Cyst distribution and excystment conditions for the dinoflagellate Peridinium penardii (Lemm.) Lemm. in a reservoir

SUMMARY. 1. Cyst distribution and a possible excystment site of the dinoflagellate Peridinium penardii (Lemm.) Lemm. in a reservoir were surveyed.2. The presence of vegetative cells of P. penardii from all lake bottom mud samples, taken from several sites throughout the reservoir and cultured in vitro, showed that viable cysts of P. penardii were ubiquitous on the bottom of the reservoir.3. Culture bottles containing bottom mud with cysts of P. penardii were suspended at several depths at four stations in the reservoir. Vegetative cells of P. penardii were found in all bottles suspended at 0.5 m and almost none deeper than 20.0 m. Water depth was a critical environmental factor in preventing excystment.4. The place where P. penardii first excysts annually was predicted to be at the head of the reservoir using the data of cyst distribution, excystment conditions and morphology of the lake basin.