Population dynamics of Chattonella (Raphidophyceae), causative flagellates of noxious red tides in Ago Bay, central Japan, with an emphasis on cyst germination

Abstract

The temporal changes in the behavior of vegetative cells and cysts of the noxious red tide flagellates Chat-tonella (C. antiqua and C. marina) were investigated in Ago Bay, central Japan, mainly in 2013. In this study, in situ germination flux (cells m−2 day−1) was measured in May, June, and October 2013, using a ‘plankton emergence trap/ chamber (PET chamber)’. Vegetative cells occurred in April and increased from May, forming a bloom in July with cyst production, and subsequently decreased thereafter. This change in the occurrence of the vegetative cells was considered to be controlled primarily by temperature and secondarily by nutrients and biotic factors. The PET chamber experiment showed that cyst germination in May contributed largely to forming the initial vegetative population to initiate bloom-ing. An experiment to elucidate the effect of storage temperature on the germinability of cysts in the sediment showed that newly produced cysts (immature dormant cysts) mature (acquire germinability) under low storage temperature (10°C) within at least one month, indicating that cysts need to experience a cold season in the sediment for their matu-ration. The PET chamber experiment conducted in October before a cold season also confirmed that cysts scarcely ger-minated from the cyst population, containing many immature dormant cysts, in the in situ sediment. However, these cysts could act as seeds to form the initial population of the subsequent bloom in the next season, since the cysts will mature by then. Consequently, Chattonella cysts were concluded to play crucial roles in the population dynamics.