since restoration) and microsporidian diversity and prevalence. Additionally, our Phase I data indicate that some parasites affect the dispersal of their amphipod hosts, suggesting that parasite infections impose significant energy and health costs on their hosts. Building on these findings, Phase II will focus on the tolerance of parasitised and unparasitised macroinvertebrate hosts to salinity and temperature in laboratory experiments (in collaboration with A23), using mortality, mobility, metabolic parameters, biomarker expression, and behaviour as endpoints. To address the effect of parasites on dispersal, we will compare the dispersal capabilities of infected and uninfected species in collaboration with A16, including terrestrial adult stages of aquatic insects by assessing dispersal distance and energy storage in relation to parasitism. Additionally, we plan to study parasite recolonisation patterns following severe degradation in the field flume system (recovery from droughts) and in the recently restored main stem of the Emscher (recovery from severe degradation). In the field flumes, we will monitor parasite recolonisation trajectories in macroinvertebrates, while in the Emscher, we will focus on fish parasites in collaboration with A25 and A19. By elucidating the effects of parasites on host tolerance and dispersal, we aim to clarify the role of parasitism in the asymmetric response of host populations during recovery phases (e.g., dispersal differences between parasitised and non-parasitised individuals). Simultaneously, we will investigate the recolonisation patterns of parasite communities, hypothesising that the establishment of parasite life cycles lags behind the alleviation of stressors. The initial absence of host-parasite interactions during early phases of recovery can lead to particularly severe health impacts on hosts upon delayed parasite infection.

Sebastian Prati (University of Duisburg-Essen)

Effects of stream degradation and recovery on metazoan parasite communities: a multiple stressor approach

Many studies underline the pivotal role of parasites in ecological processes, supporting the theory that parasite diversity is closely linked to ecosystem functions and integrity. However, it has been poorly studied how parasite diversity is directly or indirectly altered by increasing and releasing multiple stressors and how this translates into overall community composition and ecological functioning.

The aim of this project is to investigate the effects of stream degradation and restoration on metazoan parasite communities and subsequent effects on free-living organisms. Therefore, fish, snails, and benthic invertebrates will be sampled from 40 field sites of the Emscher/Boye and Kinzig catchments in non-degraded, degraded, and restored sites and screened for parasites. Additionally, samples of macroinvertebrates collected from near-natural and restored sites in the Boye system over the last ten years will be analyzed. Parasitological data will be compared between sites to provide insights into the overall community structure and potential temporal trends. The responses of parasitized and unparasitized macroinvertebrate hosts will be experimentally induced using the ExStream system.

Contact: sebastian.prati@uni-due.de

First Supervisor: Prof. Dr. Bernd Sures (Universtiy of Duisburg-Essen, Aquatic Ecology)
Second Supervisor: PD. Dr. Christian K. Feld (Universtiy of Duisburg-Essen, Aquatic Ecology)
Mentor: Dr. Sebastian Emde (Landesamt für Natur, Umwelt und Verbraucherschutz Nordrhein-Westfalen (LANUV))