No‐Effect Concentration as a Basis for Ecological Risk Assessment

Legislation around emission of chemicals in the environment aims at minimizing ecological effects. Because of our poor understanding of ecosystem dynamics it is usually unclear how effects on particular species translate into ecological effects. The interpretation of results from experiments with mesocosms is also far from obvious. This explains why No-Observed Effect Concentrations (NOECs) are frequently used in environmental risk assessments. This use is, however, under increased criticism, due to substantial statistical problems that are inherent to this concept: the problem of recognizing small effects in scattery data. A statistically nonsignificant effect does not imply that biologically significant effects are absent. Small effect concentrations have been proposed to replace NOECs in risk analysis. This approach suffers from several problems, such as, What is small?; How do small effects on species relate to ecosystem dynamics?; To what extend does the resulting value depend on model details that do not have a mechanistic basis? How do we extrapolate acute effects to chronic effects? Recently we solved the problem of estimating the No-Effect Concentration (NEC) as a model parameter from data of standardized aquatic toxicity tests (Kooijman 1993): acute and chronic survival (Bedaux & Kooijman 1994, Kooijman & Bedaux 1996a,d), body growth (fish, Kooijman & Bedaux 1996c,d), reproduction (daphnia, Kooijman & Bedaux 1996b,d) and (algal) population growth (Kooijman at al, 1996, Kooijman & Bedaux 1996d). This alternative for the NOEC does not suffer from statistical problems, while its use in risk assessments still avoids the difficulty of translating observed effects into consequences for ecosystems dynamics. Being a model parameter, the point estimate of the NEC can be provided with a confidence interval. This allows positive identification of a concentration range where no effects are to be expected, which is not possible in the NOEC approach. If, on the other hand, the null hypothesis NEC=0 (implying that the toxicant has effects in all concentrations) cannot be rejected, one might consider additional research about the effects of the compound. The method to estimate NECs is a by-product of a new process-based characterization of toxic effects. The basic idea is that the hazard rate and the parameters that quantify the energy budget of the individual are proportional to the concentration in the animal that exceeds the no-effect concentration. The inverse of the proportionality constant, which is called the tolerance concentration, quantifies the toxicity of the compound. The energy budget parameters are defined by the Dynamic Energy Budget (DEB) theory (Kooijman, 1993), which specifies the rules that organisms use for the energy uptake of resources (food) and the ensuing allocations to maintenance, growth, development and propagation. The DEB theory has been tested against a wide variety of ecophysiological data (Kooijman,