Removal of Positively Buoyant Planktothrix rubescens in Lake Restoration

The combination of a low-dose coagulant (polyaluminium chloride&mdash
&lsquo
Floc&rsquo
) and a ballast able to bind phosphate (lanthanum modified bentonite, LMB&mdash
Sink/Lock&rsquo
) have been used successfully to manage cyanobacterial blooms and eutrophication. In a recent &lsquo
Floc and Lock&rsquo
intervention in Lake de Kuil (the Netherlands), cyanobacterial chlorophyll-a was reduced by 90% but, surprisingly, after one week elevated cyanobacterial concentrations were observed again that faded away during following weeks. Hence, to better understand why and how to avoid an increase in cyanobacterial concentration, experiments with collected cyanobacteria from Lakes De Kuil and Rauwbraken were performed. We showed that the Planktothrix rubescens from Lake de Kuil could initially be precipitated using a coagulant and ballast but, after one day, most of the filaments resurfaced again, even using a higher ballast dose. By contrast, the P. rubescens from Lake Rauwbraken remained precipitated after the Floc and Sink/Lock treatment. We highlight the need to test selected measures for each lake as the same technique with similar species (P. rubescens) yielded different results. Moreover, we show that damaging the cells first with hydrogen peroxide before adding the coagulant and ballast (a &lsquo
Kill, Floc and Lock/Sink&rsquo
approach) could be promising to keep P. rubescens precipitated.