Abstract
The population density of the dinoflagellates Peridinium spp., mostly of P. penardiforme, was surveyed from May, 1978 to March, 1981 at two sites in the Ishitegawa Dam Reservoir : near the dam site and at the head of the lake near the inflow site of the Ishite River. Peridinium increased to 103-104 cells·ml-1 only at the head of the lake in winter, resulting in a red tide. This abundant winter growth was partly attributed to the low water temperature because of our findings that Peridinium grew more abundantly at 10°C than at 20° or 30°C in the cultures using filtered lake water. It was also found that phosphorus was a growth-limiting nutrient in the lake water from the culture experiments involving N, P, and Fe enrichment. The difference of nutritive water quality between the head and the other parts of the lake was considered to be due to the difference in the available phosphorus supply from the outside. It was suggested that a near-dialysis culture system was naturally constructed at the head of the lake, where little of Peridinium flowed out and PO4-P was supplied continually by the inflowing water so that Peridinium grew abundantly, especially when the water was shallow in winter.
