The precise application of risk assessment can lead to different conclusions about risk depending on how
species are grouped in the assessment. We compared the use of different risk assessment methods for
three different classes of pesticide, the herbicide diuron, the fungicide chlorothalonil, and the insecticide
permethrin for marine and estuarine species. Permethrin was the most toxic pesticide to marine and
estuarine crustaceans. Diuron was the most toxic pesticide to algae, and chlorothalonil was most toxic
to early life stages of molluscs and other invertebrates. Toxicity data (96 h LC50/EC50 values) were analyzed
using a probability distribution on the ranked toxicity values and 10th centile values were calculated
based on different groups of species and for all species combined. Our results indicate that an
assessment of risk based on smaller taxonomic groups can be informative, especially for pesticides of less
specific modes of action such as chlorothalonil.