Quantifying the efficacy of diquat dibromide in controlling Microcystis aeruginosa and Aphanizomenon flos-aquae in comparison to copper sulfate and potassium permanganate

Cyanobacterial harmful algal blooms (HABs) are an emerging problem worldwide, affecting many important freshwater systems. The use of chemical algaecides can provide an effective short-term mitigation measure to control HABs. In this study, the efficacy of diquat dibromide was examined under laboratory conditions to control two problematic toxin-releasing cyanobacteria, namely Aphanizomenon flos-aquae and Microcystis aeruginosa. Its performance was then compared to that of copper sulfate (CuSO4) and potassium permanganate (KMnO4), two commonly used algaecides. The results suggest that while all three algaecides were effective in controlling Aphanizomenon, the highest inhibition rates achieved were associated with the application of diquat dibromide. Aphanizomenon exhibited a half-life of 0.48 days with diquat dibromide dosages of 0.5 and 1 mg L-1. Both diquat dibromide and CuSO4 exhibited more effectiveness in controlling Microcystis aeruginosa as compared to KMnO4. Reductions achieved by applying 0.5 mg L-1 of diquat dibromide or 1 mg L-1 of CuSO4 exceeded 95% after 48 h of treatment. Nevertheless, diquat dibromide suppressed the net regrowth of Microcystis aeruginosa up to 70 h, while suppression with CuSO4 did not exceed 64 h even with the highest applied dosage. Irrespective of the algaecide and the application dosage, regrowth was observed for Microcystis aeruginosa but not for Aphanizomenon flos-aquae. Statistical models were proposed to simulate inhibition rates and estimate net algal regeneration.