Chemical Environment for Red Tides due to Chattonella antiqua in the Seto Inland Sea, Japan

Part 1. Growth Bioassay of the Seawater and Dependence of Growth Rate on Nutrient Concentrations

Abstract

Severe red tides due to Chattonella 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 of C. antiqua, environmental factors that affect the growth of C. antiqua were monitored around the le-shima Islands, the Seto Inland Sea, in the summer of 1986. In addition, a growth bioassay of the seawater using C. antiqua was conducted under a semicontinuous culture system. Although temperature, salinity and light intensity were optimum for the growth of C. antiqua, red tides by this species did not occur. Concentrations of NH₄⁺, NO₃^- and PO₄^3- were low (<0.4, <0.2, <0.06μM, respectively) above the thermocline (8-12m) and high below it (0.6-2, 4-8, 0.4-0.8μM, respectively). Vitamin B₁₂ concentrations did not change significantly between the surface (0m) and below the thermocline (25m) in the level of 2-4ng·l^-1. The growth bioassay revealed that in the surface waters, concentrations of N- as well as 13- nutrients were too low to support a rapid growth of C. antiqua. At the depth of 25m, 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:μ=μ_max·S_N/K_g^N·S_PO4/K_g^P+S_PO4, where SN is the concentration of NO₃^- plus NH₄⁺ (0-6μM), S_PO4 the concentration of PO₄^3- (0-0.6μM), μ_max (0. 97 d^-1) the maximal growth rate, K_g^N (1.0μM) and K_g^P (0.11μM) the half saturation constants for NO₃^- and PO₄^3-, respectively. Using the above equation with nutrient concentrations measured, the rate at which seawater supports the growth of C. antiqua can be estimated and this can be used for the assessment of chemical environments with respect to the outbreak of C. antiqua.