The growth and cyst formation of a toxic dinoflagellate, Alexandrium tamarense, at low water temperatures in northeastern Japan.

Abstract A field survey was carried out in early spring to investigate the growth physiology and efficiency of cyst formation of Alexandrium tamarense in low water temperatures. A bloom of A. tamarense occurred in a stratified water column, formed by river inflow. The in situ growth rate estimated from daily cell abundance was high, 0.33 divisions day −1 , at 7.5–9°C. New cysts began to be observed during the late growth phase. Maximum cyst flux (600 cysts cm −2 day −1 ) was observed just after maximum cell abundance occurred. PO 4 –P Chl a −1 gradually decreased and reached extremely low levels beyond the mid-growth phase of A. tamarense . As sinking cysts were also recognized at that time, it suggests cyst formation may have been induced by depletion of phosphorus source. The incidence of cyst formation ( C · I ) was 30%. The number of C · I was the same as reported previously for batch cultures under conditions suitable for vegetative growth. These results indicate that A. tamarense grows with considerably higher growth rate and transforms to cysts in high numbers, in low water temperatures in the field.