Numerical Fluctuation of Bacteria and Heterotrophic Microflagellates in Hiroshima Bay of the Inland Sea of Japan in Summer 1990

Numerical fluctuations of bacteria, heterotrophic microflagellates (HMF) and autotrophic nanoplankton (ANP) were investigated at a station in Hiroshima Bay at two days intervals during the period from 18 July to 17 August, 1990. The average density of bacteria and HMF within the water column was highest at the beginning of the investigation, decreased to the end of July, and then was relatively constant. The cyclic oscillation pattern of the population abundance of bacteria and HMF was not found in the present study. The ranges of density observed were 1.16-4.30 x 10^6 cells•ml^<-1> for bacteria (mean: 2.40 x 10^6 cells • ml^<-1>), and 3.27-9.09 x 10^3 cells • ml^<-1> for HMF (mean: 5.07 x 10^3 cells • ml^<-1>). The average numerical and biomass ratios of bacteria to HMF were 481 and 1.08, respectively, which were smaller than the ratios previously reported. This means that HMF occurred more abundantly relative to bacteria in the present study area comparing to other areas. The density of ANP was more fluctuated than that of HMF, ranging from 2.41 x 10^3 to 59.00 x 10^3 cells • ml^<-1> (mean: 12.9 x 103 cells • ml^<-1>)