Abstract : The establishment of water quality objectives (WQO) for individual chemicals is a complex process fraught with many uncertainties. WQO should (in principle) be derived from no-observed-effect-concentrations (NOEC\'s) determined experimentally on selected test species and even preferably on biological communities in field situations. In practice this is seldom the case. The number of species of different trophic levels, currently available for toxicity testing is extremely low and not representative for the biological communties in natural aquatic ecosystems; furthermore, the number of standardized test methods is very limited. Data bases on the effects of xenobiotics on aquatic biota mainly contain figures on acute tests with but a few species; worse, no information is available for approximately 90 % of all industrial chemicals.
WQO are derived from tests with (almost exclusively) pelagic species without consideration for the partitioning of pollutants to the sediments and the threat to benthic biota. WQO\'s for individual chemicals do not take into consideration either additive and/or synergistic effects of toxicants. Control of industrial emissions is mostly limited to chemical analyses imposed for a limited number of compounds; toxicity testing is avoided as much as possible for reasons of complexicity and costs.
In order to establish more realistic WQO\'s, toxicity data need to be generated with a battery of representative test species comprising pelagic as well as benthic organisms and additional (standard) test methods must therefore be worked out. Short-chronic tests should be developed and validated, in replacement of the present chronic bioassays which are too complex and too time consuming. New bioassay procedures should be user-friendly and cost-effective in order to allow large scale and routine application.
The control of the hazard of industrial emissions in the aquatic environment should be based on biomonitoring with a battery of cost-effective toxicity tests, rather than on exclusive chemical analyses. Newly developed screening bioassays such as e.g. the Microtox® test, the cyst-based 24 h LC50 Toxkit tests and the 1 hr EC50 Fluotox tests have a promising future in this regard, since they bypass the major bottleneck in ecotoxicological testing, namely the need for continuous culturing and maintenance of live stocks.