Tracking Alexandrium catenella from seed-bed to bloom on the southern coast of Korea.

Alexandrium catenella was tracked from seed-bed to bloom at a hot spot of cyst deposition on the southern coast of Korea from June 2016 to Feb. 2020. Changes in cyst abundance and germinability from sediment, as well as the vegetative cell abundance and encystment in the water column were intensively monitored. Cyst germination of ca. 73% occurred synchronously in November of 2016 to 2019, when bottom water temperature was around 15 °C. After mass germination, vegetative cells formed a seed populations at low density (<10 cells L-1) during winter. Overwintering populations initiated growth in March and then proliferated into high density (ca. 4 × 104 cells L-1) spring blooms in mid-April 2017 when moderate temperature (15 °C) was recorded. There was no bloom in spring of 2018 and 2019, but small vegetative populations developed. Decline of the spring bloom was followed by massive encystment and an increase in Noctiluca abundance. An average spring encystment ratio of 0.002 was estimated for the study years. Newly formed cysts lay dormant during the warm season lasting about six months and then seeded the next population of vegetative cells. An average contribution ratio of cells recruited from the sediment was ca. 0.09 for seeding winter populations. The range in shift ratios for spring production of a daughter cyst population to prior cyst abundance of the mother population in fall was 0.1 to 0.6 for consecutive years, depending on annual variation of local environments. Tracking mass transformation of A. catenella cysts will contribute to more effective science based management of paralytic shellfish poisoning on the southern Korean coast.