Chemical environment for red tides due toChattonella antiqua

In order to assess the roles of Fe and Cu in outbreaks ofChattonella antiqua red tide, concentrations of these metals in the surface seawater were monitored around the Ie-shima Islands in the Seto Inland Sea during the summers of 1986–1988. Bioassay of the surface seawater with respect to Fe and Cu was also conducted using a cultured strain ofC. antiqua.Concentrations of Fe and Cu in the filtered seawater (Fe^F and Cu^F) were in the range of 3.9–10.0 and 9.3–11.2 nM, respectively. The bioassay with respect to Fe revealed that Fe at the surface layer was usually insufficient to support the maximum growth rate ofC. antiqua, except whenC. antiqua was dominant in the field. However, correlations between Fe^F and the growth rate of the control cultures (Fe, EDTA=not enriched; N, P, B₁₂=enriched at optimum levels) were not apparent, probably because Fe^F did not reflect the concentration of available Fe.The bioassay with respect to Cu was coupled with the Cu^F values obtained. The results indicated that Cu at the surface layer was detoxified by complexation with natural organic ligand(s), and that pCu (=minus log of cupric ion activity) was 11.5–11.7, optimum for the growth ofC. antiqua, throughout the survey period. It is suggested that Fe, but not Cu, is a potentially important factor in regulating the natural populations ofC. antiqua in the Seto Inland Sea.