From September to November of 2021, a red tide caused by the dinoflagellate Karenia selliformis occurred in the eastern part of Hokkaido and caused extensive damage to marine organisms in that area. Although other Karenia species and diatoms were also present during the bloom, the dominance of K. selliformis suggested that competitive interactions among phytoplankton may have contributed to the formation of the mono-specific bloom of K. selliformis. To understand the mechanism responsible for the K. selliformis bloom, we used bi-algal cultures to investigate the growth interactions between K. selliformis and co-occurring species, the dinoflagellate Karenia mikimotoi and the diatom Skeletonema marinoi-dohrnii complex. Bi-algal cultures with several combinations of initial cell densities demonstrated that K. selliformis suppressed the growth of both K. mikimotoi and the S. marinoi-dohrnii complex. Simulation of growth in the bi-algal cultures using the Lotka–Volterra competition model to quantify the interactions showed that the competitive effects of K. selliformis on the co-occurring species were stronger than those of K. mikimotoi and the S. marinoi-dohrnii complex. Isoclines of constant cell densities indicated that K. mikimotoi was out-competed by K. selliformis under any combinations of initial cell densities, whereas the S. marinoi-dohrnii complex could be dominant only when the initial cell density of the S. marinoi-dohrnii complex were much greater than those of K. selliformis. These results suggested that the inhibition of growth of the co-occurring species by K. selliformis was an important factor in the formation of the mono-specific bloom of K. selliformis in Hokkaido in 2021.
Yano et al. (Mon,) studied this question.