How TK‐TD and population models for aquatic macrophytes could support the risk assessment for plant protection products

Abstract

This case study of the Society of Environmental Toxicology and Chemistry (SETAC) workshop MODELINK demonstrates the potential use of mechanistic effects models for macrophytes to extrapolate from effects of a plant protection product observed in laboratory tests to effects resulting from dynamic exposure on macrophyte populations in edge-of-field water bodies. A standard European Union (EU) risk assessment for an example herbicide based on macrophyte laboratory tests indicated risks for several exposure scenarios. Three of these scenarios are further analyzed using effect models for 2 aquatic macrophytes, the free-floating standard test species Lemna sp., and the sediment-rooted submerged additional standard test species Myriophyllum spicatum. Both models include a toxicokinetic (TK) part, describing uptake and elimination of the toxicant, a toxicodynamic (TD) part, describing the internal concentration-response function for growth inhibition, and a description of biomass growth as a function of environmental factors to allow simulating seasonal dynamics. The TK-TD models are calibrated and tested using laboratory tests, whereas the growth models were assumed to be fit for purpose based on comparisons of predictions with typical growth patterns observed in the field. For the risk assessment, biomass dynamics are predicted for the control situation and for several exposure levels. Based on specific protection goals for macrophytes, preliminary example decision criteria are suggested for evaluating the model outputs. The models refined the risk indicated by lower tier testing for 2 exposure scenarios, while confirming the risk associated for the third. Uncertainties related to the experimental and the modeling approaches and their application in the risk assessment are discussed. Based on this case study and the assumption that the models prove suitable for risk assessment once fully evaluated, we recommend that 1) ecological scenarios be developed that are also linked to the exposure scenarios, and 2) quantitative protection goals be set to facilitate the interpretation of model results for risk assessment.

Scientific Opinion on the state of the art of Toxicokinetic/Toxicodynamic (TKTD) effect models for regulatory risk assessment of pesticides for aquatic organisms

“…(, ) which is further extended in Hommen et al. (). The two macrophyte species are the standard test species representing an aquatic (floating) monocot ( Lemna spp.)…”

“…As for Lemna, Myriophyllum reproduces vegetatively; but in addition, it also flowers and produces seeds once or twice a year and allocates energy to rhizomes (Best and Boyd, ). In the extended model version (Hommen et al., ), the rhizomes are included but it is not explicit which principles determine allocation to rhizomes. Allocation of energy to flowers and seeds has not been incorporated (Hommen et al., ), as it is very minor (Best and Boyd, ).…”

“…In the extended model version (Hommen et al., ), the rhizomes are included but it is not explicit which principles determine allocation to rhizomes. Allocation of energy to flowers and seeds has not been incorporated (Hommen et al., ), as it is very minor (Best and Boyd, ). The typical ‘die‐off’ after sexual reproduction described in Best and Boyd () is, however, incorporated in Hommen et al.…”

How to use mechanistic effect models in environmental risk assessment of pesticides: Case studies and recommendations from the SETAC workshop MODELINK

“…Case study 6 (Hommen et al this issue) addressed the risk of an herbicide spray application on aquatic macrophytes in edge‐of‐field water bodies. Specific protection goals for aquatic macrophytes refer to abundance or biomass of populations and are differentiated for the ecological threshold option (i.e., effects should be negligible) and the ecological recovery option (i.e., small to medium effects of short duration, e.g., weeks to a few months) (EFSA ).…”

“…Thus, when the ecological entity (e.g., a specific population) and its attribute for an assessment have been agreed on, decision criteria have to be defined on magnitude, duration, and spatial scale of acceptable effects. Examples of such quantitative criteria are given for reduction of honeybee colony sizes (EFSA ) or in Hommen et al (this issue) for reduction of aquatic macrophyte biomass. The definition of such quantitative protection goals for the most representative ecosystem service providers has to be done by groups comprising different experts and stakeholders.…”