Impact of pesticides on hyphomycetes leaf processing and macroinvertebrate shredding activity

Pesticides are frequently applied in agricultural catchments and subsequently transported to stream recipients through e.g. tile drainage and surface runoff. This gives rise to short pulses of pesticide contamination in the stream, where lipophilic compounds rapidly adsorb to organic matter, consequently impacting stream dwelling organism feeding on the organic material.The impact of pyrethroid insecticides on stream macroinvertebrates are well studied and increased mortality and drift rate along with decreased feeding activity are well known responses to even very low concentrations (10-100 ng/L). However, the potential impact of pesticides on aquatic microbes is probably equally important. Microbial organisms are essential in organic matter breakdown, and their growth additionally increases the food quality of organic matter for macroinvertebrates. Consequently, pesticides impacting microbial organisms have the power to reduce organic matter breakdown and food quality for macroinvertebrates, thereby decreasing ecosystem decomposition rates. We exposed preconditioned leafs of beech (Fagus sylvatica) to the fungicide propiconazole (100, 1000 or 2000 μg/L) and/or the insecticide alpha-cypermethrine (100, 1000 or 2000 ng/L) for three hours. Subsequently, we studied post exposure leaf degradation for four weeks in the laboratory in the presence/absence of two macroinvertebrate shredders (Gammarus pulex and Halesus radiatus) applying a classic crossed factorial design. Preliminary results indicate decreasing microbial litter processing with increasing concentrations of either propiconazole or alpha-cypermethrine. Additionally, the binary mixture further reduced microbial litter processing compared to single compound exposures. A similar reduction in leaf litter processing was only evident during the first post exposure week when macroinvertebrate shredders were present. After four weeks no differences were apparent among treatments indicating a strong degree of functional redundancy applying this macroinvertebrate assemblage.