Seasonal Fluctuation in Abundance of Bacteria, Heterotrophic Nanoflagellates, Autotrophic Nanoflagellates and Nanodiatoms in Hiroshima Bay, the Inland Sea of Japan

The seasonal and vertical fluctuations in abundance of bacteria (BA), heterotrophic nanoflagellates (HNF), autotrophic nanoflagellates (ANF), and nanodiatoms (ND) were investigated weekly over the course of a year at a sampling station in Hiroshima Bay. They were usually most abundant in the surface layer and their numbers decreased with depth. The average densities of these microbes in the water column showed similar seasonal patterns, being high in summer and low in winter. The seasonal ranges of their average densities were: 1.01-4.89 x 10^6 cells ml^ for BA, 1.01-10.0 x 10^3 cells m1^ for HNF, 0.43-19.2 x 10^3 cells ml^ for ANF and 0.07-52.8 x 10^3 cells ml^ for ND, showing that nanoautotrophs (ANF and ND) fluctuated more widely than BA and HNF. BA and HNF fluctuated in parallel, and the annual mean ratio of BA to HNF was 881. Among nanoautotrophs, ND were more abundant than ANF in spring and summer, while it was vice versa in winter. Based on multiple regression analysis, temerature, salinity and chlorophyll (plus BA or HNF) accounted for 45-73% of the variation of these microbes