Algicidal activity of the thiazolidinedione derivative TD49 against the harmful dinoflagellate Heterocapsa circularisquama in a mesocosm enclosure

To assess the algicidal effects of the thiazolidinedione derivative TD49 on the unarmored dinoflagellate Heterocapsa circularisquama and to evaluate the response of the planktonic community and the environment to this chemical, we undertook mesocosm (1,300 L) and small-scale experiments. The reduction ratio for H. circularisquama in each experiment was dependent on the concentration of TD49. At a TD49 concentration >0.4 μM, the abundance of H. circularisquama decreased by 99 % in the small-scale experiment and by 84 % in the mesocosm during the initial 2 days. At 0.2 μM TD49, the abundance of H. circularisquama decreased by up to 85 % in the small-scale experiment, whereas the abundance in the mesocosm increased, implying the absence of an algicidal effect. The decrease in planktonic organisms, including H. circularisquama, following TD49 treatment was correlated with abrupt declines in culture pH and dissolved oxygen concentration. Following addition of TD49, there was a significant increase in the abundance of diatoms and cryptophyta species, and after 8 days, the dominant species in the TD49 treatments shifted to small pennate diatoms including Cylindrotheca and Entomoneis species. The growth of some species among the zooplankton community was promoted at low TD49 concentrations (≤0.4 μM), whereas high concentrations (≥1.0 μM) had a negative effect. This study demonstrates that TD49 is an effective agent for the control for H. circularisquama blooms and that large-scale mesocosms play a crucial role in assessing the application of algicides such as TD49 in natural environments.