Field validation of species sensitivity distributions
WCA Environment, UK
There is a requirement in the technical guidance for quality standards derived under the European Water Framework Directive (WFD) to consider evidence from field and mesocosm studies, where such data exists. The same principle can also be applied to any chemical substance for which a robust ecological threshold (e.g. PNEC) has been derived, for example through the derivation of a species sensitivity distribution. Several different approaches towards performing these types of assessments were outlined, including examples of real assessments. The advantages and limitations of various assessment approaches were considered for both whole community assessments and assessments that are targeted at particularly sensitive organisms.
In order to evaluate relationships between metal exposures and benthic community metrics, the bioavailability of the metals must be calculated for each site. Several approaches can be taken towards the assessment of PNEC values, including simplistic assessments of ecological quality at different exposure levels and the derivation of limiting functions (comparable to a traditional dose response relationship). Assessments can be based on the whole community, subsets of the community, groups of taxa, or an individual taxon. Analyses based at the level of the whole community may lack the sensitivity to identify slight effects on particularly sensitive species or families. Reducing the diversity of organisms assessed increases the uncertainty in the assessment, particularly for reference based methods. This presentation reviewed approaches towards the identification of those taxa that should be considered as sensitive to a particular pollutant.
A novel approach for bridging the gap between quality standards based on laboratory ecotoxicity studies and site-specific local aquatic communities was also outlined. This approach aims to take account of variation in the composition of ecological communities, and the effect that this may have on the sensitivity of the community to a particular pollutant. This was illustrated with an example for deriving site-specific thresholds for zinc in an area affected by historic mining activities.