Seasonal distribution of the shellfish-killing dinoflagellate Heterocapsa circularisquama in Ago Bay monitored by an indirect fluorescent antibody technique using monoclonal antibodies

Heterocapsa circularisquama is the most noxious dinoflagellate to bivalves such as oysters, short-necked clams and pearl oysters because its red tides have caused major mass mortalities. In order to reduce the negative impacts of H. circularisquama, predictions of red tide occurrences are essential, and hence it is important to grasp its population dynamics. The population dynamics of H. circularisquama, however, have not yet been clarified throughout the year. To precisely monitor population dynamics, the fluorescent antibody technique is effective. We here report on H. circularisquama population dynamics monitored with the indirect fluorescent antibody technique (IFAT) using monoclonal antibodies. Samplings were carried out once a week in summer and twice a month in other seasons at 4 points in Ago Bay, Mie Prefecture, Japan, from April 2001 to March 2005. Direct counting of the cells was also performed using a normal optical microscope. Vegetative cells of H. circularisquama monitored with IFAT were generally detected from late spring (May) to late autumn, and the cell density increased (maximum 2.33×106 cells L-1) during summer. The lowest detection level was 1.33 cells L-1 with IFAT. In contrast, monitoring by counting with a common optical microscope often failed to detect H. circularisquama cells even at a density of 103 cells L-1 or more. Water temperature was significantly correlated to the abundances of H. circularisquama, and the cells were almost always observed at a temperature of 25°C or higher (summer season), and never detected at 10°C or lower (winter and early spring) in Ago Bay. In the waters of the bottom layer, H. circularisquama cells tended to be more abundant with lower dissolved oxygen (DO) values during summer. Thus, the seasonal distribution of H. circularisquama was clarified almost perfectly together with environmental factors in Ago Bay between spring and early winter.