Competition between the cyanobacterium Microcystis aeruginosa and the diatom Cyclotella sp. under nitrogen-limited condition caused by dilution in eutrophic lake

The improvement of water quality in Lake Tega, Japan, has been carried out by dilution, causing the shift of dominant species from Microcystis aeruginosa to Cyclotella sp. in summer. The disappearance of Microcystis blooms would be related to dilution, but the detail effect has not been understood yet. In this study, the effect of nitrate concentration on the competition between M. aeruginosa and Cyclotella sp. was investigated through the single-species and the competitive culture experiments. The single-species culture experiment indicated that the half saturation constants for M. aeruginosa and Cyclotella sp. were 0.016 and 0.234 mg N L−1, representing that M. aeruginosa would possess a higher affinity to nitrate. On the other hand, the maximum growth rate for Cyclotella sp. was obtained as 0.418 day−1, which did not represent a significant difference with 0.366 day−1 obtained for M. aeruginosa. The competitive culture experiment revealed that Cyclotella sp. completely dominated over M. aeruginosa at the nitrate concentrations of 0.5 and 2.5 mg N L−1. The dominance of Cyclotella sp. could be attributed to the difference in the abilities of nitrate storage as well as nitrate uptake. One of the possibilities for the disappearance of Microcystis blooms caused by dilution as observed in Lake Tega could be due to the decrease in nitrate concentration, and the lower N:P ratio seemed not to relate to Microcystis blooms.