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.

Erika Juliana Pérez Manosalva (University of Duisburg-Essen)

Impacts of Multiple Stressors on Fungal Communities Associated with Leaf Decomposition: Structural Dynamics, Acclimatisation, and Trophic Implications

The functioning of freshwater ecosystems, particularly headwater streams, depends on the decomposition of organic matter such as leaf litter, which underpins nutrient cycling and aquatic food webs. Fungi are key microbial decomposers, initiating together with bacteria leaf decomposition and provide a high-quality food source for macroinvertebrates. However, environmental stressors can alter fungal community composition and impair their ecological function, with potential modulation by prior stress exposure and seasonal variation.

Within Phase II of RESIST, I will investigate the acclimative responses and recovery potential of aquatic fungal communities to multiple stressors, and the consequent effects on leaf decomposition. The project examines how prior exposure history influences fungal acclimatization processes and functional plasticity. To this end, fungal communities from several sites of the Boye catchment, differing in stress exposure history, will be subjected to gradients of temperature and flow (to simulate drought). Functional (e.g., decomposition, respiration, exo-enzyme activity) and structural (e.g., fungal community composition via DNA metabarcoding) endpoints will be assessed across multiple colonisation periods. In the final period, acclimatised communities be exposed to conditions mimicking their site of origin, enabling a recovery period.

Additionally, together with projects A08 and A23, we will explore the potential influence of fungal community exposure history and parasitic infection status on the food selection from gammarids, which exhibit selective feeding behaviour. To evaluate the effect of the exposure history, gammarids collected from the same field sites as previously studied will be given to select between leaf material colonised by fungal communities from different exposure histories. And, to evaluate the effect of the parasitic infection status, the same colonized leaf material will be also offered to gammarids from laboratory cultures that exhibit distinct infection profiles. The feeding rates will be related with leaf material properties such as fungal community composition (metabarcoding), fatty acid profiles (together with A25), exo-enzyme activity, ergosterol content (as a proxy for fungal biomass), and concentrations of selected organic contaminants to identify determinants of food preference.

Contact: erika.perezmanosalva@uni-due.de

First Supervisor: Dr. Verena Schreiner (University of Duisburg Essen Ecotoxicology & Research Centre One Health Ruhr)

Second Supervisor: Prof. Dr. Dr. h.c. Mark Gessner (Leibniz Institute of Freshwater Ecology & Inland Fisheries Plankton and Microbial Ecology)

Helena Soraya Bayat (University of Duisburg-Essen)

The role of individual tolerance in community assembly during degradation and recovery

Multiple stressor research aims to be able to predict the effects of multiple stressors as one of its primary goals and applications. Precise and accurate predictions of combined stressor effects can ultimately inform risk assessments and efforts to protect sensitive ecosystems most efficiently. Testing stressor combinations on relevant species can quickly spiral to incredibly time, work, and money intensive experimental designs, worsening with increasing ecological scale. This project makes use of readily available data, from single stressors on single taxa, to test if community effects of single and multiple stressors can be predicted. Community effects will also be compared across a range of contexts, notably during stressor impact and in the recovery phase as well as across geographical regions. The focus lies on three organism groups- algae, macroinvertebrates, and fish- and three stressors- salinity, temperature, and hydromorphological change.

The project tests three central hypotheses. First, that individual tolerances vary more between species in macroorganisms than microorganisms and that tolerance to salinity and temperature stressors increases with organismal body size. Second, that community change in response to a single stressor, and to a lesser extent multiple stressors, can be predicted from individual tolerance of component taxa; in the case of single stressors individual tolerances can predict the size of the response, and in the case of multiple stressors they can predict the direction of the response. Third, taxonomic and functional composition will be more predictable during stressor impact than recovery, with the lowest diversity during the phase of stressor action. The third hypothesis addresses the Asymmetric Response Concept (ARC). To test these hypotheses, a meta-analytic approach will be used to compile data from mesocosm studies on salinity, temperature, and hydromorphological stressors as well as tolerances for individual taxa from databases. Results of mesocosm experiments from around the world will be compared to the results of experiments from projects A03 and A08 within RESIST to put them in context. The results will clarify to what extent readily available tolerance data for single stressors can be used to predict multiple stressor effects on communities.

Contact: helena.bayat@uni-due.de

First Supervisor: Dr. Ralf Schäfer (University of Koblenz-Landau, Quantitative Landscape Ecology)
Second Supervisor: Prof. Dr. Sonja Jähnig (Leibniz Institute of Freshwater Ecology and Inland Fisheries (IGB), Ecosystem Research)
Mentor: Dr. Nadine Gerner (Emschergenossenschaft und Lippeverband)