Effect of thiram and of a hydrocarbon mixture on freshwater macroinvertebrate communities in outdoor stream and pond mesocosms: II. Biological and ecological trait responses and leaf litter breakdown.

Higher-tier ecological risk assessment of chemicals often relies upon studies in dynamic and/or static mesocosms. Physico-chemical and hydrological properties of each type of mesocosm result in specific chemicals fate, community functioning, and potential recovery. In the present study, macroinvertebrate abundance- and biomass-weighted biological and ecological trait matrices were used to assess the effects of a dithiocarbamate fungicide, thiram (35 and 170 µg l(-1)), and of a petroleum middle distillate (0.01, 0.4, 2 and 20 mg l(-1)) in outdoor stream and pond mesocosms. Trait sensitivity was characterized using functional diversity indices and trait modality distributions to assess the influence of the type of experimental systems and the ability of traits to disentangle chemical-induced effects from temporal and stochastic variations. In addition, leaf litter breakdown was used as an integrative functional endpoint. Regardless to the substance, treatments had a direct effect on the functional structure of benthic macroinvertebrate communities in streams but not in ponds, suggesting that global functional responses to chemicals are system-specific. Although both substances had an effect in streams, differences were noticed in the nature of the affected traits suggesting that chemical mode of action plays a role in functional alterations. This was illustrated by the link between negative effects of chemical exposure on detritivorous taxa and reduced litter breakdown rate in streams. Therefore, characterisation of macroinvertebrate biological traits associated with the measurement of a functional process such as litter breakdown may provide a comprehensive understanding of the effects occurring in mesocosms exposed to organic chemicals.