MEPS 210:41-53 (2001)  -  doi:10.3354/meps210041

ABSTRACT: Twenty-five domestic clays and Loess clay from South Korea were tested for their ability to flocculate and remove cells of Gymnodinium breve (the Florida red-tide organism) and Aureococcus anophagefferens (the New York brown-tide organism). Twelve clay samples, consisting mostly of montmorillonite, bentonite and Florida phosphatic clay displayed removal efficiencies greater than 90% against G. breve at a clay loading of 0.25 g l^-1. Further tests with IMC-P2 phosphatic clay indicated that removal rates can reach as high as 80% at 0.04 g l^-1. In contrast, the removal values at 0.25 g l^-1 against A. anophagefferens did not exceed 40% for all clays, but increased to 80% when the clay was dispersed throughout the culture at the time of addition. The removal efficiency of aluminum sulfate (alum), polyaluminum chloride (PAC), and 4 organic flocculants were significantly lower than clays against both organisms (30 to 50%). However, the addition of 5 ppm PAC lowered the amount of clay needed for removal of G. breve by 1 order of magnitude at low clay concentrations. G. breve fully recovered and remained viable at clay loadings below 0.03 g l^-1, with or without resuspension of the flocs, although their recovery and subsequent growth were delayed by 24 h compared to untreated cells. High cell mortality (up to 100%) and no recovery were observed at clay amounts of ≥0 g l^-1, even with daily resuspension of the clay/cell pellet. At intermediate clay loadings (e.g. 0.10 to 0.25 g l^-1), survival and recovery depended on several factors: clay amount, the frequency of resuspension, or the duration of contact between the cells and clays prior to the first resuspension event. Regardless of clay loading, cell mortality was extremely low (near zero) after 2.5 h of contact, but increased significantly after 12 h. Preliminary data suggest that cell death may be caused by direct physical contact between the cells and clays and not by the release of potentially cytotoxic substances from the clays or from the lysed cells. Overall, these results show that clays differ substantially in their removal efficiencies, that individual clays differ in their ability to remove different algal species, that flocculants such as PAC can significantly improve clay removal efficiencies, and that the flocculation process can also lead to cell mortality.

KEY WORDS: Harmful algal blooms (HABs) · Clay · Management · Control · Flocculation · Gymnodinium breve · Aureococcus anophagefferens