Response of Chattonella marina (Raphidophyceae) and marine plankton to yellow clay and thiazolidinedione derivative TD49 in a mesocosm enclosure

We examined the effects of the algicide thiazolidinedione (TD49) and yellow clay on Chattonella marina and assessed their ecological risk for the entire planktonic community. Mesocosm (1000 L) exposure experiments were employed to investigate time-course responses over 9 days. The growth of C. marina was controlled at ≥0.4 μM TD49 but not inhibited in yellow clay treatments. Although the algicidal activity of the 0.4-μM TD49 + 0.4 kg t−1 yellow clay treatment for C. marina was high (72.5 % at 24 h), target alga regrowth occurred. In all treatments, inorganic nutrients such as nitrate + nitrite and phosphate decreased following commencement of the experiment but were >1 μM (limitation concentration) at days 5 and 6, even though consumption pattern of those nutrients was influenced by the TD49 concentration. Depletion of silicate in initial stages played an important role in controlling the shift from diatoms including Chaetoceros and Skeletonema spp. to cryptophytes. Zooplankton were not affected by even the highest the yellow clay treatments and TD concentration of <0.8 μM, but their abundance significantly reduced after day 1 at 0.8 μM TD49. Zooplankton nauplii gradually increased to the end of the experimental period, implying that TD49 may have a limited effect on zooplankton communities. The initial dosing concentration of each substance and the fate of nutrients following algicide application were critical in determining the timing of shifts in the phytoplankton and zooplankton species composition, as well as the algicidal effect on the target alga.