Bioaccumulation and effects of a common pesticide mix found in agricultural runoffs on growth, survival, and reproduction of great pond snails ( Lymnaea stagnalis )

Mixtures of pesticides from agricultural runoff can affect non-targeted freshwater organisms and pose ecological risks even at low concentrations. Yet, ecotoxicity is rarely assessed using pesticide mixtures found at environmental concentrations under natural fluctuating habitat conditions. This study aimed at assessing the chronic toxicity of a pesticide mixture on the model species great pond snails (Lymnaea stagnalis) over a 40-day period using semi-natural outdoor mesocosms. The pesticide mixture was prepared to reflect environmental levels of agrochemicals commonly found in freshwater streams surrounding corn/soybean farming of southern Québec, Canada. It included seven pesticides (herbicides: atrazine, S-metolachlor, glyphosate; insecticides: clothianidin, thiamethoxam, imidacloprid, and chlorantraniliprole). These pesticides were tested at environmental concentrations, and at two and ten-fold reductions from these water concentrations to observe potential benefits from reduction in pesticide use. There was no effect of the pesticide mixture on overall snail survival. However, the 0.5× and 1× pesticide mixture increased average snail body mass by disproportionately decreased the proportion of smaller snails. Also, pesticide accumulation was highest in biofilm for atrazine (0.0014-0.0256 nmol/g wet wt), then s-metolachlor (0.0018-0.0126 nmol/g), glyphosate's metabolite aminomethylphosphonic acid (AMPA; 0.0002-0.0038 nmol/g) chlorantraniliprole (0.007 nmol/g), and lastly glyphosate (0.0006 nmol/g). Bioaccumulation was highest in snails for S-metolachlor (0.0185-0.0467 nmol/g wet wt), then atrazine (0.0049-0.0246 nmol/g), AMPA (0.0002-0.0074 nmol/g), chlorantraniliprole (0.0005-0.0025 nmol/g), glyphosate (0.0010-0.0021 nmol/g), imidacloprid (0.0003-0.0005 nmol/g), and lastly clothianidin (0.0001 nmol/g). We found that concentrations of pesticides in snails were determined by an interaction between biofilm pesticide concentrations and water pesticide concentrations. Our results highlight that pesticide mixtures under semi-natural conditions can produce complex interacting effects on non-target organism's body mass and bioaccumulation of chemicals that could be considered when establishing pesticide-usage policies.