ROLE OF ENCYSTMENT AND EXCYSTMENT OF PERIDINIUM-BIPES F OCCULATUM (DINOPHYCEAE) IN FRESH-WATER RED TIDES IN LAKE KIZAKI, JAPAN

The encystment flux of Peridinium bipes f. occulatum (Dinophyceae) was investigated with sediment traps from 1986 to 1990 in Lake Kizaki. Cy sts of P. bipes were formed throughout the blooms. Encystment flux of P. bipes in the pelagic zone was usually lower than those at shallow s ites, and the density of P. bipes cysts in lake sediment was higher in the shallow region than in the pelagic zone. However, in the shallowe r region, the concentration of P. bipes cysts varied widely, possibly due to high rates of encystment and excystment. Peridinium bipes encys tment occurred between 15-degrees and 25-degrees-C in the laboratory, with very little cyst formation below 10-degrees-C. Though cyst format ion was observed in continuous darkness, the rate increased with irrad iance. Under continuous darkness, no excystment was observed at any te mperature from 5-degrees to 25-degrees-C. Eighty-one percent of the cy sts illuminated at 105 muE.m-2.s-1 excysted after 13 days incubation a t 15-degrees-C, and lower irradiances decreased germination success. R esults from laboratory experiments suggest that light is a critical fa ctor in the germination of P. bipes cysts. Bottom depth thus can have a significant effect on germination because cysts only can excyst from depths where light is sufficient. The shallow region of the lake is t hus very important as a seed bed for P. bipes during early spring. Cys ts deposited in deeper waters may not ever germinate unless they are r esuspended and transported to shallow areas where light reaches the bo ttom.