Semiweekly Variation of Spring Population of a Mixotrophic Ciliate Myrionecta rubra (= Mesodinium rubrum) in Keum River Estuary, Korea

doi:2005.20.3.207

Algae > Volume 20(3); 2005 > Article

Algae 2005;20(3): 207-216. doi: https://doi.org/10.4490/algae.2005.20.3.207

Wonho Yih^*, Geumog Myung, Hyung Seop Kim¹, Hae Jin Jeong²

Department of Oceanography, Kunsan National University, Kunsan 573-701,
¹Kunsan Regional Maritime Affairs and Fisheries Office, MOMAF, Kunsan 573-030 and
²School of Earth and Environmental Science, Seoul National University, Seoul 151-747 Korea

ABSTRACT

Myrionecta rubra, a mixotrophic ciliate, is a cosmopolitan red tide species which is commonly found in neritic and estuarine waters. M. rubra had long been listed as an “unculturable protist” until 2 different laboratory strains were finally established in 2 research groups at the beginning of this century, enabling us to perform initiative investigation into various aspect of the live M. rubra strains (Gustafson et al. 2000; Yih et al. 2004b; Johnson and Stoecker 2005). Field sampling was carried out on high tide at 2 fixed stations around Kunsan Inner Harbor (St.1 near the Estuarine Weir and St.2 off Kunsan Ferry Station) every other day for 4 months from mid-February 2004 to understand detailed figure of the recurrent spring blooms of M. rubra following the onset of the water gates operation of the Keum River Estuarine Weir on August 1994. With its maximum abundance of 272 cells mL?1 in St.1, fluctuation pattern of the M. rubra population at the 2 stations was strikingly similar. Notable growth of M. rubra population started on late April, to cause M. rubra red tides during one month from mid-May in which “exceptionally low salinity days” without its red tide were intermittently inserted. High abundance of M. rubra over 50 cells mL?1 was recorded at samples with their water temperature and salinity higher than 15°C and 4.0 psu, respectively. During pre-bloom period when salinity fluctuation is moderate and the water temperature is cooler than 15°C, Skeletonema costatum, a chain-forming centric diatom, was most dominant. Cyanobacterial species such as Aphanizomenon flos-aquae and Phormidium sp. replaced other dominant phytoplankters on the days with “exceptionally low salinity” even during the main blooming period of M. rubra. To summarize, M. rubra could form spring blooms in Keum River Estuary when the level of salinity fluctuation was more severe than that for the dominant diatom Skeletonema costatum and milder than that for the predominance by freshwater cyanobacteria. Therefore, optimal control of the scale and frequency of freshwater discharges might lead us to partially modify the fluctuation pattern of M. rubra populations as well as the period of spring blooms by M. rubra in Keum River Estuary. Sampling time interval of 2 days for the present study or daily sampling was concluded to be minimally required for the detailed exploration into the spring blooms by M. rubra populations in estuaries with weirs like Keum River Estuary.

Key words: Estuarine Weir, freshwater discharge, Keum River, laboratory strain, mixotrophic ciliate, Myrionecta