Immediate and prolonged effects of hydrogen peroxide to a natural plankton community from a tropical reservoir: ecotoxicological aspects and compositional changes

Cyanobacterial blooms compromise water quality worldwide, demanding nutrient input control. However, once established, other strategies are necessary to control the growth of cyanobacteria. Oxidation processes like hydrogen peroxide (H₂O₂) offer a potential solution, however, research on the prolonged effect of the treatment is limited, including planktonic community recovery post-H₂O₂ exposure. Additionally, the efficiency of H₂O₂ on cyanobacteria and its impact on the plankton community has been mostly investigated in temperate regions. Here, we investigated the impact of single H₂O₂ additions at varying concentrations (1, 10, and 100 mg L⁻¹) on a natural phytoplankton and bacterioplankton community from a southern tropical eutrophic reservoir over 21 days in laboratory conditions. We aimed to evaluate the differential susceptibility of cyanobacteria and green algae, the possible phytoplankton regrowth, the changes in the bacterioplankton composition and the effect of residual H₂O₂ or its by-products in organisms of different trophic levels. Initially, cyanobacteria (mainly Microcystis and Cyanobium) were the main contributors to chlorophyll [Chl] concentrations, followed by green algae. Within 7 days, phytoplankton abundance decreased in all conditions, with pronounced effects in H₂O₂ treatments within the first 48 h. H₂O₂ exposure led to residual Chl concentrations for cyanobacteria and green algae, and after fresh culture medium addition, green algae were able to regrow in < 10 mg L⁻¹ treatments, but not cyanobacteria. In lower H₂O₂ concentrations Microcystis, Cyanobium, Limnothrix, and Planktothrix resisted, while in the higher concentration, Cyanobium and Mastigocladopsis persisted. In the bacterioplankton community, H₂O₂ addition led to a higher abundance of taxa from the hgcI clade, Comamonadaceae family, Craurococcus-Caldovatus and Staphylococcus. Ecotoxicological assays revealed transient effects of H₂O₂ on Daphnia similis survival, while Danio rerio remained unaffected in concentrations up to 100 mg L⁻¹. Our findings demonstrate that green algae exhibit greater resistance and resilience to H₂O₂ compared to cyanobacteria, with transient effects on zooplankton at concentrations up to 10 mg L⁻¹. This research underscores the complexity of managing cyanobacterial blooms and emphasizes the need for comprehensive strategies considering ecological impacts.