EXPERIMENTAL-DESIGNS FOR AQUATIC MESOCOSM STUDIES - A COMPARISON OF THE ANOVA AND REGRESSION DESIGN FOR ASSESSING THE IMPACT OF TETRACHLOROPHENOL ON ZOOPLANKTON POPULATIONS IN LIMNOCORRALS

Two experimental designs were compared for assessing the impact of a commercial 2,3,4,6-tetrachlorophenol formulation (DIATOX(R)) on zooplankton abundance in a set of limnocorrals. The first design, based on blocked analysis of variance (ANOVA) procedures, used treatments of 0, 0.75, and 1.50 mg/L DIATOX with three replicates of each. The second design, based on regression procedures, had eight treatments ranging from 0 to 7.3 mg/L DIATOX with no replication. For the ANOVA experiment, a conventional ANOVA by day revealed significant impact on all zooplankton taxa investigated. Impact and time to recovery were greater with the 1.50-mg/L DIATOX treatment, and rotifers were more sensitive than macrozooplankton. A repeated-measures ANOVA, used to correct for pretreatment variability, revealed additional differences as significant. The ANOVA design permits easy assessment of impact at the chosen toxicant levels, but no conclusion can be drawn regarding the no-effect concentration (NEC) for major zooplankton taxa. For the regression experiment, linear dose-response relationships were used to estimate the NEC by inverse regression analysis. NECs for major zooplankton taxa and dominant species ranged from 0.28 mg/L DIATOX for immature Copepoda to 0.50 mg/L DIATOX for combined Cladocera. In 9 of 14 cases, however, the estimated EC50 less-than-or-equal-to NEC, an indication that an increased number of lower test concentrations were needed. The regression design offers a promising alternative to the common ANOVA design and allows for the estimation of specified end points such as the NEC and EC50.