Pesticides are broadly applied in current agriculture practices globally and may end up in interconnected water bodies i.e. ditches, ponds, and lakes via numerous routes such as spray drift, runoff and leaching. Given the fact that they are inherently designed to harm biota, pesticides may pose risks to a range of aquatic organisms. Non-target organisms may be exposed to fluctuating concentrations with successive pulses of pesticide contaminants. In general, pesticide risks are often assessed by performing laboratory experiments and/or semi-field experiments evaluating a continuous and single application, respectively, which does not necessarily correspond to the exposure pattern of realistic applications (e.g. time-varying exposure). This mismatch is one of the main challenges in contemporary ecological risk assessment. Evaluation of the potential adverse effects of multiple pulsed pesticide exposure on non-target aquatic organisms is therefore considered to be of importance and should also become a part of the standard European registration procedure.
This thesis aims to compare the effects of different time-variable exposure regimes, having the same Time Weighted Average (TWA) but different peak concentrations, of a pesticide on aquatic species and communities (Chapter 1). For the risk assessment of pesticides, an imperative question is addressed about which type of concentration, the TWA or the peak, is more appropriate to assess the longer-term risks of pesticides. In addition, this thesis also uses empirical approaches to establish rules-of-thumb to extrapolate from one type of exposure pattern to the other.
Chapter 2addresses the issue whether peak or TWA21d concentrations should be used in the aquatic risk assessment of insecticides when the predicted or measured exposure is variable in time. Therefore, in this chapter I aimed to compare the effects as observed in cosm experiments on the basis of the peak concentration of their exposure profile as well as their TWA21d concentration using three sensitive endpoints, i.e. microcrustaceans, macrocrustaceans and insects. This comparison was performed for individual insecticides and also for groups of chemicals sharing the same toxicological mode-of-action. To achieve this aim, a review of the empirical PERPEST database was performed, which contains classified effects of insecticides on various endpoints as observed in freshwater model ecosystems that evaluate the effects of pesticides. The PERPEST (Predicting the Ecological Risks of PESTicides in freshwater ecosystems) model uses this database to predict the effects of a particular concentration of a pesticide on various community endpoints. Since the PERPEST data base only contains the peak concentrations of the exposure profiles evaluated in the cosm experiments, all cosm studies were re-reviewed in order to obtain the TWA21d. In order to facilitate a comparison across insecticides, the exposure concentrations were expressed as toxic units (TU). On the basis of these TUs, threshold values were assumed to be equivalent for compounds with a similar mode-of-action. TUs were calculated by dividing the concentrations evaluated in the cosm study by the Hazardous Concentration 50% (HC50), which was calculated as the geometric mean of all acute toxicity values of the insecticide for aquatic arthropods. For acetylcholinesterase inhibiting insecticides, we found that when comparing peak and TWA21d concentrations,direct effects became apparent at TWA21d concentrations that were a factor of 5 lower than their respective peak exposure concentrations.We therefore recommend an extrapolation factor of 5 to extrapolate safe peak concentrations to safe TWA concentrations, especially when the threshold value is based on a study evaluating a single application of an acetylcholinesterase inhibiting compound.For acetylcholinesterase inhibiting insecticides, TWA21d concentrations can be used as good predictors for long-term effects on sensitive endpoint groups in the risk assessment process, sincesomewhat clearer dose-response relationships were obtained for all endpoints in case of TWA21d exposures when compared to peak exposures. For pyrethroids, no clear dose-response relationship was found, neither when the comparison was scaled on peak concentrations, nor when scaled on TWA21d exposures. For moulting inhibiting insecticides, the peak and TWA21d concentrations may have equal importance in order to standardise the effects.
In Chapter 3 I compared the effects of different time-variable exposure regimes having the same TWA concentration but different peak concentrations of the organophosphate insecticide chlorpyrifos on freshwater invertebrate communities. The experiment was performed in outdoor microcosms by introducing three different regimes: a single application of 0.9 µg a.i./L; three applications of 0.3 µg a.i./L with a time interval of 7 d; and continuous exposure to 0.1 µg a.i./L for 21 d. Our results indicated that the application of chlorpyrifos resulted in decreased abundances of species belonging to the arthropod community, with the largest adverse effects reported for the mayfly Cloeon dipterum and cladocerans Daphnia gr. longispina and Alona sp., while smaller effects were observed for other insects, copepods, and amphipods. At the population-level, most species showed the same effect magnitude at the end of the experimental period, indicating that the TWA concentration of chlorpyrifos is predictive for its long-term effects on arthropod species. The mayfly C. dipterum, however, only responded to the single-application treatment, which could be explained by the toxicokinetics of chlorpyrifos in this species. Intrinsic sensitivity is a product of the processes of toxicokinetics (TK: uptake, biotransformation and elimination of the compound) and toxicodynamics (TD: internal damage, individual recovery and threshold) of a compound (Chapter 4). Therefore, differences in field responses of species to time-variable exposure profiles may relate to differences in the TKTD of chlorpyrifos in these species. At the end of the experimental period the invertebrate community showed approximately the same effect magnitude forall time-variable exposure regimes of treatments.These results suggest that for this combination of concentrations and duration of the TWA, the TWA concentration is more important for most species than the peak concentration for the assessment of long-term risks of chlorpyrifos.These results support the recommendations of the ELINK workshops, which suggest that for long-term effects the TWA concentration may be more relevant than the peak concentration.
In order to assess the effects of time-varying pulses of pesticides, the development of models that can describe the toxicokinetic (TK) and toxicodynamic (TD) of a chemical in individuals of a species is of major importance. This is because non-target organisms may be exposed to fluctuating concentrations or sequential pulses of pesticides in the environment. Furthermore, recovery of individuals after being exposed to pesticides, will occur as part of the TD processes, but is not routinely taken into account in risk assessment. The Threshold Damage Model (TDM) is a process-based model for predicting the acute effects of pulsed pesticide exposure on the survival of aquatic invertebrates and consists of a TK part in which uptake and elimination are described, and of a TD part accounting for processes such as damage, individual recovery, and internal thresholds.
Chapter 4presents data from a series of laboratory experiments with the model substance chlorpyrifos, which were used to parameterize the TD part of the TDM model for four different species. The experiment quantified mobility and survival of the four freshwater species Chaoborus obscuripes, Cloeon dipterum, Plea minutissima andDaphnia magna after two subsequent 24 h pulses of chlorpyrifos with an intermediate time interval that either allowed for the elimination of the compound and potential individual recovery between successive pulses or not. The killing rate constant, recovery rate constant, and the threshold for damage were estimated by fitting the TDM to the experimentally observed survival data using estimates for the TK parameters for the same species from the literature. The species C. obscuripes andD. magna showed an immediate decrease in mobility and a delayed effect in survival whereas C. dipterum and P. minutissima responded immediately to the exposure with both endpoints. C. obscuripes was the only species showing no individual recovery. In general, the effect of the pulses was smaller if the intervals between pulses allowed for elimination and potential recovery. The experimental data were successfully fitted by the TDM model, however, not all parameters were estimated equally robustly. This expresses the need for further data collection and development of TKTD models for different species and compounds. Improved TKTD models could be combined with individual-based models to provide more accurate and detailed model predictions of direct effects of pesticides on immobility and mortality and how these direct effects propagate to population recovery in order to link the different levels of biological organisation.
Chapter 5presents a study which aimed at evaluating the effects of different time-varying exposure patterns of the strobilurin fungicide azoxystrobin on freshwater microsocosm communities. These exposure patterns included two treatments with a similar peak but different TWA concentrations, and two treatments with similar TWA but different peak concentrations. The experiment was carried out in outdoor microcosms under four different exposure regimes; (1) a continuous application of 10 µg/L (CAT10) for 42 days, (2) a continuous application of 33 µg/L (CAT33) for 42 days, (3) a single application of 33 µg/L (SAT33), and (4) a treatment with four applications with a time interval of 10 days of 16 µg/L (FAT16). Multivariate analyses demonstrated significant changes in zooplankton community structure in all but the CAT10 treated microcosms relative to that of controls. The largest adverse effects were reported for zooplankton taxa belonging to Copepoda and Cladocera. By the end of the experimental period (day 42 after treatment), community effects were of similar magnitude for the pulsed treatment regimes, although the magnitude of the initial effect was larger in the SAT33 treatment. This indicates that for long-term effects the TWA is more important for most zooplankton species in the test system than the peak concentration. Azoxystrobin only slightly affected some species of the macroinvertebrate, phytoplankton and macrophyte assemblages. The overall No Observed Ecologically Adverse Effect Concentrations (NOEAEC) in this study was 10 µg/L.
Chapter 6discusses the findings of this thesis. I aim to compare the effects of time-variable exposure regimes as observed in the cosm experiments described in this thesis as well as in the reviewed cosm studies published in the open literature in terms of peak and TWA concentrations on aquatic communities and ecosystems and draw conclusions from all the results presented in this thesis.