Inhibition of massive blooms of fish-killing harmful dinoflagellate Cochlodinium polykrikoides related with abnormal high temperature by large amount of Changjiang River discharge along the Tongyeong coast (South Korea)

Abstract

For the past 20 years, blooms of Cochlodinium polykrikoides have been consistently present along the Tongyeong coast, Korea, but they abruptly disappeared in 2016. Despite extensive studies on this dinoflagellate, the cause of this abrupt decline remains largely unknown. To understand these phenomena, physico-chemical and biological data were collected along the Tongyeong coast through a biweekly field survey from June to September. The environmental differences between 2016 and previous years (2012-2015) were investigated. In August, the water temperatures (c.a.,30°C) and the salinity (c.a., 30) were outside the optimum ranges for C. polykrikoides growth in the study area. Moreover, the averages of these factors were significantly different from previous years (p<0.001). In 2016, the amount of Changjiang River discharge, which can affect coastal environments via ocean currents, was relatively larger than in the past four years, reducing the salinity and increasing the water temperature in August. Increased stratification and diatom dominance, both negatively associated with C. polykrikoides growth, were consistently observed during the study period. In the Goheung-Oenarodo area, which is the western part of the southern Korean coastal waters (KCW), C. polykrikoides blooms were observed, but they did not expand to the entire southern KCW, unlike in previous years. At that time, the strong winds from the Malakas typhecline remains largely unknown. To understand these phenomena, physico-chemical and biological data were collected along the Tongyeong coast through a biweekly field survey from June to September. The environmental differences between 2016 and previous years (2012-2015) were investigated. In August, the water temperatures (c.a.,30°C) and the salinity (c.a., 30) were outside the optimum ranges for C. polykrikoides growth in the study area. Moreover, the averages of these factors were significantly different from previous years (p<0.001). In 2016, the amount of Changjiang River discharge, which can affect coastal environments via ocean currents, was relatively larger than in the past four years, reducing the salinity and increasing the water temperature in August. Increased stratification and diatom dominance, both negatively associated with C. polykrikoides growth, were consistently observed during the study period. In the Goheung-Oenarodo area, which is the western part of the southern Korean coastal waters (KCW), C. polykrikoides blooms were observed, but they did not expand to the entire southern KCW, unlike in previous years. At that time, the strong winds from the Malakas typh