Effects of intrusion and retreat of deep cold waters on the causative species of red tides offshore in the South Sea of Korea

To explore the effects of eutrophic deep waters offshore on red-tide outbreaks, waters were collected from 0, 20, and 40 m depths at an offshore station in the South Sea of Korea, and the growth rate of each major red-tide dinoflagellate Prorocentrum donghaiense , Tripos furca , Alexandrium fraterculus , and Margalefidinium polykrikoides in the waters was measured. No species grew at dissolved inorganic nitrogen concentrations (DIN) of < 7.0–8.4 µM, but the growth rates of all four species rapidly increased and became saturated at 12–15 µM DIN. On July 31, 2020, the DIN was 4.1–4.6 µM in 0–10 m depth waters but 8.9–18.4 µM in 20–50 m depth waters. Under these circumstances, considering that calculated reachable depths of P. donghaiense , T. furca , A. fraterculus , and M. polykrikoides were 10, 15, 24, and 52 m, respectively, A. fraterculus and M. polykrikoides can reach the deep waters and grow, but P. donghaiense and T. furca cannot. However, if the deep waters intrude and ascend by 10–20 m relative to that on July 31, four species can reach deep waters and grow, whereas if the deep waters retreat and descend by 10–20 m, only M. polykrikoides can reach the deep waters and grow. Red-tide outbreaks by M. polykrikoides in the South Sea in 2014 occurred after the deep water retreat. Thus, the position of the eutrophic deep waters, affected by their intrusion and retreat and solar insolation, can affect outbreaks of harmful M. polykrikoides red tides.