A common fungicide impairs stream ecosystem functioning through effects on aquatic hyphomycetes and detritivorous caddisflies

Fungicides can reach streams through runoff or adhered to leaf litter, and have the potential to adversely affect processes such as litter decomposition and associated communities. This study investigated the effects of chlorothalonil, a widely used fungicide, on litter decomposition, detritivorous invertebrates (larvae of the insect Sericostoma pyrenaicum) and aquatic hyphomycetes (AHs), using stream microcosms. We considered the single and combined effects of two exposure modes: waterborne fungicide (at two concentrations: 0.125 mu g L-1 and 1.25 mu g L-1) and litter previously sprayed with the fungicide (i.e., pre-treated litter, using the application dose concentration of 1250 mu g L-1). We also assessed whether fungicide effects on invertebrates, AHs and decomposition varied among litter types (i.e., different plant species), and whether plant diversity mitigated any of those effects. Invertebrate survival and AH sporulation rate and taxon richness were strongly reduced by most combinations of fungicide exposure modes; however, invertebrates were not affected by the low waterborne concentration, whereas AHs suffered the highest reduction at this concentration. Total decomposition was slowed down by both exposure modes, and microbial decomposition was reduced by litter pre-treatment, while the waterborne fungicide had different effects depending on plant species. In general, with the exception of microbial decomposition, responses varied little among litter types. Moreover, and contrary to our expectation, plant diversity did not modulate the fungicide effects. Our results highlight the severity of fungicide inputs to streams through effects on invertebrate and microbial communities and ecosystem functioning, even in streams with well-preserved, diverse riparian vegetation.We thank Richard Pearson and two anonymous reviewers for their comments on the manuscript. This study was derived from AC's PhD thesis and AA's MSc thesis. AC was supported by a fellowship of the National Secretariat of Science, Technology and Innovation (SENACYT) and the National Research System of Panama (SNI). SM was supported by a postdoctoral grant from the University of the Basque Country. Additional funding was obtained from the Spanish Ministry for Science, Innovation and Universities and FEDER (project BioLoss, RTI2018095023-B-I00) and Basque Government funds (IT951-16)