Improvement on species sensitivity distribution methods for deriving site-specific water quality criteria

Species sensitivity distribution (SSD) is the most common method used to derive water quality criteria, but there are still issues to be resolved. Here, issues associated with application of SSD methods, including species selection, plotting position, and cutoff point setting, are addressed. A preliminary improvement to the SSD approach based on post-stratified sampling theory is proposed. In the improved method, selection of species is based on biota of a specific basin, and the whole species in the specific ecosystem are considered. After selecting species to be included and calculating the cumulative probability, a new method to set the critical threshold for protection of ecosystem-level structure and function is proposed. The alternative method was applied in a case study in which a water quality criterion (WQC) was derived for ammonia in the Songhua River (SHR), China.