The distribution and biomass of Peridinium bipes f. occultatum were observed in the autumns of 1991, 1992 and 1994 at the head of the Shorenji Dam reservoir (Mie prefecture, Japan). Summarizing observations and some other information, we discussed processes related to the accumulation of Peridinium biomass. In the study area, biomass of Peridinium increased at a similar rate to growth day by day, and roughly 2/3 of the daily maximum biomass fluctuated in a given day. The horizontal ‘import’ and ‘export’ of Peridinium biomass by flows mainly change the biomass of red tide at the upstream end of the reservoir. Flow control in the reservoir, as well as growth control, will also be important in suppressing freshwater red tide.

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