Abstract
The present states of picoplankton were surveyed in 41 Japanese reservoirs of which nutrient level varied from oligotrophic to eutrophic. Based on the results, the effects of nutrients and artificial circulation were investigated. In all reservoirs surveyed, we found occurrence of picoplankton. Summer average cell numbers ranged from 2,800 to 518,000 cells/mL. In the conditions of T-P level from 10 to 60µg/L, summer and thermocline, cell numbers of picoplankton were abundant. In the reservoirs artificial circulation installed, the abundance of picoplankton tend to decrease to half level compared to no circulation reservoirs. Based on the results obtained, future managements policy of dam reservoir was discussed.
