Phytoplankton composition associated with physical and chemical variables during summer in the southern sea of Korea: Implication of the succession of the two toxic dinoflagellates Cochlodinium (a.k.a. Margalefidinium) polykrikoides and Alexandrium affine

Harmful algal blooms (HABs) of the dinoflagellates Cochlodinium and Alexandrium are known to have serious negative impacts on marine ecosystems and the aquaculture industry. We investigated the appearance and succession patterns of both HAB species in the southern sea of Korea and performed a bioassay using C.polykrikoides and A.affine cultures to evaluate their competitive relationship. From June to August in 2017, the surface water temperature increased from 17.4 °C to 26.8 °C, the salinity decreased from 34.2 to 30.8, and strong stratification developed. The nutrient concentrations remained low in the surface layer and high in the bottom layers. C.polykrikoides dominated in July; however, A.affine reached bloom levels throughout the study area in August. The growth experiments were carried out on each species at six water temperature (15, 20, 22.5, 25, 27.5 and 30 °C) and six salinity (20, 24, 26, 28, 30 and 32) levels. The maximum growth rate for C. polykrikoides (0.31 day−1) was measured at 25 °C and a salinity of 32, and for A. affine (0.43 day−1) at 25 °C and a salinity of 30. At all salinity levels, C. polykrikoides cells died within 2 days when temperature were 30 °C, while A.affine maintained high growth rates (0.35–0.39 day−1). Further, C. polykrikoides is salinity sensitive at the lowest salinity level of 20 in 25 °C and 27 °C, but A. affine did not respond sensitively to changes in salinity. In contrast, A. affine did not grow at low temperature of 15 °C, but C. polykrikoides could survive in high salinity range (26–32). In co-cultivation at a 1:1 cell inoculation ratio, A.affine had a competitive advantage over C. polykrikoides at all temperatures; however, when the inoculation ratios were varied, the species with a higher initial cell density showed a competitive advantage at all temperatures except 30 °C. Combined the field and bioassay results indicated that A. affine may have an advantage over C. polykrikoides under relatively higher temperature and low salinity conditions, which is important when evaluating the species succession and blooms in the southern sea of Korea during summer. • Succession phenomenon from C. polykrikoides to A. affine in Korean coastal area. • Influence of Changjiang diluted water in the southern sea of Korea, during summer in 2017. • A. affine had better environmental tolerance than C. polykrikoides to higher water temperature and lower salinity ranges. [ABSTRACT FROM AUTHOR]