Dietary exposure of stormwater contaminants in biofilm to two freshwater macroinvertebrates

Aquatic habitats in urban environments are exposed to complex contaminant mixtures that may harm aquatic biota. The impact of contaminant transfer from contaminated biofilm through aquatic food webs is still understudied, as is the current state of knowledge on dietary exposure of urban contaminants to biota residing in stormwater ponds. Our overall objective was to characterize urban pesticide accumulation in a common aquatic food source (biofilm) in stormwater ponds and to investigate the potential toxicity of that food source by testing the responses of two freshwater macroinvertebrates to experimental exposure. We conducted two dietary bioassays using biofilm collected from 15 stormwater ponds in Brampton, Ontario: an acute exposure with the mayfly Neocloeon triangulifer, and a chronic exposure with the freshwater snail Planorbella pilsbryi. We screened for 542 current-use and legacy pesticides to measure pesticide burden (the number of pesticides detected) and the concentration of pesticides in the biofilm. We also quantified chlorophyll-a, pheophytin, and ash-free dry weight content which we used as indicators of biofilm quality. We found no correlations between pesticide burden and chlorophyll-a, pheophytin, or ash-free dry weight of the biofilm diets. Compared to control diets, biofilms collected from stormwater ponds caused a reduction in survival and growth endpoints for both test species, indicating that biofilm-consuming invertebrates living in stormwater ponds may be experiencing risks previously unaccounted for by traditional ecological risk assessments. Pesticide occurrences in biofilm diets did not relate to mayfly survival, growth, or biomass production. Nor were they related to snail growth. This suggests that other contaminants in stormwater-cultivated biofilm are contributing to the observed effects. Snail survival and biomass production were negatively related to pesticide burden in the diets. This implies that duration of exposure may influence the degree and manifestation of pesticide toxicity via dietary exposure.