PARENT SESSION
5A Assessing and predicting toxicant effects in an ecologically complex world
9:00 AM to 7:00 PM, Monday, 07 May 2001

(M/MF193) Confirmation of the SSD concept by experimental data and field distribution patterns of species in The Netherlands.

van Wijnen, Harm¹, Posthuma, Leo¹, de Zwart, Dick¹, van den Brink, Paul², ^{1 2}

ABSTRACT- The Species Sensitivity Distribution-concept (SSD concept) is used in environmental decision making, i.e. in derivation of Environmental Quality Objectives (EQOs) and in Ecological Risk Assessment (ERA). In view of the societal implications of a wrong concept, or unjustified application in certain conditions only, an answer was sought to the question whether SSD-based predictions reflect the truth. Can the concept per se be confirmed? Are there use limitations or rules of thumb to be taken into account when decisions involve SSD-based results? To investigate this, SSDs based on laboratory toxicity data were compared to toxic effects observed in (semi-)field conditions for various compounds, with different mode of action and pertaining to both the aquatic and terrestrial compartment. The comparisons showed that the statistical distribution of species sensitivities may be similar when uncertainties are small (number of species incorporated, acute direct effects, limited influence of possible ecological compensation mechanisms, etc.), suggesting confirmation of the concept per se. The more the situation deviates from ideal, the more careful the uncertainties should be scrutinized. The data suggest that separate SSDs should be derived where possible for each toxic mode of action (in relation to sensitivity of exposed taxa), that exposure uncertainties should be addressed in particular for the soil compartment, and that prediction accuracy is limited to direct effects. Despite uncertainties, the EQO-use of SSDs has as yet yielded EQOs (5th percentile) that generally seem to offer sufficient ecosystem protection. For ERA use, ecological insights are crucial for correct SSD-based conclusions.

Key words: Species Sensitivity Distribution (SSD), multi-substance Potentially Affected Fraction (ms-PAF), multivariate analysis