Red tides in Masan Bay, Korea in 2004–2005: I. Daily variations in the abundance of red-tide organisms and environmental factors

• Red tide events in Masan Bay from June 2004 to May 2005 occurred 36 times for a total of 195 days.
• Daily sampling is necessary to detect all red-tide events because some red tides persisted only one to a few days.
• Water temperature is likely to affect the causative species of red tides in Masan Bay.
• Some red-tide species formed red tides when inorganic nutrient concentrations were lower than the reported values of K1/2.
• Some red-tide species had a significant positive correlation with bacteria and other algae. Their mixotrophic nature may enable these species to grow and form red tides.

To investigate red tides in Masan Bay, Korea, in which red tides have frequently occurred, we measured the abundance of red-tide organisms at a fixed station daily from June 2004 to May 2005. We daily measured physical, chemical, and biological properties. During the study period, 36 red-tide events occurred. Of these, 7 events were overwhelmingly dominated by cryptophytes, 5 by phototrophic dinoflagellates, 2 by diatoms, 2 by raphidophytes, 1 by a mixotrophic ciliate, and the rest by mixtures of several taxonomic groups. The durations of the red-tide events ranged from 1 to 40 days and total duration was 195 days. Most of the red tides occurred between June and September 2004 and between January and March 2005. The maximum abundance and biomass of total phototrophic dinoflagellates were 27,183 cells ml−1 and 3516 ng C ml−1, respectively, while those of total diatoms were 71,538 cells ml−1 and 10,981 ng C ml−1, respectively. Furthermore, the maximum abundance and biomass of total raphidophytes were 90,010 cells ml−1 and 10,177 ng C ml−1. The biomass of total phototrophic dinoflagellates had significant positive correlations with salinity, pH, dissolved oxygen, euglenophytes, raphidophytes, cyanobacteria, and heterotrophic bacteria, but negative correlations with temperature, nitrite plus nitrate and phosphate concentrations. In addition, the biomass of raphidophytes had a significant positive correlation with temperature, pH, and heterotrophic bacteria, but a negative correlation with salinity and the phosphate concentration. This evidence suggests that red-tide dynamics dominated by these phototrophic dinoflagellates and raphidophytes may be mainly affected by potential prey concentrations rather than inorganic nutrient concentrations. Daily sampling is necessary to explore red-tide dynamics in Masan Bay because the generation time of the causative species is ∼0.5–3 days.