Microcosm Studies (Extended Toxicity Studies)

Single-species Laboratory Studies

Extended toxicity studies can be conducted with algae, invertebrates or fish. These studies may differ from conventional toxicity tests in one or more ways, such as: inclusion of sediment, to allow the chemical to partition between water and sediment as it would do in a natural environment; variation of chemical concentration over time, to simulate exposure patterns typically observed in the field when chemicals disappear due to degradation or water movement; periodic re-introduction of organisms, to simulate natural immigration and allow observation of population recovery (invertebrates or algae only, not fish).

The objective of these studies is to demonstrate that the direct toxic effects of the chemical (as measured in standard toxicity tests) are mitigated by factors in a natural environment. For example, many compounds rapidly adsorb to sediment when dosed into a water-sediment system. At the same application rate, a water-sediment system may reduce toxicity by reducing the bioavailability of the compound in the water-sediment system compared to a water-only system. Or, to take a different example, if chemical concentrations in the water decrease over time, it may be possible to demonstrate recovery of invertebrate populations. Many regulatory authorities acknowledge that effects are not ecologically significant if population recovery can occur within a reasonable period of time (usually defined as one growing season). Thus, these extended toxicity studies can be used to justify a higher Predicted No-Effect Concentration (PNEC) for the chemical, resulting in a more favorable assessment of ecological risk.