Statistical comparisons of the no-observed-effect concentration and the effective concentration at 10% inhibition (EC10) in algal toxicity tests

Batch-type algal toxicity tests were conducted to evaluate the low toxic ef fects of metal toxicants. Both the no-observed-effect concentration (NOEC) and the effective concentration at 10% inhibition (EC10) were calculated an d compared. The results of this study indicate that, for algal toxicity tes ts, NOEC. offers better protection to the test organisms than EC10. In addi tion, the consistency of NOEC is found to depend on the response endpoints measured. A cut-off-value approach is proposed to determine whether NOEC or EC10 should be chosen for estimating low toxic effects. For a specific tox icity test, the average cut-off value directly indicates the magnitude of t he square root of the within-group-variance. A precise test associated with small within-group-variances also produces relatively small average cut-of f value (say, less than 10% of the growth reduction). This average value pr ovides useful information on the minimum protection that can be offered by the NOEC.