Excystment patterns of the freshwater dinoflagellate Peridinium bipes (Dinophyceae) in Juam Reservoir, Korea

Abstract

We examined seasonal variations in vegetative populations and cyst germination of the dinoflagellate Peridinium bipes Stein in reservoir water samples collected from August 2003 to March 2005, a period that included a sudden bloom of this species (September 2003 to March 2004). Monthly variations in cyst abundance and germination were tracked, and the effects of environmental factors (water temperature, pH, light and nutrients) on cyst germination were measured in the laboratory under ambient field conditions. During the bloom period, the cyst abundance of P. bipes in sediment samples fluctuated from 4 to 427 cells g(-1) (wet weight), and did not show season-dependent variation. During the same period, the number of vegetative cells of P. bipes in water samples varied from 0 to 9.79 x 10(2) ml(-1). Laboratory experiments revealed a maximum germination rate at 15.6 degrees C, and effective germination was observed at the naturally occurring pH values of 6 to 8, but not at pH 9. Cysts obtained from samples collected at higher temperatures (over 15 degrees C) germinated more quickly than those seeded at lower temperatures, while cysts collected in fall and early winter had a higher cumulative excystment rate than those collected in spring and summer, suggesting that cysts deposited at higher temperatures may act as a seed population for the winter blooms. These findings collectively indicate that germination of R bipes was mainly affected by water temperature and light intensity, and not nutrient levels and pH, and further show that the bloom of P. bipes observed in Juam Reservoir was likely promoted by the presence of sufficient nutrients, relatively high excystment rates and active growth occurring under low temperature conditions.