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Y,jk = µ + B; + D; + BD;; + Tk + BT;k + DTA + BDT;;k <br />where Y;jk is the count mean of the i`' backwater on the j`' date from the k`h treatment, B; is the <br />backwater (block) effect, Dj is the date effect, and Tk is the treatment. F-ratios for treatment <br />effects were calculated using the mean square of the block by treatment interaction as the <br />denominator. Normal probability plots showed that the treatment means met the assumption of <br />normality. <br />We also wanted to evaluate the effects of treatments on the diversity of copepods and <br />chironomids. Since the maximum number of samples processed varied (see above), we used <br />the rarefaction method to obtain the expected number of cyclopoid copepod species and <br />chironomid genera in a sample size of 30. Rarefaction estimates the number of species <br />expected in a random sample of individuals taken from a collection (see Krebs 1989 for <br />methodology). <br />Next, we grouped all taxa into one of four trophic categories: collector/gatherers, <br />herbivores, omnivores, and invertebrate predators. A multivariate analysis of variance <br />(MANOVA) and univariate ANOVA tests were then performed on the log transformed data <br />for both benthic and planktonic organism counts (from weeks 1, 3, and 5) using the same <br />model as in the weighted ANOVA tests. This was to test the effect of the treatments on trophic <br />structure of the whole invertebrate community and on the density of specific trophic levels <br />within the community. <br />Finally, mean densities for control treatments, with 95 % confidence intervals, were <br />calculated for the major benthic invertebrate taxa collected on each of the sampling dates. The <br />average densities, with 95 % confidence intervals, were calculated for closed and perforated <br />5