Spring bloom of Alexandrium tamarense in Chinhae Bay, Korea

Abstract

Chinhae Bay, one of the most eutrophic coastal embayments in Korea, is known for recurring red-tide blooms. To understand bloom development of the toxic dinoflagellate Alexandrium tamarense, weekly sampling was carried out from March to May of 1996 and 1997 in 2 regions, reflecting different flow regimes: one region (2 sites) was more susceptible to the influence of freshwater discharge than the other (2 sites). Nutrient concentrations remained high at nearly all depths throughout the study periods, suggesting that A. tamarense was not nutrient-limited for growth. A. tamarense cell density comprised < 5% of the total phytoplankton cell density at all times, in accordance with previous claims that this motile phytoplankton is unlikely to out-compete other spring bloom phytoplankton in high nutrient-level environments. The blooms occurred in a relatively wide temperature window (11 to 18C SST), but in a narrow range of salinity (31 to 33 psu), and were associated with a stratified water column. Prolonged high temperature or reduced water column stability were ascribable for the demise of the blooms. In cases when blooms disappeared, even under favorable environmental conditions, life-cycle transition via sexual reproduction may have been responsible for their demise. Physical dilution processes, such as advection and dispersion, appeared to determine the magnitude of blooms, as supported by the occurrence of larger blooms and higher in situ growth rates in locations less subjected to the direct influence of river runoff or tidal currents.