Effect of phosphorus fluctuation caused by river water dilution in eutrophiclake on competition between blue-green alga Microcystis aeruginosaand diatom Cyclotella sp.

Tega-numa (Lake Tega) is one of the eutrophic lakes in Japan. For the improvement of water quality in Lake Tega, the NorthchibaWater Conveyance Channel was constructed in 2000, which transfer water from Tone River into the lake. After 2000, thedominant species of diatoms, mainly Cyclotella sp., have been replacing blue-green algae, mainly Microcystis aeruginosa in LakeTega. This transition of dominant species would be due to the dilution, but the detail mechanism has not been understood yet. Thisstudy examined the relationship between phosphorus fluctuation caused by river water dilution to Lake Tega and dominance of algalspecies, M. aeruginosa or Cyclotella sp. based on the single-species and the mixed-species culture experiments. The single-speciesculture experiment showed that the half-saturation constant and uptake rate of phosphorus were one order lower and seven times higherfor M. aeruginosa than those for Cyclotella sp. These findings implied that M. aeruginosa would possess a potential for the growthand survival over Cyclotella sp. in the phosphorus limited condition. The superiority of M. aeruginosa was reflected in the outcome ofthe mixed-species culture experiment, i.e., dominance of M. aeruginosa, even phosphorus concentration was lowered to 0.01 mg-P/L.Therefore, it could be concluded that the decrease in phosphorus concentration due to the river water dilution to Lake Tega would beinterpreted as a minor factor for the transition of dominant species from M. aeruginosa to Cyclotella sp.