Periphyton as an integrative indicator of pesticide bioaccumulation, nutrient pollution and fatty acid alterations in agricultural streams

Agricultural activities are a major source of pesticides and nutrients in freshwater ecosystems. However, little is known about the bioaccumulation and toxic impacts of pesticide mixtures on periphyton, an often-overlooked community of benthic microalgae. This study investigates pesticide bioaccumulation in periphyton and its links to nutrient enrichment and shifts in fatty acid profiles, highlighting its role as an integrative indicator in agricultural streams. Periphyton colonized on artificial substrates in three watercourses in southern Sweden was sampled and analyzed over a three-month summer period, representing sites with varying pesticide contamination. Periphyton bioaccumulated up to 30 pesticides, 17 of which were also present in surface water, with distinct profiles between matrices. Bioaccumulation was persistent over time and showed site-specific patterns. Eighteen pesticides exceeded the REACH threshold for very bioaccumulative substances (BCF > 5000). Diflufenican was identified as a high-risk compound, characterized by a high BCF and with persistent, bioaccumulative and toxic properties. Elevated nutrients coincided with greater algal abundance, notably diatoms, and higher levels of essential fatty acids, including eicosapentaenoic acid and docosahexaenoic acid, suggesting nutrient enrichment strongly shapes periphyton and may obscure subtle pesticide effects. Assessing pesticide accumulation in periphyton demonstrates its value as a passive sampler providing complementary insight into chemical exposure and ecological status beyond surface-water monitoring.