TR 073 – The Value of Aquatic Model Ecosystem Studies in Ecotoxicology

TR 073 : The Value of Aquatic Model Ecosystem Studies in Ecotoxicology | December 1997

The process of risk assessment of substances aims at safeguarding the integrity of complex environments and ecosystems.  In this context, a No-Effect Concentration for environmental organisms needs to be predicted (=PNEC) on the basis of a limited amount of ecological and ecotoxicological data available.  Most of the substance-specific data have been generated on single species under laboratory conditions, and empirically-derived assessment factors are currently used for the extrapolation to the real environment.  It is the purpose of this report to explore in detail the value of aquatic model ecosystem studies in predicting the effect of substances in the "real world" ecosystem.

The relevant scientific literature was thoroughly screened and the various types of studies found were described separately for the three broad groups of studies, i.e. static freshwater, flowing freshwater and marine systems. Large differences exist among the reported studies concerning the test conditions chosen, particularly location, duration, size and complexity.  The Task Force concluded that it seems inappropriate at this stage, to recommend a single standard test design or a set of designs.  Each study should be tailored to address the specific issues or data requirements that have arisen from earlier stages of testing.

To enable safe concentrations to be forecast by means of assessment factors backed up by sound scientific data, a two-step procedure was followed:

• Prediction from chronic single-species No-Observed-Effect-Concentrations (NOECs) to model ecosystem NOECs;
• Prediction from model ecosystem NOECs to field NOECs (=PNECs).

To establish the potential usefulness and the role of model ecosystems in risk assessment, NOECs obtained from well-designed model ecosystem studies were compared with NOECs obtained from laboratory single-species tests on the one hand and with field studies on the other.

A database has been assembled containing high quality published information on the toxicity of substances in ecosystem studies and those from chronic single-species tests.  Those ecosystem studies which provided values for both NEOCs as well as the corresponding Lowest-Observed-Effect-Concentrations (LOECs) were selected following a critical review of the literature.  From a total of 1108 data points only 248 studies fulfilled this criterion.  They covered 34 substances.  The data from single-species tests were extracted from the ECETOC Aquatic Toxicity (EAT) database, complemented with company and additional literature data of comparable quality.

The ratios between the most sensitive single-species NOECs and the most sensitive multi-species NOECs were compared, irrespective of ecological relevance.  This was considered to be a reasonable conservative approach for the derivation of assessment factors.  In the evaluation of model ecosystem studies for a particular substance however, it is necessary to select from the various endpoints recorded, the lowest one which is ecologically significant.  Such evaluations, performed with three substances in this report, also demonstrate the high degree of conservatism of the above assessment factor.

For the prediction from chronic single-species NOECs to model ecosystem NOECs, the median value for the ratios (which ranged from 0.02-77.5 with log-normal distribution) was found to be 1.45 with a 90%ile value of 8.14. This suggests that an assessment factor of about 8 for the extrapolation from the lowest chronic single-species NOEC-value to a NOEC-value in a model ecosystem would be safe.

For the second step a comparison was made between results from model ecosystems and results from field studies.  The conclusion was that results from the model ecosystem studies of sufficient complexity could be considered as realistic for the real world situation.

This means that an assessment factor of 8 is equally suitable for the prediction of a safe environmental concentration (PNEC) on the basis of chronic single-species NOECs.