1National Research Institute of Fisheries and Environment of Inland Sea, Fisheries Research Agency, Hiroshima 739-0452, Japan, 2The Oceanic Institute, Makapu'u Point 41-202 Kalanianaole Hwv Waimana'lo Hawai'i 96795, USA, 3Tohoku National Fisheries Research Institute, Fisheries Research Agency, Miyagi 985-0001, Japan
The resting cysts of Alexandrium spp., macro- and meiobenthos, and the bottom environment were monitored to investigate the influence of biological and physicochemical factors on the distribution of Alexandrium spp. cysts in Hiroshima Bay during the bloom period of Alexandrium tamarense from March to May 1998. The mean density of Alexandrium cysts was 60~1,116cysts/g wet sediment at five stations during the investigation period. High cyst density was observed at three nearshore stations where high concentrations of organic substances in bottom sediments were recorded. The density of Alexandrium cysts showed no clear correlation to the density of macrobenthos in March, but increased at the stations with a high pollution index and low density of macrobenthos in April. In May the density of Alexandrium cysts decreased at the station where the macrobenthos density increased, which might suggest that predation and/or bioturbation by the macrobenthos caused the decrease of Alexandrium cysts. No influence of meiobenthos on the density of cysts was observed.