A method is described that allows the measurement of river periphyton chlorophyll a and pheophytin a both on rocks and in soft sediments. The method involves successive extractions directly from frozen samples with boiling methanol. Pigments are estimated by optical density measurements before and after acidification. Methanol is superior to acetone for the distinction of chlorophyll a and pheophytin a by this means.
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Tett et al. (1975) studied this question.