Harmful Algal Bloom (HAB) mitigation using ball clay: Effect on non-target organisms

A mitigation strategy for harmful algal blooms (HABs) is to remove cells from the water column by physically trapping them through flocculation with clay particles and subsequently settling algal cells to the bottom. Of prime consideration is the effect of clay addition on the non-target benthic and water column organisms. This study examined the effects of ball clay (using the previously derived optimum concentration of 1 g L-1) on non-harmful microalgae (Isochrysis galbana Parke) and diatom (Skeletonema tropicum Cleve), mussel (Perna viridis Linnaeus) and fishes such as Milkfish (Chanos chanos Forsskal), Sea bass (Lates calcalifer Bloch) and Rabbitfish (Siganus guttatus Bloch). Results show 50 % mortality of Isochrysis galbana (IC50) at 4.5 x 10(18) g L-1 of clay loading after 48 h. This implies that very high concentrations of clay are needed before toxic level is reached. The same is true with Skeletonema tropicum (IC50 = 40 g L-1 ball clay). Exposure of Perna viridis to 1 g L-1 ball clay showed accumulation of clay within 0.5 h after clay addition but with subsequent removal after 1-2 h through pseudofeces production. 75 % to 100 % survival was observed when juvenile and adult Chanos chanos, Lates calcalifer and Siganus guttatus were exposed to ball clay. Overall, the non-target organisms tested were not significantly affected by ball clay addition.