Using species sensitivity distributions to determine boundaries for chemical pollution

Abstract

Species sensitivity distributions (SSDs) and distributions of toxicity values are used for determining toxicological and ecotoxicological thresholds, the HC5 (hazardous concentration for 5% of the species) and the TTC (threshold of toxicological concern), respectively. An SSD describes the sensitivity distribution of a range of species towards a single toxicant, while the TTC is based on the toxicity distribution of a range of different compounds towards the same species or group of species. Both concepts can be combined by using the distribution of HC5 values for a group of substances in order to estimate the TTC. HC5- as well as TTC-values are usually based on toxicity data from standard single species assays. However, the ecological impact of a compound is the result of the reaction of a range of interacting species. In order to explore the impact of using data from ecological communities instead of single species, we performed a comparison of the TTC based on HC5-values with the TTC based on community ecotoxicological data (threshold of concern for community toxicity, TCCT). This study was performed by using toxicity data for Photosystem II- inhibiting herbicides to single algal species and algal communities. Single-species based SSDs and the corresponding HC5 values were established for seven different herbicides. Community ecotoxicity data were collected for 17 herbicides. The resulting thresholds were 6.8 nmol/L for the TTC based on HC5 values and 3.5 nmol/L for the TTC based on community data. This indicates that indirect ecological effects and species interactions do not seem to play a major role, as long as photosynthesis is in focus.