Toxicity of clay flocculation of the toxic dinoflagellate, Karenia brevis, to estuarine invertebrates and fish

The environmental effects of clay flocculation used to remove red tide dinoflagellate blooms from the water column are relatively unknown on benthic organisms. The primary objective of this study was to determine the laboratory-derived toxicities of clay flocculation of the toxic dinoflagellate, Karenia brevis, for four common estuarine test species. Phosphatic clay mixed with a coagulant (polyaluminum hydroxy chloride (PAC)) was not acutely or chronically toxic in most cases to infaunal amphipods (Leptocheirus plumulosus and Ampelisca abdita), grass shrimp embryos (Palaemonetes pugio) and larval sheepshead minnows (Cyprinodon variegatus). K. brevis alone (density RANGE = 3880–5060 cells ml−1; brevetoxin (Btx) RANGE = 19.8–140.7 μg l−1) was very toxic to C. variegatus and, to a lesser extent, L. plumulosus. The addition of clay–coagulant did not usually reduce this toxicity. The combination of clay, coagulant and K. brevis cells when settled over a natural sediment were usually as toxic to the benthic test species as K. brevis alone. This result suggests that clay flocculation of K. brevis blooms will neither increase, nor decrease toxicity to benthic organisms relative to that attributable to an untreated bloom. Validation of this conclusion, however, is required since it is based on laboratory-derived, single species toxicity data using media collected from a simulated red tide event. The determination of environmental effects on indigenous benthic biota in near-coastal areas during a natural red tide event, prior to and after treatment with clay flocculation, would provide the perspective needed for a more realistic hazard assessment of this possible control procedure.