Distribution and Abundance of resting cysts of Alexandrium tamarense and/or A. catenella (Dinophyceae) in Tokyo Bay, Japan

Sediment samples collected at 33 stations in Tokyo Bay during the autumn of 1999 were examined to determine the distribution and abundance of resting cysts of Alexandrium tamarense and/or A. catenella. Extremely low concentrations of cysts were observed in Tokyo Bay compared with previously investigated sites in the Seto Inland Sea and Lake Hamana which have almost the same marine environmental conditions as found in Tokyo Bay. Cysts were only found in the upper 2.0–3.0 cm of sediment at three stations along the northwestern coast of the bay, at concentrations ranging from 0.8–2.1 cysts cm 3 of wet sediment. All of the observed cysts consisted of the outer thecal integument with no vegetative contents and are therefore concluded to be unable to play a role in the seeding of blooms. Thus, currently the possibility of paralytic shellfish poisoning, caused by a bloom of A. tamarense and/or A. catenella initiated from benthic cysts, is considered to be low in Tokyo Bay. However, commercial shellfish fisheries and recreational shellfish gathering are expected to remain important activities in this bay. Careful monitoring of toxic dinoflagellates and shellfish toxicity is necessary, because invasion by toxic dinoflagellates into Tokyo Bay via ballast water or oceanic water are potential events in the future.