Causation of reversal simultaneity for diatom biomass and density of Phormidium tenue during the warm season in eutrophic Lake Barato, Japan

The diatom biomass of Lake Barato, as measured from July to September, decreased simultaneously with an increase in filament density of Phormidium tenue after 1997. There was a high negative correlation between the diatom biomass and the densities of P. tenue (r2 = 0.928). Although total nitrogen (TN) and total phosphorus (TP) concentrations decreased from 1996, TN:TP ratio increased from 1997 because the TP concentration became markedly lower. The decrease in diatom biomass might have been due to the loss in phosphorus available for algae. Because the increase in density of P. tenue might have been due to the decrease in diatom biomass, experiments using a growth inhibitor for diatoms were performed to examine whether the density of cyanobacteria increases without diatom growth. Samples of the lake water collected in three seasons (August and October 1998, May 1999) were incubated with and without germanium (Ge) as a growth inhibitor of diatoms. The increase in density of P. tenue was inhibited concurrent with the increase in diatom biomass in the first and middle stages of incubation without the addition of Ge in August 1998 and May 1999. In contrast, a higher density of P. tenue was observed in the incubation with diatom growth inhibited by Ge over the same period. These results suggest that diatoms have an effect in restraining the growth of P. tenue.