Mixotrophic dinoflagellate red tides in Korean waters: Distribution and ecophysiology

• From 1981 to 2009, 693 mixotrophic dinoflagellate red-tide events occurred in Korean waters.
• Of these, Cochlodinium polykrikoides red tides accounted for ∼37%.
• Eighty-five percent of mixotrophic dinoflagellate red tides in Korean waters occurred from June to September, in the high water-temperature season.
• Eighty-six percent of the outbreak events were recorded in southern coastal waters, 12% in eastern coastal waters, and only 2% in western coastal waters of Korea.
• DIN and DIP are likely to be the primary factors influencing the frequency of mixotrophic dinoflagellate red tides in Korean waters.

We investigated the outbreaks of red tides dominated by dinoflagellates in the coastal waters of Korea from 1981 to 2009. During the study period, 693 mixotrophic dinoflagellate red-tide events occurred. Of these, 36.8% were Cochlodinium polykrikoides red tides, with Akashiwo sanguinea, Ceratium furca, and Prorocentrum minimum also frequently forming red tides. Eighty-five percent of mixotrophic dinoflagellate red tides in Korean waters occurred from June to September, in the high water-temperature season. The monthly frequency of outbreaks of mixotrophic dinoflagellate red tides increased from twice in January–March to 228 times in August, but it decreased to 2–9 times in November and December. Eighty-six percent of the outbreak events were recorded in southern coastal waters, 12% in eastern coastal waters, and only 2% in western coastal waters of Korea. There were wide temperature and salinity ranges during red tides associated with A. sanguinea, C. furca, and P. minimum, whereas for C. polykrikoides and Gonyaulax polygramma the temperature and salinity ranges were narrow. The concentrations of dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphate (DIP) in 1996–2000 were higher than the reported values for the half-saturation concentrations of uptake of nitrate and phosphate for most of the causative mixotrophic dinoflagellates. However, the concentrations of DIN and DIP have decreased gradually since 2000 and the frequency of mixotrophic dinoflagellate red tides has also decreased. Thus, DIN and DIP are likely to be the primary factors influencing the frequency of mixotrophic dinoflagellate red tides in Korean waters. Five novel dinoflagellate species have been discovered in Korean coastal waters since 2005 and their morphological and molecular characteristics have been described as new genera and species. It is thus necessary to monitor their dynamics to investigate whether they can form red tides.