A critical examination of the protection level for primary producers in the first tier of the aquatic risk assessment for plant protection products

Background: The aim of environmental risk assessment (ERA) for pesticides is to protect ecosystems by ensuring that specific protection goals (SPGs) are met. The ERA follows a prospective tiered approach, starting with the most conservative and simple step in risk assessment using the lowest available endpoint. For aquatic primary producers, however, the recommendation to use the lowest endpoint (referring to 50% inhibition in: “biomass” –area under the curve- E b C 50 , yield E y C 50 or growth rate E r C 50 ) and a default assessment factor (AF) of 10 has been changed in 2015, by switching to the sole use of the growth rate inhibition endpoint (E r C 50 ). This study examines the implications of this change on the level of conservatism provided by the tier 1 risk assessment and evaluates whether it ensures a suitable minimum protection level. Results: Our analysis shows that replacing the lowest endpoint with the growth rate inhibition endpoint while maintaining the AF of 10 significantly reduces the conservatism level of the tier 1 risk assessment. Comparing protection levels achieved with different endpoints reveals that the current assessment is less protective than the previously agreed assessment. To ensure a similar level of protection, we recommend to increase the AF to a minimum of 24 in the tier 1 risk assessment based on E r C 50 . Independently of the endpoint selected for the tier 1 risk assessment, several issues in the general risk assessment of pesticides for aquatic primary producers contribute to uncertainties when assessing the protection levels, e.g. a lack of appropriate comparison of the surrogate reference tier with field conditions, the suitability of E r C 50 for certain macrophytes species, and the regulatory framework's failure to consider realistic conditions in agricultural landscapes with multiple stressors and pesticide mixtures. Conclusions: We advise to consider adjusting the risk assessment in order to reach at least the previously agreed protection level for aquatic primary producers. In view of multiple uncertainties pointing at a higher risk in the field as assumed in the ERA, continuing using an endpoint with a higher value and without adjustment of the assessment factor jeopardize the aim of halting biodiversity loss in surface waters.