Generation of a Chattonella antiqua bloom by imposing a shallow nutricline in a mesocosm

Abstract

A mesocosm was installed in the Seto Inland Sea where outbreaks of Chattonella antiqua (red tide) have frequently been observed. An artificial, shallow (6‐m depth) nutrient stratification analogous to that observed with C. antiqua in nature was imposed. The initial bottom temperature of 20–22°C was optimum for C. antiqua excystment as seeding, and the surface temperature of 25–27°C was optimum for growth. Excess nitrogen and phosphate compared with silicate changed the phytoplankton community from a mixture of diatoms and dinoflagellates to one of dinoflagellate dominance. Absence of copper toxicity was essential for growth of C. antiqua. Cessation of vertical circulation promoted accumulation of dinoflagellates at the water surface, and selective grazing toward small‐size dinoflagellates by zooplankton controlled the population of dinoflagellates.C. antiqua could reach a depth of 7.5 m during diel vertical migration, and its migration speed was ~0.8 m h‒1; upward and downward speeds were the same. Outbreaks of C. antiqua were favored by artificial eutrophication and subsequent maintenance of a stable, shallow nutricline at a depth (6 m) such that C. antiqua could take up nutrients in the nutrient‐rich lower layer at night.