Effects of suspended and sedimented clays on juvenile hard clams, Mercenaria mercenaria, within the context of harmful algal bloom mitigation

Increased interest in using ecologically inert clays to flocculate, sediment, and thus mitigate harmful algal blooms at nearshore mariculture sites has prompted studies on the effectiveness of this method on prolific blooms, such as those caused by the neurotoxic dinoflagellate Karenia brevis in the Gulf of Mexico. Potential repercussions of this control strategy revolve around the increased flux of suspended particles to the benthos. Juvenile suspension-feeding bivalves are potentially vulnerable as they could suffer burial, a decrease in clearance rates, and/or an increase in pseudofeces production in response to suspended clay, leading to reduced growth and delay in attaining size refuge from predators. Here we assess lethal and sublethal effects on juvenile hard clams, Mercenaria mercenaria, in a 2-week flume application of phosphatic clay to simulated blooms of the nontoxic dinollagellates Heterocapsa triquetra and Prorocentrum micans. Flow regimes simulated two contrasting, "worst-case" field conditions where (1) low flow allowed complete sedimentation and formation of a benthic sediment layer, and (2) high flow allowed complete particle suspension. No clam mortalities occurred in either treatment. The sedimentation treatment showed variable growth inhibition in shell and/or tissue, but effects were not apparent compared to controls (no sediment layer), and clams rapidly resumed siphon contact with the overlying water column. In contrast, a strong growth effect (similar to90% reduction in shell and tissue growth) occurred in particle-suspension trials compared to no-clay controls. These results suggest that repeated clay applications in the field are likely more detrimental to clams in a high-energy environment leading to prolonged in situ resuspension of clay than in an environment favoring sedimentation.