No-observed-effect concentrations in batch and continuous algal toxicity tests

In this study, we compare the no-observed-effect concentrations (NOECs) bas ed on different response parameters, using batch and continuous algal toxic ity tests. For both batch and continuous tests, parameters based on total c ell Volume (TCV) were found to be less sensitive than those related to cell densities. The above observation mainly occurred because, under the stress es from metal toxicants evaluated in this and a previous study, the mean ce ll volume (MCV) of algae increased considerably. The increase of MCV compen sates for the effects brought about by the reduction in cell density and ev entually results in less Variation in TCVs. This study shows that parameter s based on cell density are quite sensitive and ideal for the estimation of NOECs. In addition, comparison of the NOEC values derived from different c ulture techniques shows that the continuous method generally yields lower N OEC values than that obtained by the batch tests. The results of this study also indicate that the NOEC provides more protection to the test organism than the effective concentration at 10% growth reduction (EC10). For toxici ty test methods that produce small variations among replicates, the NOEC is still a good indicator of low toxic effect. Furthermore, for the continuou s algal toxicity test, a relatively simple approach is proposed to determin e the NOEC Values based on the algal culture's control charts. The proposed method produced identical results as those based on conventional hypothesi s-testing methods.