Termination of a toxic Alexandrium bloom with hydrogen peroxide

The dinoflagellate Alexandrium ostenfeldii is a well-known harmful algal species that can potentially cause paralytic shellfish poisoning (PSP). Usually A. ostenfeldii occurs in low background concentrations only, but in August of 2012 an exceptionally dense bloom of more than 1 million cells L⁻¹ occurred in the brackish Ouwerkerkse Kreek in The Netherlands. The A. ostenfeldii bloom produced both saxitoxins and spirolides, and is held responsible for the death of a dog with a high saxitoxin stomach content. The Ouwerkerkse Kreek routinely discharges its water into the adjacent Oosterschelde estuary, and an immediate reduction of the bloom was required to avoid contamination of extensive shellfish grounds. Previously, treatment of infected waters with hydrogen peroxide (H₂O₂) successfully suppressed cyanobacterial blooms in lakes. Therefore, we adapted this treatment to eradicate the Alexandrium bloom using a three-step approach. First, we investigated the required H₂O₂ dosage in laboratory experiments with A. ostenfeldii. Second, we tested the method in a small, isolated canal adjacent to the Ouwerkerkse Kreek. Finally, we brought 50 mg L⁻¹ of H₂O₂ into the entire creek system with a special device, called a water harrow, for optimal dispersal of the added H₂O₂. Concentrations of both vegetative cells and pellicle cysts declined by 99.8% within 48 h, and PSP toxin concentrations in the water were reduced below local regulatory levels of 15 μg L⁻¹. Zooplankton were strongly affected by the H₂O₂ treatment, but impacts on macroinvertebrates and fish were minimal. A key advantage of this method is that the added H₂O₂ decays to water and oxygen within a few days, which enables rapid recovery of the system after the treatment. This is the first successful field application of H₂O₂ to suppress a marine harmful algal bloom, although Alexandrium spp. reoccurred at lower concentrations in the following year. The results show that H₂O₂ treatment provides an effective emergency management option to mitigate toxic Alexandrium blooms, especially when immediate action is required.