Impairment and recovery of stream ecosystems experiencing multiple stressors: Responses of leaf- associated fungi, microbial activities and litter decomposition
The proposed project addresses impacts of three common stressors and their combinations on litter- colonising decomposers and consequences for the decomposition of leaf litter (Coarse Particulate Organic Matter; CPOM) in streams. Efforts will focus on responses of fungi and litter decomposition in experiments conducted in the indoor AquaFlow and the outdoor ExStream mesocosm systems, complemented by field decomposition experiments and a survey of aquatic fungi in two catchments with different legacies. We predict moderate temperature increases to stimulate fungal activity, accelerate decomposition, and shift fungal community structure. Salinization is expected to affect fungal community structure, but to stimulate activity and decomposition only at low ion concentrations, resulting in a hump-shaped stress response. These relationships are predicted to result in varying outcomes in multiple-stressor scenarios, depending on the exact stressor intensity, indicating that reliable predictions require quantitative assessments. Indirect mechanisms are predicted to dominate impacts of hydromorphological changes (except during complete drought and high flow).
Consequences for litter decomposition and associated processes (i.e. functions) are likely to be less pronounced than for fungal community structure assessed in the experiments and field survey, due to buffering effects in diverse communities. Experiments in the two mesocosm systems will be designed to assess single and combined effects of the above stressors, as well as responses to subsequent stressor decrease, with the AquaFlow experiments being guided by the pollution-induced community tolerance (PICT) concept adopted from ecotoxicology. Responses will be assessed after stressor increase and release, the latter to determine recovery of community structure and functions.
Response variables determined in the experiments include fungal biomass and production, sporulation and community composition in addition to litter mass loss, microbial respiration and selected enzyme activities, notably that of pectinases. Short-term experiments in the AquaFlow system will focus on microbial respiration and fungal sporulation, which informs about general fungal activity (total number of spores produced per unit time) and the fitness of individual species (absolute and relative number of spores produced by individual fungal species). This species-specific information will be obtained by microscopic analyses of the characteristic spores of litter-degrading fungi in streams (aquatic hyphomycetes) and compared to species identifications by amplicon sequencing conducted in A04. Sporulation will also be used in combination with stable isotope probing and amplicon sequencing (collaborations with A02 and A04) to identify active fungi.