<i>Microcystis</i> bloom control using hydrogen peroxide and floating sodium percarbonate algaecide Lake Guard Oxy in Florida

Harmful algal blooms (HABs) pose a critical threat to aquatic ecosystems, human health, and economies globally, necessitating effective mitigation strategies. This study investigated the efficacy of two hydrogen peroxide treatments, liquid hydrogen peroxide and sodium percarbonate algaecide Lake Guard Oxy (LGOxy), in the field during a <i>Microcystis</i> bloom in the Caloosahatchee River, Florida, USA. Initial results indicated bloom suppression following both the liquid hydrogen peroxide and LGOxy treatments as 79% and 46% reduction in cyanobacterial relative abundance in 16S rRNA gene amplicon sequencing, 81% and 90% reduction in chlorophyll-a, and 91% and 97% reduction in <i>Microcystis</i> colony density. Both treatments were effective in reducing cyanobacterial abundance in the short term, from immediately after application to the following day. However, their effects did not persist longer term (several days to a week), likely because water exchange occurred through the connection to a flowing system. LGOxy exhibited a more targeted impact on <i>Microcystis</i> than other cyanobacterial species, likely because <i>Microcystis</i> forms thin surface blooms that remain in contact with the buoyant LGOxy particles. We also observed increased upregulation of the expression of nitrogen fixation genes in diazotrophs, such as <i>Dolichospermum</i>, based on transcriptomic analysis, particularly following LGOxy application. Ultimately, three treatments within 2 weeks (one liquid application and two LGOxy treatments) were not sufficient to fully eliminate the cHAB from the riverine study site after 14 days. This underscores the limitations of hydrogen peroxide-based treatments when target HABs are already established and the challenges facing deployment in open or actively flowing hydrologic systems.IMPORTANCEHarmful algal blooms (HABs) pose a significant threat to aquatic ecosystems, public health, and global economies, necessitating the development of effective mitigation strategies. Hydrogen peroxide has emerged as an environmentally friendly algaecide suitable for large-scale algal bloom control. Traditionally, hydrogen peroxide has been applied in liquid form; however, solid peroxide formulations are increasingly preferred by water managers due to their advantages in storage, transport, and handling safety. Despite these practical benefits, a direct comparison of the efficacy of liquid versus solid hydrogen peroxide formulations for algal bloom mitigation remains limited. This case study evaluated the effectiveness of two hydrogen peroxide-based treatments, liquid hydrogen peroxide and Lake Guard Oxy, during a field application targeting a <i>Microcystis</i> bloom in the Caloosahatchee River, Florida, USA.