Bottom water hypoxia enhanced by vertical migration of the raphidophyte Chattonella sp. in the Ariake Sea, Japan

In general, the formation of hypoxic water masses in coastal waters is caused by the restriction of vertical oxygen transport due to enhanced stratification, and the increase in oxygen consumption due to the decomposition of organic matter supplied from phytoplankton blooms. However, the direct relationship between phytoplankton blooms and hypoxia has been rarely reported. In this study, we clarified the relationship between the development of a hypoxic water mass (< 3 mg L⁻¹) and a bloom of the raphidophyte Chattonella sp. in the Ariake Sea, Japan through field surveys and experiments to measure the water oxygen demand. The process of development of and recovery from the hypoxic water mass in a spring-neap tidal cycle was observed. From the spring to the neap tide, high-salinity, high-density water from offshore intruded into the bottom layer of the inner part of the bay, strengthening the stratification and developing a hypoxic water mass. In this period, the bacterial oxygen demand was dominant during the day in the bottom layer. At night, oxygen consumption increased due to the respiration of Chattonella that had accumulated in the bottom layer, reaching five times the amount during the day. In the following spring tides, the stratification weakened and the hypoxic water mass disappeared. However, water oxygen demand was the highest, suggesting the influence of the decay of Chattonella that caused the bloom. These results indicate that the increase in oxygen consumption at night due to the diurnal vertical migration of Chattonella greatly influenced the development of the hypoxic water mass.