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<br />, . <br /> <br />w <br />'=' <br />o <br />'-" <br /> <br />The optical density of acetone extracts of pigments from periphyton colonies <br /> <br />which developed on submerged slides was measured with a spectrophotometer at <br /> <br />663 mm before and after acidification. Both cblorophyl a and pheophyten a, a <br /> <br />degradation product of chlorophyl a, concentrations were calculated as described <br /> <br />by Lorenzen, 1967. <br /> <br />The ratio between the organic matter and chlorophyl d was utiliz.ed to <br /> <br />determine the condition of the periphyton. Normally, periphyton com~unities <br /> <br />are dominated by algae. Discharge of degradable non-toxic organic wastes into <br /> <br />Surface waters usually results in a greater biomass of non-chlorophyllous, <br /> <br />heterotrophic organisms than of algas which is reflected in a rise in the total <br /> <br />biomass = chlorophyl a ratio. This ratio is termed the Autotrophic I."lcex (AI) <br /> <br />and may be calculated by the formula. <br /> <br />Al "" Biomass (dry weight organic matter) <br />cblorophyl a <br /> <br />Where periphyton mass and c.i1oropbyl a are expressed in mg/m2 of surface area <br /> <br />from which the sample was taken. <br />