Analysis of Growth Limiting Factors Causative of Freshwater Red Tide by Dinoflagellate Peridinium bipes f. occultatum

Since the early 1980s freshwater red tides caused by dinoflagellates of the genus Peridinium have frequently occurred mainly on man-made lakes in western Japan. The Kinki district in particular has often experienced red tide blooms of Peridinium bipes f. occultatum. In order to establish the cause of this phenomenon, authors carried out AGP assays by using sample waters taken from three sites, and made an analysis of growth limiting factors of P. bipes in the reservoirs. Clonally isolated and axenic strains of P. bipes were utilized for the AGP assay of waters from the Asahi Reservoir (Nara Prefecture), the Goukawa Reservoir (Wakayama Prefecture) and the Shimokotori Reservoir (Gifu Prefecture) in 1989-90. The growth of P. bipes was markedly stimulated by the phosphorus which enriched surface water from these reservoirs. Growth was further enhanced when nitrogen was added together with phosphorus. This suggests that phosphorus is the primary growth-limiting factor for P. bipes in these reservoirs. During 1982-89 the annual mean values in concentrations of total phosphorus and total nitrogen in the Asahi Reservoir had a positive correlation to the daily annual total number of red-tide blooms and with the inflow volume. It is assumed, therefore, that the inflow from the river is the main source of phosphorus supplied to the Asahi Reservoir, and that phosphorus as the primary growth-limiting factor controls the annual frequency in red-tide blooms of P. bipes in this reservoir.