Different Nutrient Availabilities of Surface and Bottom Water under Nutrient-depleted Conditions during Bloom Formation of the Toxic Dinoflagellate Alexandrium tamarense in Osaka Bay, Japan

Alexandriium tamarense is a toxic dinoflagellate known to produce neurotoxins cause paralytic shellfish poisoning to human and marine animals. To understand the growth dynamics of A.tamarense, the seasonal changes in A,tamarense vegetative cells and environmental factors were evaluated using monthly field observations at two fixed stations in Osaka Bay, Japan, from January to May 2008. Additionally, a bioassay with axenic A.tamarense clonal cultures was performed to determine the growth potentials and growth-limiting nutrients of seawater samples collected during the field observations. The density of A.tamarense increased from February to April, and depletions of dissolved phosphate and silicate were observed in the surface layer during this period. The bioassay showed that phosphorous limitation occurred at the surface water of one station during March and April, while nitrogen limitation occurred in the bottom water. Moreover, at the other station, the growth potentials of the bottom water were higher than those of the surface water during February and April. Thus, the differences of nutrient availabilities between surface and bottom water during spring in Osaka Bay potentially allow A.tamarense to grow with nutrients uptake from bottom water by vertical migration.