The purpose of this test is to determine the effects of a substance on the growth of freshwater microalgae and/or cyanobacteria, which play an important role as primary producers in aquatic systems. Exponentially growing test organisms are exposed to the test substance in batch cultures over a period of normally 72 h. In spite of the relatively brief test duration, effects over several generations can be assessed.
The test organisms are exposed to a dilution series of the test substance within the relevant concentration range (derived from a range finding pre-test) and incubated under defined light and temperature conditions for 72 h. Three replicates for each test concentration and six control replicates without test substance are studied in parallel. Several species of non-attached microalgae and cyanobacteria may be used for this test (e.g. Desmodesmus subspicatus, Raphidocelis subcapitata). Growth of the test organisms is analysed after 24, 48 and 72 h incubation by measuring a surrogate parameter for biomass, such as cell counts, fluorescence, optical density etc. The test endpoint, inhibition of growth, is calculated by comparing the logarithmic increase in biomass during the exposure period (average specific growth rate) for each test concentration with growth rate in the control batch. Dose-effect relationships are calculated by means of regression analysis with log-probit-transformation of the resulting parameter pairs, and from this the 50% effect concentration regarding growth (ErC50) is determined. In addition, the 50% effect concentration of the yield (biomass at the end of the exposure period minus the biomass at the start of the exposure period) and the lowest observed effect concentration (LOEC) and the no observed effect concentration (NOEC) are statistically determined.