Population development strategy of two dinoflagellates Cochlodinium polykrikoides and Alexandrium affine during the summer in the coastal environments of the southern sea of Korea

Abstract

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 coastal environments of southern 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 from17.4°C to 26.8°C, the salinity decreased from 34.15 to 30.8, and strong stratification developed. The nutrient concentrations maintained low level in the surface layer and high in the bottom layers. First, C. polykrikoides dominated in July however, abundance of A. affine reached bloom levels in whole study areas 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) conditions. In growth experiments, both species did not grow well at 15°C, and the growth rate also increased with water temperature, showing the maximum growth rate at 25°C (C. polykrikoides: 0.31 d-1, A. affine: 0.43 d-1). However, C. polykrikoides did not grow at low salinity of 20 at 25°C and 27.5°C, and A. affine did not show any difference in growth rate by salinity (p < 0.001). At 30°C, cultured C. polykrikoides cells died within 2 days, while A. affine1