2019 Volume 66 Issue 1 Pages 1-10
A red tide causative dinoflagellate, Karenia mikimotoi, appeared in a dense concentration in Kusudomari, Kujukushima waters, Japan in June 2015. A PVC-made mesocosm was installed during the red tide period to examine possible factors controlling diel vertical migration of K. mikimotoi, and the following parameters were measured during 19–20 June: vertical profiles of water temperature, salinity, chlorophyll fluorescence, photon flux density, Fv/Fm, concentrations of DIN and PO4-P, cell densities of K. mikimotoi and Prorocentrum shikokuense. During the cloudy daytime, cells of K. mikimotoi were mainly found in the subsurface (0.5–2 m), while the distribution layer went down to the deeper layer (4–5 m) under intense sunlight. Interestingly, under such strong light conditions, the K. mikimotoi population always stayed beneath the P. shikokuense layer. Averaged photon flux density at the K. mikimotoi layer, where the highest chlorophyll fluorescence (>15) were observed, was 100±43 µmol m-2 s-1, a value almost consistent with the suitable range for K. mikimotoi growth. Meanwhile, at night, cells of K. mikimotoi were mostly found in the nutrient-rich bottom. This study confirmed the suggestions of previous resarchers that K. mikimotoi cells accumulate at a depth that provides a suitable light regime in daytime and remain at the bottom layer at night to absorb nutrients. Moreover, the current study demonstrated K. mikimotoi stayed beneath the dense P. shikokuense layer, which might provide a shade-effect for K. mikimotoi and enable them to grow in shallower layers.
