Effect of marine heatwaves on bloom formation of the harmful dinoflagellate Cochlodinium polykrikoides

Abstract

In 2018, the bloom of harmful dinoflagellate Cochlodinium polykrikoides occurred under abnormally high water temperature (WT) conditions caused by heatwaves in Korean coastal water (KCW). To better understand C. polykrikoides bloom at high WTs in 2018, we conducted field survey and physiological and genetic experiments in laboratory using two different strain of CP2013 and CP2018. The heatwave increased the WT from 24.1℃ to 29.2℃ for two weeks, leading to strong stratification even in mid-July (p < 0.01, Chi square = 94.656, Kruskal–Wallis test). Under early stratification conditions, patch blooms formed more earlier than the average outbreak in the last 17 years in KCW, despite high WT reaching 30℃. In laboratory experiments, although there were no genetic differences in the LSU rDNA, both strains showed a significant different growth response to high WTs; above 28℃, CP2013 did not survive, but CP2018 was able to grow, suggesting that CP2018 had potential growth capacity at high WTs. However, the growth rate of CP2018 was low at 30°C. In addition, the blooms of C. polykrikoides in 2018 lasted only 3 weeks, an exceptionally short compared to average duration of past 20 years. The negative correlation between the average WT and duration of C. polykrikoides bloom in previous 17 years (R2=0.52, p<0.01) supports that high WT approaching 30℃ is not favorable for C. polykrikoides in KCW. Thus, in relation to heatwaves, our findings indicated that early stratification condition plays a critical role in developing C. polykrikoides blooms, but maintaining bloom are negatively affected under high WT conditions.