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 Yeong Du Yoo a , Hae Jin Jeong a, *, Jae Seong Kim b , Tae Hoon Kim c , Jong Hyeok Kim d , Kyeong Ah Seong d , Seung Hyun Lee a , Nam Seon Kang a , Jong Woo Park d , Jaeyeon Park e , Eun Young Yoon e , Won Ho Yih d a School of Earth and Environmental Sciences, College of Natural Sciences, Seoul National University, Seoul 151-747, Republic of Korea b Water and Eco-Bio Corporation, Kunsan National University, Kunsan 573-701, Republic of Korea c IGD Corporation, 499, Gunja-dong, Siheung-si, Gyeonggi-do, 429-823, Republic of Korea d Department of Oceanography, College of Ocean Science and Technology, Kunsan National University, Kunsan 573-701, Republic of Korea e Advanced Institutes of Convergence Technology, Suwon, Gyeonggi-do 443-270, Republic of Korea 1. Introduction Red tides occur in the coastal and offshore waters of many countries and in oceanic waters (Jeong, 1995; Horner et al., 1997; Imai et al., 2001; Sordo et al., 2001; Anderson et al., 2002; Alonso- Rodriguez and Ochoa, 2004; Seong et al., 2006; Jeong et al., 2013a,b; Kang et al., 2013; Lee et al., 2013; J.Y. Park et al., 2013). They can alter the balance of food webs and cause large-scale mortalities of fish and shellfish (Smayda, 1990; Glibert et al., 2005; Anderson et al., 2012; Fu et al., 2012). Studies of red-tide formation and persistence suggest that grazing pressure may play an important role in bloom dynamics (Watras et al., 1985; Turner, 2006; Jeong et al., 2011a; Kim et al., 2013). In particular, grazing by heterotrophic protists is believed to contribute to the decline of red tides (Holmes et al., 1967; Eppley and Harrison, 1975; Jeong, 1995, 1999; Kamiyama et al., 2000; Stoecker and Gustafson, 2002; Johnson et al., 2003; Kim and Jeong, 2004; Tillmann, 2004). Heterotrophic protists such as heterotrophic dinoflagellates (HTDs) and ciliates are known to be effective grazers on red-tide organisms (Jeong and Latz, 1994; Hansen et al., 1997; Jeong et al., 1999, 2001a, 2003b, 2007, 2008, 2010, 2011b; Tillmann, 2004; Yoo et al., 2010, 2013; T.G. Park et al., 2013). These heterotrophic protists usually divide 1–3 times each day after feeding on optimal or sub-optimal red-tide organisms and thus are often abundant during red tides (e.g. Jeong et al., 2010). They can have considerable grazing impact on red-tide organisms, controlling their popula- tions (Lessard and Swift, 1985; Calbet et al., 2003; Jeong et al., Harmful Algae 30S (2013) S89–S101 A R T I C L E I N F O Keywords: Ciliate Food web Grazing Harmful algal bloom Heterotrophic dinoflagellate Red tide A B S T R A C T 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 mm) 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. ß 2013 Elsevier B.V. All rights reserved. * Corresponding author. E-mail address: hjjeong@snu.ac.kr (H.J. Jeong). Contents lists available at ScienceDirect Harmful Algae jo u rn al h om epag e: ww w.els evier.c o m/lo cat e/hal 1568-9883/$ – see front matter ß 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.hal.2013.10.009