Succession phenomenon of Cochlodinium polykrikoides and Alexandrium affine related with changes of coastal environments in the southern sea of Korea in summer

Abstract

Among the harmful algal blooms (HABs) species, the dinoflagellate Cochlodinium, and Alexandrium species have been studied widely, due to their serious damages to the aquaculture industry. The present study investigated the appearance and succession patterns of HABs in the Korean coasts, and performed the bioassay using Cochlodinium polykrikoides and Alexandrium affine cultures to evaluate their competitive relationship and explain the succession phenomenon of HABs. From Juneto August, surface water temperature increased from 20.3°C to 26.8°C and the salinity decrease from 33.9 to 31.8 psu, and the stratification developed strongly. In this time, nutrient concentrations were kept low in surface layer, then dinoflagellate HABs species, C. polykrikoides, dominated in zone I, but A. affine dominated both zone I and II in middle August. In growth experiments, C. polykrikoides did not grow at 20 psu of 25 °C and 27°C, and it was dead within 2 daysin all cultivation of 30°C. The maximum growth rate (μmax) of C. polykrikoides was 0.31 d-1 at 25°C of 32 psu. A. affine showed negative growth rates at 15°C, but it grew well in all salinity at 20 °C to 30 °C. The maximum growth rate (μmax) of A. affine was 0.43 d-1 at 25 °C of 30 psu. In co-cultivation, A. affine had a competitive advantage at all temperatures, which of 1: 1 ratio of initial cell density of both species. In co-cultivations of gradient initial cellccession patterns of HABs in the Korean coasts, and performed the bioassay using Cochlodinium polykrikoides and Alexandrium affine cultures to evaluate their competitive relationship and explain the succession phenomenon of HABs. From Juneto August, surface water temperature increased from 20.3°C to 26.8°C and the salinity decrease from 33.9 to 31.8 psu, and the stratification developed strongly. In this time, nutrient concentrations were kept low in surface layer, then dinoflagellate HABs species, C. polykrikoides, dominated in zone I, but A. affine dominated both zone I and II in middle August. In growth experiments, C. polykrikoides did not grow at 20 psu of 25 °C and 27°C, and it was dead within 2 daysin all cultivation of 30°C. The maximum growth rate (μmax) of C. polykrikoides was 0.31 d-1 at 25°C of 32 psu. A. affine showed negative growth rates at 15°C, but it grew well in all salinity at 20 °C to 30 °C. The maximum growth rate (μmax) of A. affine was 0.43 d-1 at 25 °C of 30 psu. In co-cultivation, A. affine had a competitive advantage at all temperatures, which of 1: 1 ratio of initial cell density of both species. In co-cultivations of gradient initial cell