Complex effects of dissolved organic matter, temperature, and initial bloom density on the efficacy of hydrogen peroxide to control cyanobacteria.

Harmful cyanobacterial blooms plague reservoirs and lakes used for a variety of purposes, such as recreation and drinking water. Chemical controls are frequently used to mitigate the occurrence of cyanobacterial blooms given that many are fast-acting and effective at reducing cyanobacterial abundance. Recent research has identified hydrogen peroxide (H 2 O 2 ) as an environmentally friendly alternative to algaecides that have typically been used, such as copper sulfate. To build on past studies, these experiments sought to further understand how well H 2 O 2 treatments reduce cyanobacteria in complex eutrophic conditions, as well as to assess treatment effects on a non-target phytoplankter, a green alga. We assessed the effectiveness of H 2 O 2 (at treatments of 2-16 mg L -1 ) under varying environmental conditions in a controlled laboratory setting, including (1) dissolved organic matter (DOM) concentrations (humic acid; 0-60 mg L -1 ), (2) temperature (20, 25, and 32 °C), and (3) initial algal biomass (chlorophyll-a; 82-371 µg L -1 ). In contrast to our expectations, neither DOM concentration nor temperature meaningfully impacted the effectiveness of H 2 O 2 at reducing cyanobacteria. However, initial algal biomass as well as H 2 O 2 treatment dose greatly influenced the effectiveness of the algaecide on cyanobacteria. Treatments of ≥ 8 mg H 2 O 2 L -1 on algal biomass were significantly buffered with higher DOM and lower temperature, and the biological significance of these findings should be explored further. Across all experiments, H 2 O 2 concentrations of 0.03-0.12 mg H 2 O 2 L -1  µg chlorophyll L -1 were effective at significantly reducing cyanobacteria with varying effects on algal biomass. Thus, water resource managers are encouraged to consider how ambient levels of phytoplankton biomass may affect the ability of H 2 O 2 to control cyanobacterial blooms prior to treatment.