Ecological significance of hazardous concentrations in a planktonic food web

Species sensitivity distributions (SSDs) are statistical distributions that are used to estimate the potentially affected fraction (PAF) of species at a given toxicant concentration, the hazardous concentration for that fraction of species (HCPAF). Here, we use an aquatic food web model that includes 14 phytoplankton and 6 zooplankton species to estimate the number of species experiencing a biomass reduction when the food web is exposed to the HCPAF and this for 1000 hypothetical toxicants and for PAF=5–30%. When choosing a 20% decrease as a cut-off to categorize a species’ biomass as affected, 0–1 and 2–5 out of the 20 species were affected at the HC5 and HC30, respectively. From this, it can be concluded that the PAF is a relatively good estimator of the number of affected species. However, when phytoplankton species experiencing ≥20% biomass increase were also classified as affected, the number of affected species predicted by the food web model varied strongly among toxicants for PAF >5, with 2–16 out of 20 species affected at the HC30. Phytoplankton species with extreme (both high and low) values for uptake rates and light limitation constants experienced smaller effects on their biomass than phytoplankton species with more average parameter values. We conclude that, next to measures of toxicity, ecological characteristics of species may help understanding ecological effects occurring in ecosystems also.