At the start of this august, a dangerously high concentration of the dinoflagellate Alexandrium Ostenfeldii was found within a creek near Ouwerkerk, Zeeland. This species is known to produce very toxic substances (saxitoxine among others), and with concentrations of millions of cells per liter this could pose a threat to humans and wildlife. Luckily these algae could be isolated in time. But how to get rid of them?
In a cooperative effort between Rijkswaterstaat, Arcadis, the University of Amsterdam and the local authorities a newly developed method was tested. By adding low concentrations of hydrogen peroxide the bloom could be destroyed. The CytoSense instrument owned by Rijkswaterstaat was used for analyzing the effects of the treatment. By closely monitoring the concentrations and activity of the species using the Cytosense, the effects of the treatment could be tracked in real time. The potential of the CytoSense measurements to tune the exact amounts of H2O2 was demonstrated. The decline of Alexandrium cell numbers could be followed in realtime, and their deteriorating physiological state was evident from the images and fluorescence profiles. After two days the concentrations of Alexandrium Ostenfeldii was brought back to only 1% of its original value, making the water fit for mixing with other surface waters again. Since peroxide breaks down into water and oxygen, it poses no long-term threats to the ecosystem.