Seasonal variations in chlorophyll a and cell density of micro-, nano-, and picophytoplankton were studied in northern part of eutrophic Hiroshima Bay to estimate the contribution of each size phytoplankton to total phytoplankton biomass and cell density. The highest percentages of micro- and nanophytoplankton to total chlorophyll a was observed during winter to spring and summer to autumn, respectively. However, percentage of picophytoplankton to total chlorophyll a showed little variation throughout the year. During January to April and June to August, nanophytoplankton alone or micro- and nanophytoplankton were dominant accounting for 50-90% of total cell density in every year. However, in other months, picophytoplankton accounted for 50-95% of total cell density. Picocyanobacteria were the dominant component of picophytoplankton community during the whole year, and the important contributor to total phytoplankton cell density in Hiroshima Bay. Chlorophyll a concentration of nanophytoplankton increased when NO3-N concentration increased with the inflow of river water due to rainfall. Chlorophyll a concentration of picophytoplankton increased with the decrease in NO3-N by the growth of nanophytoplanlton.
