Red tides in Masan Bay, Korea in 2004–2005: II. Daily variations in the abundance of heterotrophic protists and their grazing impact on red-tide organisms

• From June, 2004 to May, 2005, 29 heterotrophic dinoflagellate, 16 tintinnid, and several naked ciliate taxa were found.
• Heterotrophic protists were abundant in red-tide periods, but rare in non-red tide periods.
• The range of the grazing coefficients attributable to a dominant heterotrophic protist on a red-tide organism was large.
• Heterotrophic protistan grazers may at times have considerable grazing impact on the populations of co-occurring red-tide organisms.

To investigate the role of heterotrophic protists in the dynamics of red tides in Masan Bay, Korea, we measured the abundance of heterotrophic dinoflagellates, ciliates, and heterotrophic nanoflagellates in daily samples collected from June 2004 to May 2005. The abundance of heterotrophic dinoflagellates, tintinnid ciliates, naked ciliates, and heterotrophic nanoflagellates were high when red tides occurred, with maximum biomass of 1916, 1263, 1013, and 141 ng C ml−1, respectively. The HTDs Gyrodinium dominans/Gyrodinium moestrupii, Gyrodinium glaucum, Protoperidinium bipes, and Pfiesteria-like dinoflagellates (PLDs) as well as naked ciliates (≤50 μm) were present nearly all year and their maximum biomass was 235, 48, 298, 1020, and 1013 ng C ml−1, respectively. PLDs were the most abundant taxa during red tides dominated by Akashiwo sanguinea, Heterocapsa rotundata, summer populations of Prorocentrum minimum, Heterosigma akashiwo, Eutreptiella gymnastica, and cryptophytes, while G. dominans/G. moestrupii were most abundant during red tides dominated by Ceratium furca and Dinophysis acuminata. Naked ciliates were most abundant during red tides dominated by Cochlodinium polykrikoides, Prorocentrum triestinum, and winter populations of P. minimum. The maximum calculated grazing coefficients for each dominant heterotrophic protistan grazer on their respective co-occurring red-tide organisms in Masan Bay ranged from 0.0 to 6.8 h−1. The results of the present study suggest that populations of certain heterotrophic protistan grazers may have considerable potential grazing impact on populations of red-tide organisms in Masan Bay.