The prevalence and acceptability of mesocosm studies submitted for macrophytes in pesticide risk assessment

Mesocosms can be used in higher tier aquatic risk assessments to assess the impact of plant protection products (PPPs) on macrophytes. However, it is unclear whether these expensive and time-consuming higher tier studies influence regulatory outcomes. This review highlights common shortcomings in the experimental design of mesocosm studies, with the aim of maximizing the regulatory value of future mesocosm studies. Fourteen mesocosm studies, which have been submitted for the regulatory risk assessments for macrophytes in the EU or Great Britain, were identified and reviewed. Results show that only five of the 14 mesocosm studies were deemed acceptable by the regulatory authorities, suggesting that mesocosm studies are not currently being used to their full potential. Issues with the submitted studies include not following a realistic PPP exposure profile (including incorrect dose timings and dilutions), only using one macrophyte morphology, not leaving enough time for the macrophytes to establish, and a lack of replicates, which increases variability within treatments. Glyceria maxima and Myriophyllum spicatum were frequently the most sensitive macrophyte species, whereas dry weight was often the most sensitive and least variable endpoint. Although mesocosms provide the opportunity for recovery and community responses to be observed, such information has not been used by regulatory authorities. Future regulatory mesocosm studies can build on the shortcomings highlighted here, providing a greater chance of regulatory impact.