Chemical environment for red tides due toChattonella antiqua in the seto inland sea,Japan

Severe red tides due toChattonella antiqua occur sporadically during summer in the Seto Inland Sea, Japan, and cause significant damage to the fishing industry. In order to assess the chemical environment with respect to the outbreak ofC. antiqua, environmental factors that affect the growth ofC. antiqua were monitored around the Ie-shima Islands, the Seto Inland Sea, in the summer of 1986. In addition, a growth bioassay of the seawater usingC. antiqua was conducted under a semicontinuous culture system. Although temperature, salinity and light intensity were optimum for the growth ofC. antiqua, red tides by this species did not occur. Concentrations of NH ₄ ⁺ , NO ₃ ^? and PO ₄ ^3? were low (<0.4, <0.2, <0.06 µM, respectively) above the thermocline (8–12 m) and high below it (0.6–2, 4–8, 0.4–0.8 µM, respectively). Vitamin B₁₂ concentrations did not change significantly between the surface (0 m) and below the thermocline (25 m) in the level of 2–4 ng·l^?1. The growth bioassay revealed that in the surface waters, concentrations of N- as well as P- nutrients were too low to support a rapid growth ofC. antiqua. At the depth of 25 m, neither N, P nor B₁₂ limited the growth rate. In order to obtain more quantitative information on the growth rate as a function of the concentrations of N- and P- nutrients,C. antiqua was grown in a semicontinuous culture system by changing nutrient concentrations systematically. The observed growth rate (μ) can be approximated as follows: $$mu = mu _{max } .frac{{S_N }}{{K_g ^N + S_N }}.frac{{S_{PO4} }}{{K_g ^P + S_{PO4} }},$$ whereS _N is the concentration of NO ₃ ^? plus NH ₄ ⁺ (0–6 µM),S _PO, the concentration of PO ₄ ^3? (0–0.6 µM), μ_max (0.97 d^?1) the maximal growth rate,K ₀ ^N (1.0 µM) andK ₀ ^P (0.11 µM) the half saturation constants for NO ₃ ^? and PO ₄ ^3? , respectively. Using the above equation with nutrient concentrations measured, the rate at which seawater supports the growth ofC. antiqua can be estimated and this can be used for the assessment of chemical environments with respect to the outbreak ofC. antiqua.