Effects of Pesticide Mixtures and Environmental Factors on Benthic Diatom Communities: A Microcosm Approach

Protecting aquatic ecosystems from pesticide pollution is a critical regulatory task. Benthic diatoms, being part of aquatic biofilms, are commonly used to indicate nutrient pollution. However, their potential as bioindicators for pesticides and other co-occurring environmental stressors remains poorly assessed. To address this, we conducted a flow-through chamber experiment designed to reflect realistic field exposure scenarios. We exposed biofilms to 44 treatments combining environmentally relevant pesticide mixtures including herbicides, fungicides, insecticides (44 levels of measured Toxic Units (TU_mix) ranging from 2 × 10^-5 to 4), in combination with three environmental stressors (light intensity, temperature, nutrient levels) over a 25-day period. Upon termination, we examined the diatom community composition using Next-Generation Sequencing along with community-level biofilm descriptors (β-diversity, effective photosynthetic activity, chlorophyll-a content, ash-free dry weight, bacterial abundance). While we did not find any significant responses to pesticides at the community level, we identified species-specific patterns of sensitivity or tolerance to pesticides and other experimental factors, thereby identifying them as potential bioindicators. These candidate species represent a first step toward a diatom-based index for pesticide stress assessment in Swiss streams. These findings highlight the need for realistic, multifactorial experiments to assess the impact of stressors in aquatic ecosystems.