Poster Presentation Society for Freshwater Science 2025 Annual Meeting

Floc and Sink technique associated with hydrogen peroxide to remove harmful cyanobacterial blooms: An in-situ mesocosm approach (118988)

Marcela Miranda 1 , Felipe Siqueira Pacheco 1 , Natália P Noyma 2 , Erick Drummond 2 , Caio Graco-Roza 3 , Leonardo de Magalhães 2 , Vera Huszar 4 , Maíra Mucci 5 , Jean Ometto 6 , Ernani Pinto 7 , Miquel Lürling 5 , Marcelo M Marinho 2
  1. Department of Ecology and Evolutionary Biology., Cornell University, Ithaca, NY, USA., Ithaca, New York, Estados Unidos
  2. Department of Plant Biology, University of Rio de Janeiro State, Rio de Janeiro, Brazil
  3. Department of Geosciences and Geography, University of Helsinki, Helsinki, Finland
  4. National Museum, Federal University of Rio de Janei, Rio de Janeiro, Brazil
  5. Department of Environmental Sciences, Wageningen University, Wageningen, The Netherlands
  6. Impacts, Adaptation and Vulnerabilities Division, National Institute for Space Research (INPE), São José dos Campos, São Paulo, Brazil
  7. Center for Nuclear Energy in Agricultur, University of São Paulo, Piracicaba, São Paulo, Brazil

Eutrophication control and mitigation of harmful cyanobacterial blooms remain critical challenges for water quality management. While reducing nutrient loads is the most effective preventive measure, it is not always feasible, necessitating rapid curative interventions. This study assessed the effectiveness of the Floc and Sink technique combined with hydrogen peroxide (H₂O₂) for removing harmful cyanobacterial blooms in a shallow eutrophic tropical pond. A mesocosm experiment was conducted using poly-aluminum chloride (PAC) as a coagulant and local red soil (RS) as ballast. Two treatments were applied: PAC+RS and H₂O₂+PAC+RS, with the latter aiming to prevent biomass resuspension. Optimal doses of PAC, RS, and H₂O₂ were determined through preliminary laboratory tests. Both treatments significantly reduced chlorophyll-a, total nitrogen, and total phosphorus concentrations. However, the addition of H₂O₂ led to a pronounced decline in photosystem II (PSII) efficiency during the first two days, indicating cyanobacterial cell damage. This resulted in cyanotoxin release, detected at high concentrations. Despite the initial biomass removal, the effect was not sustained. After seven days, live cyanobacterial flocs resuspended and proliferated in the water column. Over the 28-day study period, neither treatment effectively suppressed cyanobacterial blooms in the long term. These findings suggest that while the Floc and Sink technique, with or without H₂O₂, provides a temporary reduction in biomass, it does not prevent bloom resurgence in shallow tropical systems.Continuous monitoring of cyanobacterial biomass and PSII efficiency is recommended to determine  the optimal timing for reapplication of H₂O₂ alongside the Floc and Sink technique. This approach may help extend the suppression of cyanobacterial biomass in eutrophic systems such as Mapro Pond, contributing to improved water quality management strategies.