Mesocosm effect concentrations

Microcosm and/or mesocosm effect concentrations (MEC) as calculated by means of logistic regression... 

Further, the validation of a model needs the definition of the criterion for establishing that a model has been validated. How well should a model predict effects precisely, and what are the bounds between which one calls a model (sufficiently) valid It also needs the definition of the context against which a model is to be considered valid. For example, validation of the SSD model has generally been based on whether the so-called hazardous concentration for 5% of the species (HC5) is a concentration that is conservative (sufficiently protective) compared to the no-effect concentration in multispecies mesocosm or field tests. In that sense, the model has performed well for both aquatic and terrestrial systems (e.g., Emans et al. 1993 Okkerman et al. 1993 Posthuma et al. 1998 Versteeg et al. 1999 van den Brink... 

Predicted no-effect concentrations (PNECs) are traditionally used in risk assessments and form the basis of most EQS values. There are 3 basic approaches to the derivation of PNECs and resultant EQS values. The traditional method uses standard toxicity data and applies an AF to the most sensitive endpoint to derive a protective concentration. The SSD approach utilizes all available toxicity data to derive a value that is protective of a given percentage (e.g., 95%) of the species and is increasingly being used by many countries, often with a small AF placed on, for example, a predicted HC5 (hazardous concentration to 5% of species, i.e., the 5th percentile of the SSD) based on chronic data. Finally, model ecosystems such as lentic mesocosms can be used to derive safe values, again usually with a small AF. 

Standards that are derived using SSDs for the soil ecosystem can in some cases be validated in the held. The overview by Posthuma et al. (2002) reported on some validation studies in which it was shown that the HC5 was lower than the no-effect concentration of studied ecosystems (i.e., in mesocosm or held conditions). An array of further studies has been published since that time. However, held studies are often difhcult to interpret in terms of dose-response relationships. This difficulty in interpreting held data is sometimes due to soil heterogeneity and a highly variable soil ecosystem. Nevertheless, held soils are relevant test systems and represent a more realistic environment. Although causality may be difhcult to assess, the use of pragmatic methods, derived from an expert judgment process, can improve the overall accuracy of standards. 

Considering evidence from both field and mesocosm studies, it may be concluded that certain groups of aquatic macroinvertebrates are sensitive to pyrethroids and that there can be changes, in the short term, at the population level and above with exposure to environmentally realistic concentrations of them. It should be possible to pick up effects of this kind in natural waters using ecological profiling, for example, the River Invertebrate Prediction and Classification System (RIVPACS). There is... 

The Community-Level Aquatic Systems Studies Interpretation Studies (CLASSIC) guidance document, which deals with the interpretation of results of microcosm and mesocosm tests in the risk assessment procedure of pesticides, recommends that regulatory model ecosystem experiments be conducted in spring to midsummer (Giddings et al. 2002). On the basis of the limited number of model ecosystem experiments described above, it seems that threshold concentrations for effects observed in early-season studies are reasonably predictive for threshold concentrations later in the season. Above these threshold concentrations, however, the intensity and duration of the responses (direct and indirect effects) may vary during different periods of the year. Consequently, the extrapolation of NOECcommunity values from one season to another seems to be possible with lower uncertainty than hazard estimates of higher concentrations in which both direct and indirect effects are involved. 

Recently, metapopulation models have been successfully applied to assess the risks of contaminants to aquatic populations. A metapopulation model to extrapolate responses of the aquatic isopod Asellus aquaticus as observed in insecticide-stressed mesocosms to assess its recovery potential in drainage ditches, streams, and ponds is provided by van den Brink et al. (2007). They estimated realistic pyrethroid concentrations in these different types of aquatic ecosystems by means of exposure models used in the European legislation procedure for pesticides. It appeared that the rate of recovery of Asellus in pyrethroid-stressed drainage ditches was faster in the field than in the isolated mesocosms. However, the rate of recovery in drainage ditches was calculated to be lower than that in streams and ponds (van den Brink et al. 2007). In another study, the effects of flounder foraging behavior and habitat preferences on exposure to polychlorinated biphenyls in sediments were assessed by Linkov et al. (2002) using a tractable individual-based metapopulation model. In this study, the use of a spatially and temporally explicit model reduced the estimate of risk by an order of magnitude as compared with a nonspatial model (Linkov et al. 2002). 

The median acute HC5 value is lower than the effect class 2 concentration observed in microcosm and mesocosm experiments treated once with a pesticide. The corresponding lower-limit HC5 value was, with high certainty, lower than reported effect class 1... 

Mesocosm no-effect data on a rapidly dissipating compound such as a pyrethroid insecticide may not be suitable for a chronic EQS applied to a river. Furthermore, most existing micro- and mesocosm studies are inappropriate for EQS derivation if fish are the most sensitive species because fish have generally been excluded from such tests. There is consequently a need for evidence-based decision making for interpretation of nonstandard mesocosm studies. Microcosm and mesocosm tests can, however, be used directly for EQS derivation if algae, macrophytes, and invertebrates are appropriately represented in the test systems and if they concern substances subject to transient exposure. They are then directly applicable for the derivation of M AC-EQSs. For this purpose, the NOEAEC can be used as it represents the highest initial concentration that causes no ecologically relevant effects. 

Microcosms and mesocosms can also directly be used to derive AA-EQSs if the test substance concentration was relatively constant over time (due to stability of the test item or due to experimental manipulation) and the study duration was long enough to detect possible long-term effects (usually at least 8 weeks, but this will depend on the life cycle of sensitive taxa). These test systems are more difficult to... 

If the chemical composition of the samples is known or at least partly known (in a stepwise TIE approach) or existing data allow for QSAR calculation, the samples can be ranked by TUs. Arts et al. (2006) studied, in 12 outdoor ditch mesocosms, the effects of sequential contamination with 5 pesticides in a regression design. They applied dosages equivalent with 0.2%, 1%, and 5% of the predicted environmental concentration (PEC) subsequently over 17 weeks. Endpoints recorded over 30 weeks included community composition of macroinvertebrates, plankton, and macrophytes, and leaf litter decomposition as functional ecosystem parameters. TUs were calculated in relation to acute toxicity data for the most sensitive standard species Daphnia magna and Lemna minor. Principal response curves (PRCs), a special form of constrained PCA, and Williams test (NOEC, class 2 LOEC) were used to identify the most sensitive taxa. Next to direct effects on certain species, also indirect effects, for example, how the change in abundance of a sensitive species affects the abundance of another, more tolerant species, can be detected only in mesocosm or in situ experiments. All observed effects were summarized in effect classes in a descriptive manner. 

Test the hypotheses listed in Step 8. Hypothesis can be tested using a variety of field, mesocosm, or laboratory test methods. In an ideal situation it should be possible to make predictions based upon known concentrations and then sample that field site in order to confirm effect or no-effect. It may be necessary to rework the risk assessment in order to reduce uncertainty, or a stressor-habitat-effect linkage may be incorrect. Testing the risk predictions allows feedback into the assessment process improving future predictions. 

Engel, A. et al., 2005. Testing the direct effect of CO2 concentration on abloom of the coccolithophorid Emiliania huxleyi in mesocosm experiments. Limnology and Oceanography, 50, 493-507. 

---