Effects of polyaluminum chloride and lanthanum-modified bentonite on the growth rates of three Cylindrospermopsis raciborskii strains

In tropical and subtropical lakes, eutrophication often leads to nuisance blooms of Cylindrospermopsis raciborskii. In laboratory experiments, we tested the combined effects of flocculant polyaluminum chloride (PAC) and lanthanum-modified bentonite (LMB) on the sinking and growth rates of three C. raciborskii strains. We tested the hypothesis that the combination of PAC and LMB would (1) effectively sink C. raciborskii in a test tube experiment and (2) impair C. raciborskii growth, irrespective of the biomass of the inoculum (bloom) and the strain in the growth experiment. We tested the recommended (LMB1) and a three-times higher dose of LMB (LMB3). The combined addition of PAC and LMB enhanced the sedimentation of all C. raciborskii strains. Moreover, both the PAC and LMB doses decreased the phosphate concentration. PAC and LMB1 decreased the growth rate of all strains, but the efficacy depended on the biomass and strain. The combined addition of PAC and LMB3 inhibited the growth of all strains independently of the biomass and strain. We conclude that a low dose of PAC in combination with the recommended dose of LMB decreases C. raciborskii blooms and that the efficiency of the technique depends on the biomass of the bloom. A higher dose of LMB is needed to obtain a more efficient control of C. raciborskii blooms.