Structural and biological trait responses of diatom assemblages to organic chemicals in outdoor flow-through mesocosms

• Diatom community traits and structure respond to a fungicide and to hydrocarbons.
• Diatom ecological guilds were sensitive to the exposure to both types of chemicals.
• Exposure to hydrocarbons resulted in a shift of diatom community taxonomic composition.
• Bioindication indices were poorly sensitive to the exposure to organic chemicals.
• Abundance of abnormal diatom forms did not significantly increase in the exposed communities.

The sensitivity of diatom taxonomy and trait-based endpoints to chemicals has been poorly used so far in Environmental Risk Assessment. In this study, diatom assemblages in outdoor flow-through mesocosms were exposed to thiram (35 and 170 μg/L), and a hydrocarbon emulsion (HE; 0.01, 0.4, 2 and 20 mg/L). The effects of exposure were assessed for 12 weeks, including 9 weeks post-treatment, using taxonomic structure and diversity, bioindication indices, biological traits, functional diversity indices, indicator classes and ecological guilds. For both chemicals, diversity increased after the treatment period, and responses of ecological traits were roughly identical with an abundance increase of motile taxa tolerant to organic pollution and decrease of low profile taxa. Bioindication indices were not affected. Traits provided a complementary approach to biomass measurements and taxonomic descriptors, leading to a more comprehensive overview of ecological changes due to organic chemicals, including short- and long-term effects on biofilm structure and functioning.