Seasonal variation in the abundance of the colony-forming diatom <i>Thalassiosira diporocyclus</i> in Minamata Bay and its growth characteristics

Seasonal variation in the abundance of the large mucilage colony-forming diatom Thalassiosira diporocyclus was investigated from October 2020 to January 2022 in Minamata Bay. This diatom is a red-tide forming harmful species that causes the bleaching of seaweed culture (Pyropia thalli) due to nutrient exhaustion. The growth characteristics of T. diporocyclus were also examined in incubation experiments using a culture strain. Vegetative cells of T. diporocyclus were consistently detected in the water column throughout the year, making this first report that they have survived in the summer. The cell density of T. diporocyclus exhibited clear seasonal variation, with a peak (4.8×103–8.5×103 cells L−1) in November and decrease in December in 2020 and 2021. High cell densities were observed in a narrower range of water temperature (21.0°C–21.2°C) and salinity (33.0–33.1). Nutrient concentrations increased throughout the water column in September, when stratification disappeared. However, the cause of the population decline in December remained unclear. Incubation experiments demonstrated positive growth of T. diporocyclus at 15°C–27°C and salinity levels of 25–35 (0.63±0.02–1.29±0.02 d−1). Thalassiosira diporocyclus could actively grow in almost all seasons in Minamata Bay. These results suggest that T. diporocyclus has an autochthonous life cycle in Minamata Bay.