Abstract
Growth-limiting nutrients of the red tide flagellateHeterosigma akashiwoin Osaka Bay, the Seto Inland Sea, were examined using the method of algal growth potential. The growth yield ofH. akashiwowas enhanced by addition of chelated iron to filtered seawater, but not by the addition of nitrogen, phosphorus or vitamins. An areal survey conducted in June 1981 demonstrated that the concentration of soluble iron in the surface seawater was highest in the southeastern part of Osaka Bay where salinity was less than 30 %o and iron was correlated with the concentration ofH. akashiwo. Increased supply of chelated iron caused by the increase in river runoff during the rainy season is a crucial factor involved in outbreaks of H. akashiwo red tides in Osaka Bay.
