Single and mixed effects of an herbicide and fungicide on green and brown fluvial food chains

Leaf litter and algae are the most important basal resources for invertebrate primary consumers in freshwater ecosystems. These basal source-consumer links can be affected by chemical pollutants, such as pesticides, that are widely used in agricultural activities. Pesticides are usually present in complex mixtures which potentially modifies the toxicological effect of the single compounds. In this study, we focused on the effects of two pesticides: terbuthylazine (1 µg/L), an herbicide that inhibits photosynthesis, and tebuconazole (13 µg/L), a fungicide inhibitor of ergosterol biosynthesis in fungi. Our aim was to evaluate the effects of these two pesticides on stream biofilms (photosynthetic activity and chlorophyll-a) and leaf litter (mass loss, fungal biomass, C:N content) as well as diet-related effects on the consumption and growth rates of the snail Physella acuta (grazer feeding strategy) and the crustacean Echinogammarus sp. (shredder). We conducted a 14-day experiment using artificial channels where biofilm, leaf-litter and invertebrates were exposed to control conditions and to both pesticides, individually and in combination. The pesticides did not affect leaf litter measures or leaf litter processing by the shredder, either separately or in combination. However, both pesticides and their mixture reduced the chlorophyll-a concentration in the biofilm when grazers were present. Indirect, but likely also direct, effects of the fungicide on grazers reduced their respiration rate and increased their growth rate. These effects indicate that low concentrations of pesticides can have unexpected effects when considering top-down interactions, mainly on biofilms under the effects of grazing. Our pesticide concentrations were below those detected in some European surface waters, so it cannot be ruled out that more negative responses would exist if higher concentrations were maintained over time.