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For diatom study, we used 2 cm3 samples taken every 10 cm. For light microscope study, samples were prepared following the method proposed by Schrader and Gersonde (1978). Qualitative and quantitative diatom analyses were done at 1000× magnification using a Zeiss-Axioscope with phase-contrast illumination. Counts were carried out on permanent slides of acid-cleaned material (Mountex mounting medium). Several traverses across the coverslip were examined, depending on valve abundance. At least two coverslips per sample were scanned in this way. Diatom counting of two replicate slides indicated that the analytical error of concentration estimates is ≤15%. The counting procedure and definition of counting units for diatoms to the lowest possible taxonomic level followed those proposed by Schrader and Gersonde (1978).