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Control of Florida Red Tides Using Phosphatic Clay


Red-tides of the dinoflagellate Karenia brevis are a serious and recurrent problem in Florida coastal waters, causing severe impacts on public health, marine life, and regional economies. This study evaluated the effectiveness, practicability, and possible impacts of clay dispersal as a means of controlling these phenomena. The method, used effectively in Japan and Korea to protect fish mariculture, is based on the co-flocculation between microalgae and mineral particles, leading to the formation of larger aggregates, which settle faster and further entrain cells during descent. In modified jar tests, phosphatic clays from varying stages in beneficiation, pond age, and geographic origin, all displayed > 80% removal efficiency (RE), with ≥ 0.25 g/L at 9,000 cells/mL. Phosphate rock only showed 10% RE. Lower clay amounts combined with 5 ppm of polyaluminum chloride (PAC) increased RE by one order of magnitude, while alum and two cationic polymers were less effective. Cell removal > 70% required a minimum of 1,000 cells/mL at ≥ 0.10 g/L. Post-treatment mortality depended on clay concentration, resuspension frequency, and the duration of clay/cell contact prior to first resuspension. Bulk chemical analysis showed elevated Cr content relative to standard sediments, while analysis of the supernatant revealed significant phosphate release (moderated by PAC), and possibly 210Pb. Flume experiments defined flow speeds that influence cell removal (settling) and erodibility (including the effect of consolidation over time and PAC addition). The primary logistical and engineering challenges of possible field implementation were associated with handling and processing of wet clays, and finding a suitable method of clay dispersal at larger scales. The highest costs were estimated in clay transport, followed by ship/crew hiring. In summary, this study demonstrated the efficacy of phosphatic clays to remove and kill Karenia brevis in laboratory experiments, the physicochemical, hydrodynamic and scale conditions that can influence cell removal and sedimentation, balanced by the possible impacts on seawater chemistry, sediment properties, and economic costs. These findings provide crucial information needed to assess this bloom control strategy, but also indicate a need to examine this concept further under well-defined field conditions.