Factors influencing maintenance and decline of a diatom bloom in the Yatsushiro Sea, Japan

Short-term growth dynamics of diatoms were examined by daily sampling and microcosm experiments under in situ environmental conditions in the late rainy season (July) for 10 days and by weekly sampling during summer (June–September) at a fixed point in the Yatsushiro Sea, Japan. During the daily sampling period, a bloom of diatoms such as Skeletonema spp., Thalassiosira spp., and Chaetoceros spp. (>104 cells mL−1) occurred in the surface and subsurface layers during the first half of the period, but the population of diatoms declined and sank to the bottom by an average of about 1 m day−1 during the second half of the observation period. Daily changes in net growth rates of the three dominant diatom genera (ranging from −3.8 to 3.0 day−1), which were estimated by daily 24-h microcosm incubations, roughly corresponded with those in ambient cell densities in the surface layer. The relationship between net growth rates and environmental factors suggest that the growth dynamics of diatoms were regulated by irradiance and river discharge (i.e., salinity and phosphate concentration) during the observation period. The field observations using daily microcosm experiments revealed that the maintenance and decline of a diatom bloom on a daily/weekly time scale are controlled by the rapid growth and sinking responses of diatoms following abrupt changes in coastal environments in the Yatsushiro Sea.