Thermotaxic diel vertical migration of the harmful dinoflagellate Cochlodinium (Margalefidinium) polykrikoides: Combined field and laboratory studies

The harmful dinoflagellate Cochlodinium polykrikoides, a species that causes mass mortality of farmed fish, uses diel vertical migration (DVM) as an ecological strategy. In summer 2018, a bloom of C. polykrikoides occurred on the southern coast of Korea when the surface water temperature exceeded 29 degrees C, as a result of a marine heatwave. To understand the effect of high temperature conditions on the DVM of C. polykrikoides, vertical profiles of environmental variables and the occurrence of the dinoflagellate were investigated through a 48 h field survey. In addition, a thermally stratified environment (6-12 degrees C difference between the surface and bottom layers) was established in a laboratory study to investigate the effect of temperature difference between water layers on the DVM of C. polykrikoides. In the field, most of the C. polykrikoides population was at a depth of 3-6 m during the day, where the water temperature was significantly lower (p < 0.01; Chi square = 57.98; Kruskal-Wallis test) than in the surface layer (0 m), and only the water temperature at 0 m was not correlated with weighted mean depth of C. polykrikoides, suggesting the usage of DVM to avoid high temperature stress. According to our field and laboratory results, there was a trend of greater DVM velocity by thermotaxis when moving from unfavorable water temperature (30 degrees C hot and 12 degrees C cold) to favorable water temperature for growth (optimal 24 degrees C) of C. polykrikoides. Our findings suggest that thermotaxic DVM is an important ecological strategy used by C. polykrikoides to optimize environmental conditions for growth through vertical positioning and changing migration velocity.

Automated summary

This study investigated the effect of high temperature conditions on the diel vertical migration (DVM) of the harmful dinoflagellate Cochlodinium polykrikoides, which causes mass mortality of farmed fish. The field and laboratory results indicate that C. polykrikoides uses thermotaxic DVM as an important ecological strategy to optimize environmental conditions for growth through vertical positioning and changing migration velocity.