Abstract:

The toxic dinoflagellate — Alexandrium tamarense (Lebour) Balech, formed resting cysts in f/2 media with low nitrate concentrations. Among the concentrations tested, f/20 NO₃^- was the most effective to induction with an encystment percentage of 2.0 in batch culture. About 73.2% and 17.6% of cysts were produced on 8 and 9 d after transferring. Newly formed cysts developed accumulation body 3 d later and kept forming mucilaginous substance, which might help their dispersal and survival. The mandatory dormancy period of resting cysts was 15 and 10 d when stored at 15 and 20 ℃ respectively. The cysts could germinate without temperature change, with germination of 75.6% 20 d after formation at 20 ℃. The Alexandrium cyst density in the surface sediment of DG-26 station reached above 25 cysts/g in May and November of 2002, and dropped to 4.5 and 0.9 cysts/g in August of 2002 and February of 2003, suggesting that Alexandrium cysts might have germinated in spring and autumn 2002. Cysts produced during the bloom returned to water column soon, whatever the season and water temperature were. The cyst density in the surface sediment at DG-26 station in May, 2003 was only 3.3 cysts/g and the cysts were newly formed. In the Yangtse River estuary, the inoculum size was not a major factor to determine the outbreak of A. tamarense bloom.

Key words: cyst, Alexandrium tamarense , Yangtse River estuary, bloom