Abstract:
There is a growing international trend towards the protection of
freshwater resources from pollution by imposing instream guidelines
and specified waste-discharge conditions. Current methods
for devising freshwater quality guidelines are based on species
sensitivity distributions (SSDs) that are used to identify pollutant
concentrations, ensuring the protection of a modelled percentage
of species (95% protection is a common goal). SSDs are derived
from the toxicity test results of as many taxa as possible for each
polluting substance. Waste-discharge licences can be for single
substances, specified in terms of chemical concentrations, and
derived in conjunction with instream guidelines; or for complex
mixtures, specified in terms of toxic units. In both cases toxicity test
results are the core data used. The emphasis on SSDs calls into
question the species constituting the test populations. It is likely
that SSDs based in part on the responses of local organisms will
achieve superior site-specific ecological protection. Until the early
1990s, there were very few data on the tolerances of South African
freshwater organisms. In the intervening decade, the Unilever
Centre for Environmental Water Quality at Rhodes University has
developed a toxicity database that, to date, records the responses
of 21 South African freshwater taxa to 26 single-substance pollutants
or mixtures. This is the most comprehensive database of South
African toxicity responses available and has been used in the drawing
up of methods and guidelines to protect water resources. This
paper aims to make these data available and to describe applications
of the data using selected case studies.
