Microcosm Studies (Indoor Microcosms)

Indoor Multi-species Studies

Model ecosystems (microcosms) containing water, sediment, and communities of plants and invertebrates from natural ponds can be established in aquaria in the laboratory, and can be used to measure higher-level ecological effects of chemicals. A typical microcosm size is 200 L, although we have had success with smaller and larger microcosms as well. Fish are usually not included in indoor microcosms. The microcosms are usually allowed to "mature" for at least 4 weeks before the chemical is applied, and effects are measured for up to 16 weeks after chemical application. A typical study design includes 5 exposure levels (chemical concentrations) plus controls, in duplicate or triplicate; thus 12 to 18 microcosm units. Effects on phytoplankton, periphyton, zooplankton, or macroinvertebrates can be measured, depending on the objectives of the study.

Indoor microcosm studies are intended to measure chemical effects on populations and communities under simulated natural conditions. The results may help the registrant in two ways:

First, it is usually found that aquatic populations and communities can tolerate chemical exposures higher than the LC50 or NOEC of sensitive test species, due to factors such as population recovery and replacement of species. Microcosm results can therefore be used to justify a higher PNEC, as described above for extended toxicity studies.
Second, results of microcosm studies are considered by most regulatory authorities to be more representative of natural conditions than results of conventional toxicity tests, which justifies use of a smaller "application factor" or "safety factor" when comparing the PNEC to the Predicted Exposure Concentration (PEC).