Effects of tropical cyclones and biological interactions on red tide dynamics by the ichthyotoxic dinoflagellate Cochlodinium polykrikoides in the Korean coastal waters
한국연안에서 태풍과 생물학적 상호작용이 코클로디니움 적조에 미치는 영향에 대한 연구

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자연과학대학 지구환경과학부
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서울대학교 대학원
AlexandriumCochlodinium polykrikoidesEcologyGrazingTaxonomyTyphoonsHarmful algal bloom
학위논문 (박사)-- 서울대학교 대학원 : 지구환경과학부 해양학전공, 2016. 2. 정해진.
Red tides are discoloration of the sea surface due to microalgal blooms. The mixotrophic dinoflagellate Cochlodinium polykrikoides often forms red tide patches in the coastal waters of many countries and has sometimes caused large-scaled mortality of fish in both cages and natural environments. This dinoflagellate has caused losses of USD $1-60 million to Korean aquaculture industry every year. The mechanism of the outbreak, persistence, and decline of Cochlodinium red tides have not been fully understood yet, although, ecophysiological characteristics of C. polykrikoides have been well documented.

Here, I explored effects of some critical physical and biological factors such as tropical cyclones, competing diatoms, and novel mixotrophic dinoflagellate grazers on the red tide dynamics of C. polykrikoides.

Korea usually experiences several tropical cyclones (typhoons) in C. polykrikoides red tide period every year. Tropical cyclones are generally accompanied by strong winds and heavy rains and thus can change some critical factors affecting red tide dynamics. Strong winds often generate intensive turbulence which can inhibit growth of red tide organisms. Several studies reported that after the passage of tropical cyclones, the dominant red tide species in the water column were switched from flagellates to diatoms. Thus, tropical cyclones are likely to physically and biologically affect on the red tide dynamics of C. polykrikoides. However, effects of tropical cyclones on Cochlodinium red tides have not been well documented yet. Therefore, I explored the effects of tropical cyclones on the outbreak, persistence, and decline of Cochlodinium red tides by analyzing the daily maximum wind speed and daily maximum Cochlodinium cell abundance during the 14 tropical cyclone cases in South Sea of Korea in 2012-2014. I found that Cochlodinium red tides disappeared when daily maximum wind speeds exceeded 14 m s-1, but were not markedly affected when daily maximum wind speeds were less than 5 m s-1. Thus, this study suggests that daily maximum wind speeds of tropical cyclones may differentially affect the outbreak and persistence of Cochlodinium red tides.

I explored the effects of competing diatoms on dynamics of Cochlodinium and found that some competing diatoms reduced the growth and swimming speed of Cochlodinium through both physical contact and chemical stress when the concentration of the diatoms exceeds certain levels. This evidence suggests that the outbreak of Cochlodinium red tides can be prevented or delayed when the concentration of diatoms is high.

No effective mixotrophic dinoflagellate grazer on C. polykrikoides has been found yet, whereas several ciliates and heterotrophic protistan grazers have been reported. During Cochlodinium red tides in the coastal waters of eastern Korea, in 2014, I isolated a Alexandrium cell and established a clonal culture. This Alexandrium was revealed to be a mixotrophic dinoflagellate that exclusively feeds on C. polykrikoides. Based on the results from morphological and genetic analyses, the Alexandrium strain was revealed to be a new species, to be named as Alexandrium pohangense n. sp.

To explore the roles of Alexandrium pohangense in marine ecosystems, I measured the growth and ingestion rates of A. pohangense as a function of the concentration of only prey C. polykrikoides in the laboratory. The maximum ingestion rate of A. pohangense on C. polykrikoides was 7 cells predator-1 d-1, and the maximum mixotrophic growth rate reached 0.5 d-1, while the autotrophic growth rate was 0.1-1. Furthermore, by combining the results from the feeding experiments and abundance data obtained in field, I estimated grazing impact of A. pohangense on populations of C. polykrikoides in the natural environments. The grazing coefficients attributable to A. pohangense on co-occurring C. polykrikoides were up to 1.57 d-1. Thus, up to 79 % of the C. polykrikoides populations could be removed by A. pohangense population in a day.

Conclusively, through field observation, diverse feeding experiments in the laboratory, and morphological and molecular analyses, I found that tropical cyclones, competing diatoms, and a new mixotrophic dinoflagellate A. pohangense can significantly affect the red tide dynamics by C. polykrikoides.
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College of Natural Sciences (자연과학대학)Dept. of Earth and Environmental Sciences (지구환경과학부)Theses (Ph.D. / Sc.D._지구환경과학부)
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