Hazard assessment in freshwater ecosystems.

Hazard assessment of chemicals in freshwater environments depends on comparing concentrations that are expected to occur in water and sediment, i.e. expected environmental concentrations (EEC), with those that are estimated to have no biological effects, i.e. the no-observed effect concentrations (NOEC). The difference between these two estimates is the margin of safety. The EEC can be estimated from data for chemical release rates, physicochemical properties and environmental parameters that affect transport and transformation. The NOEC can be estimated from the results of toxicity tests using aquatic plants, invertebrates and fish. When making these estimates it may be necessary to extrapolate from relatively limited laboratory data to the real world. Inevitably, this involves some degree of uncertainty. Such uncertainty can often be resolved by carrying out controlled field tests, using small, outdoor enclosures (microcosms), relatively large, outdoor ponds (mesocosms) and experimental streams. In this paper the advantages and disadvantages of various experimental approaches and systems will be reviewed.