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Clay-based compounds are globally the most advanced and widely used method of direct suppression of marine harmful algal blooms, and are currently undergoing investigation as an option to control Karenia brevis blooms in Florida. Before clay may be accepted for widespread use, there are multiple concerns and challenges that must be addressed regarding the environmental safety of this method, such as effects on water quality, the fate of toxins, and potential impacts of clay treatment to non-target organisms. To contribute to ongoing assessments of clay as a potential control method for K. brevis blooms, we conducted experiments with a formulation of kaolinite clay modified with polyaluminum chloride known as Modified Clay II (MC II). In these experiments, we evaluated water chemistry dynamics and physiological responses in several bottom-dwelling marine species with ecological and economic significance, including blue crab (Callinectes sapidus), sea urchin (Lytechinus variegatus), and hard clam (Mercenaria campechiensis). First, we conducted an experiment with blue crabs in 20 L aquarium tanks (N = 48), exposing the animals to cultured K. brevis (1 x 106 cells L-1) and MC II (0.5 g L-1) and measuring mortality and reflexes over 192 hours. In our second experiment, K. brevis (1x106 cells L-1) and MC II (0.2 g L-1) were applied to 1,400 L mesocosms (N = 9) containing blue crabs, sea urchins, and hard clams, which were observed over 96 hours. In our final experiment, we modified the methods of the previous experiment to again examine K. brevis (1x106 cells L-1) and MC II (0.2 g L-1) with the same model species in 1,400 L mesocosms (N = 12) over 72 hours. In these two mesocosm experiments, we observed cell and toxin removal, changes in water quality characteristics including nutrients and carbonate chemistry, and measured mortality, respiration rate, reflexes, and internal toxin content. Our results were congruent across our three experiments. Treatment with MC II significantly reduced cell concentrations but did not reduce toxin concentrations in the water column. We found no notable impacts of clay treatment to reflexes, respiration rates, or internal toxin content for either of our three species. No significant differences in mortality were found for our three species, excluding crabs in the first mesocosm study, which were found to have pre-existing infections that confounded our results. Analyses of nutrients indicates MC II may remove dissolved phosphorus from the water column, and the potential to improve water quality which may make this formulation of clay desirable to managers. Overall, treatment with this formulation of clay did not appear to induce any significant measured effects on the model species within the observed time frames of these experiments. Clay appears to be a promising option to treat K. brevis blooms given its low cost, ease of application, and negligible impacts to the environment, and its use may relieve the damaging effects of K. brevis blooms by preventing mortalities that would otherwise occur were blooms allowed to persist. We therefore recommend that clays, including MC II, be considered for additional laboratory and field tests, with the goal of obtaining further information on potential ecological impacts so that managers and researchers can make informed decisions on the use of bloom control technologies in Florida waters.

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