Effects of Vertical Water-mixing against Freshwater Algal Bloom.

The vertical mixing of water within a water column by an underwater mixer to prevent freshwater algal bloom (freshwater red tide) by dinoglagellate, Peridinium bipes was studied at Tonoyama reservoir in the Kii Peninsula. A mesocosm was created by enclosing water of the reservoir with water-proof vinyl sheet fences. Water conditions, such as water temperature and DO, were kept uniform throughout the mesocosm by the vertical water flow generated with the underwater mixer. In the summer of 1993, the usual large freshwater algal bloom did not appear in the reservoir due to the unusually cold temperature, high rainfall and repeated large volume discharges. We had to conduct this experiment at low P. bipes cell densities; however, a comparison of the cell density in the mixing area with those in the control area and outside of the mesocosm indicated a suppressed reproduction rate.