Size-fractionated phytoplankton biomass in coral reef waters off Sesoko Island, Okinawa, Japan

The seasonal dynamics of size-fractionated phytoplankton biomass in a coral reef area were investigated off Sesoko Island, Okinawa, Japan. Chlorophyll (Chl) a and nutrient concentrations were low almost all year round, showing no clear seasonal variability with values similar to those in some other coral reef areas. Picoplankton dominated the phytoplankton community; average contributions of pico-, nano- and microplankton to the total Chl a biomass were 52, 34 and 11%, respectively. However, in 1998, when the seawater ttemperature was extremely high and coral bleaching occurred due to the ENSO event, low Chl a coupled with high nutrient concentrations were observed. This was associated with a decrease in the picoplankton size fraction. We suggest that the high seawater temperature in 1998 decreased the growth rate of the picoplankton; however, the micro- and nanoplankton may also be limited by other factors such as low nutrient concentrations.