Mechanisms and thresholds of fungicide effects on microbial communities and organic matter processing

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In the first project phase, we examined effects of fungicides on organic matter processing in streams. We found that the fungicide toxicity gradient in the field study was associated with changes in microbial communities and a decline in microbial organic matter processing. In the second project phase, we will scrutinise effect mechanisms of fungicides on microbial communities and the potential propagation to organic matter processing. In addition, we will reconcile the mismatch regarding threshold levels between field observations and the few studies on fungicides under controlled conditions (e.g. microcosms, mesocosms). In the first work package, we scrutinise the effect mechanisms of fungicides on microbial leaf processing. The experiment follows a complex test design that considers crucial elements of the field situation, namely repeated exposure, feeding pressure by shredders and multiple cycles of colonisation-decomposition. The experiment will be run in 6 places worldwide using typical invertebrate shredders and leafs (consortium of laboratories from Australia, Canada, Germany, Denmark, Costa Rica and Sweden) and will employ an exposure pattern with mixtures derived from a global analysis of fungicide concentrations in surface waters. In addition, the experiment will feature several turnovers of organic material in terms of leaf material, which means that potential effects can cumulate. The latter has been demonstrated in a preliminary experiment. The measured endpoints include the leaf processing rate, invertebrate growth and microbial community composition and biomass. We will derive effect thresholds for fungicides and hypothesise that the effects cumulate over time and that this explains differences in effect thresholds between laboratory and field studies.The second work package aims to establish a stronger link between fungicide toxicity and potential changes of microbial communities and organic matter processing. It features a field study with each of 6 streams with high and low fungicide toxicity, where one pristine upstream site at the edge of the forest and one downstream site in the viticultural area will be selected (i.e. a total of 24 sites). The field survey will span the pre-fungicide application and the following fungicide application period and includes the comparison of microbial communities and microbial processing between upstream and downstream sites. Leaf bags colonised in pristine upstream reaches and in downstream reaches in the viticultural area will be employed to identify (1) differences in the community composition and organic matter processing and its relation to the fungicide application period and fungicide toxicity and (2) the implications of the pollution state for microbial colonisation. Overall, this follow-up project will provide mechanistic explanation and further evidence of causality of fungicide toxicity with ecological endpoints.