IN-SITU EVALUATION OF OPTIONS FOR CHEMICAL TREATMENT OF HEPATOTOXIC CYANOBACTERIAL BLOOMS

Closed-bottom limnocorrals were placed in a hardwater lake in central Alberta to compare the effect of two alternative approaches to chemica l removal of toxic phytoplankton blooms. Reglone A, which lyses phytop lankton cells, and lime-alum, which precipitates intact phytoplankton cells out of the water column were both effective in removing phytopla nkton from the water column. Our results were consistent with laborato ry studies in that treatment with Reglone A removed phytoplankton (pri marily cyanobacteria) blooms with a concomitant increase in dissolved microcystin (exo-MCYST) and phosphorus concentrations in the surroundi ng water whereas lime-alum treatment did not. Maximum exo-MCYST concen trations in the water phase of the lime-alum treated limnocorrals were 32-fold lower than those recorded in the Reglone-treated Limnocorrals . Treatment with lime alone caused a sharp rise in pH (to >10), and th e observed increase in exo-MCYST was likely due to pH shock. Exo-MCYST concentration in the Reglone-treated enclosures remained high for the duration of the experiment (>5 days). As microcystin did not partitio n onto lake sediments in laboratory studies, our limnocorrals results were probably a good indication of microcystin dynamics in lakes. Thus , the use of chemicals such as lime-alum that precipitate out intact p hytoplankton cells lessens the potential health risk where microcystin s are present.