Competitive growth characteristics between Microcystis aeruginosa and Cyclotella sp. accompanying changes in river water inflow and their simulation model

Abstract

SummaryWhen the water quality in Lake Tega was the worst among Japanese lakes in the 1970s, cyanobacteria (mainly the genus Microcystis) were observed to be the dominant species in the summer. Since 2000, for water quality improvement, a large amount of the Tone River water was discharged into Lake Tega, resulting in an improvement in the water quality together with transition of algal dominant species from cyanobacteria to diatoms (mainly the genus Cyclotella). Although several factors including nutrient concentration and daily renewal rate (d) could have been related to the succession of algal dominant species, these effects have not been understood very well. This study investigated the effects of the daily renewal rate and nitrate‐nitrogen (hereafter nitrogen) concentration (N) on the competition between the cyanobacterium Microcystis aeruginosa and the diatom Cyclotella sp. through monoculture and monoxenic culture experiments. Based on the experimental results, a simulation model was constructed to predict the competitive growth pattern of each species. Monoxenic culture experiments showed that M . aeruginosa outcompeted Cyclotella sp. under conditions of N = 0.5 (1.0 mg‐N L−1) and d = 5, 15, and 20%. The domination of M . aeruginosa could be attributed to smaller values for the half‐saturation constant and the minimum nitrogen cell quota for this species with respect to Cyclotella sp. However, Cyclotella sp., possessing higher values of nitrogen cell quota and nitrogen uptake rate, dominated by an increasing nitrogen concentration (N = 1.0 mg L−1) and daily renewal rate (d = 30%). The competitive growth patterns of M . aeruginosa and Cyclotella sp. could roughly be predicted by the simulation model. These results suggested that the daily renewal rate as well as the nitrogen concentration, could affect the competition and be influential factors as to which species dominates between M . aeruginosa and Cyclotella sp.