Population‐level effects and recovery of aquatic invertebrates after multiple applications of an insecticide

Abstract

Standard risk assessment of plant protection products (PPP) combines "worst-case" exposure scenarios with effect thresholds using assessment (safety) factors to account for uncertainties. If needed, risks can be addressed applying more realistic conditions at higher tiers, which refine exposure and/or effect assessments using additional data. However, it is not possible to investigate the wide range of potential scenarios experimentally. In contrast, ecotoxicological mechanistic effect models do allow for addressing a multitude of scenarios. Furthermore, they may aid the interpretation of experiments such as mesocosm studies, allowing extrapolation to conditions not covered in experiments. Here, we explore how to use mechanistic effect models in the aquatic risk assessment of a model insecticide (Modelmethrin), applied several times per season but rapidly dissipating between applications. The case study focuses on potential effects on aquatic arthropods, the most sensitive group for this substance. The models provide information on the impact of a number of short exposure pulses on sensitive and/or vulnerable populations and, when impacted, assess recovery. The species to model were selected based on their sensitivity in laboratory and field (mesocosm) studies. The general unified threshold model for survival (GUTS) model, which describes the toxicokinetics and toxicodynamics of chemicals in individuals, was linked to 3 individual-based models (IBM), translating individual survival of sensitive organisms into population-level effects. The impact of pulsed insecticide exposures on populations were modeled using the spatially explicit IBM metapopulation model for assessing spatial and temporal effects of pesticides (MASTEP) for Gammarus pulex, the Chaoborus IBM for populations of Chaoborus crystallinus, and the "IdamP" model for populations of Daphnia magna. The different models were able to predict the potential effects of Modelmethrin applications to key arthropod species inhabiting different aquatic ecosystems; the most sensitive species were significantly impacted unless respective mitigation measures were implemented (buffer zones resulting in reduced exposure). As expected the impact was stronger in shallow ditches as compared to deeper pond scenarios. Furthermore, as expected, recovery depended on factors such as temperature (affecting population growth rate and number of generations) and the occurence of nonimpacted aquatic ecosystems (their frequency and connectivity). These model predictions were largely in line with field observations and/or the results of a mesocosm study, providing additional evidence on the suitability and reliability of the models for risk assessment purposes. Because of their flexibility, models may predict the likelihood of unacceptable effects-based on previously defined protection goals-for a range of insecticide exposure scenarios and freshwater habitats.

How to Evaluate the Quality of Toxicokinetic—Toxicodynamic Models in the Context of Environmental Risk Assessment

“…In turn, TKTD models are sometimes used as building blocks in population models (for GUTS, see, e.g., Dohmen et al 2016). The TK models used are also applied by themselves to analyze body-residue data over time.…”

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

“…Case study 5 (Dohmen et al this issue) explored risk of a rapidly dissipating insecticide (pyrethroid type), which was applied several times during the growing season, to 3 representative aquatic invertebrates. As exposure was pulsed, populations may recover after pulses, and therefore the Ecological Threshold as well as the Ecological Recovery Option (EFSA ) were addressed: effects on population densities should be negligible or should last no longer than days to weeks at the edge of fields and up to several days in protected areas and overall watersheds.…”

“…There was consensus during the workshop that MEMs can be used for both vertebrates and invertebrates to answer specific questions that have arisen after standard and higher‐tier studies. For example, they can be used to better describe exposure and effects of organisms (see case study 1, Ducrot et al this issue) or they can be used to extrapolate effects and recovery of vulnerable populations from a community‐level study (e.g., a mesocosm study) to other habitat types or landscapes (e.g., case studies 4 and 5, Dohmen et al this issue).…”

Impaired ecosystem process despite little effects on populations: modeling combined effects of warming and toxicants

“…In addition to organic chemicals, metal pollution, mostly as a result of mining, remains one of the biggest stressors in aquatic ecosystems (Byrne et al, 2011). These toxicants can act lethally by reducing survival of exposed organisms (Rubach et al, 2011;Galic et al, 2014;Dohmen et al, 2016), or sublethally, by impacting physiological processes and fitness-related traits, such as feeding (Zubrod et al, 2010;Agatz et al, 2013), growth ( Alvarez et al, 2006), and reproduction (Kl€ uttgen et al, 1996;Agatz et al, 2012). In turn, reduced fitness can compromise population resilience and ecosystem service delivery, but the extent to which this occurs differs based on different processes driving population dynamics and on environmental contexts, for example, temperature (Chapman, 2002;Calow & Forbes, 2003;Forbes et al, 2008).…”