An assessment of the toxicity of pesticide mixtures in periphyton from agricultural streams to the mayfly Neocloeon triangulifer

Ontario’s streams are susceptible to pesticide pollutants which are transported outside of the intended area of application from surrounding agricultural fields. Periphyton are a vital dietary component for grazing organisms in aquatic ecosystems but can also accumulate substantial levels of pesticides from the surrounding water. Consequently, grazing aquatic organisms are likely subjected to pesticide exposure through the consumption of pesticide contaminated periphyton. The objectives of this study were to determine if pesticides partition into periphyton in riverine environments across Southern Ontario, and if so, to determine the toxicity of pesticides in periphyton when fed to the grazing mayfly Neocloeon triangulifer. Ten sites across Southern Ontario were sampled between May-September 2021. Sites with low, medium, and high pesticide exposure based on historic water quality monitoring data were selected to incorporate a pesticide exposure gradient into the study design. Artificial substrate samplers were utilized to colonize periphyton in situ, which was then analyzed for the presence of ~500 pesticides. Results from this study demonstrate that periphyton is capable of accumulating pesticides in agricultural streams. A novel, 7-day toxicity test method was created to investigate the effects of pesticides partitioned into periphyton when fed to N. triangulifer. Periphyton collected from the field sites were fed to N. triangulifer and survival and biomass production were recorded. Survival and biomass production significantly decreased when fed periphyton colonized in streams with catchments dominated by agricultural land use (p<0.05). However, the relationship observed between pesticide concentration and survival or biomass production was not consistent. Using field colonized periphyton allowed us to assess dietary toxicity of environmentally relevant concentrations of pesticide mixtures, however nutrition and taxonomic composition of the periphyton may vary between sites. We recommend future research to simulate individual pesticide exposures to periphyton in the laboratory prior to performing these novel dietary toxicity assessments.