Effects of the fungicide pyrimethanil on biofilm and organic matter processing in outdoor lentic mesocosms

The effect of the fungicide pyrimethanil (0.7 mg L-1) on biofilm development and alder leaf litter decomposition in aquatic ecosystems was assessed in outdoor lentic mesocosms immediately and 274 days after pyrimethanil application. Pyrimethanil decreased ergosterol concentrations (an indicator of fungal biomass) and the abundance and richness of the macroinvertebrate community associated with decomposing leaves. However, because neither fungi nor macroinvertebrates were main factors contributing to decomposition in this particular system, organic matter processing rates were not affected. After 274 days, pyrimethanil concentration in the water column was <= 0.004 mg L-1 but richness, biomass and composition of the invertebrate community associated with decomposing leaf-litter still showed the effect. The comparison of ergosterol (a molecule existing on both algae and fungal cell membranes), with chlorophyll (an indicator of algal biomass) associated with biofilm suggests that pyrimethanil may decrease fungal biomass and alter the relative abundance of algae and fungi on biofilm developing in control-and treated-mesocosms.