Comparison of algal harvest and hydrogen peroxide treatment in mitigating cyanobacterial blooms via an in situ mesocosm experiment.

The use of short-term, fast-acting curative treatments to rapidly suppress the proliferation of upcoming cyanobacterial blooms without negative side effects on overall water quality is important for environmental regulatory agencies. A 15-day in situ mesocosm experiment was conducted to evaluate the effects of algal harvest at different intensities and the effect of hydrogen peroxide on the mitigation of cyanobacterial blooms, subsequent algal growth and phytoplankton community structure. The results indicate that filtration through a 30-μm-pore-size net could remove most of the Microcystis colonies, leading to a decline in algal biomass. However, algal harvest at 30% and 60% intensities tended to promote cyanobacterial growth under nutrient-replete conditions, and the mitigation effect only lasted a few days, since cyanobacteria biomass exhibited no significant difference between the control and those two treatments on Day 6. When the algal harvest intensity was 90%, the cyanobacterial biomass remained at a relatively low level for 15 days. The average Microcystis colony size rapidly returned to the initial level after an initial decline across all the algal harvest intensities, indicating that algal harvest should be repeatedly performed within a short time period to mitigate Microcystis blooms. Furthermore, removing Microcystis colonies by filtration led to increased diversity in the phytoplankton community, as the proportion of non-Microcystis cyanobacteria increased with harvest intensity. This result might pose a challenge for cyanobacterial bloom control over the long term if filamentous cyanobacteria become dominant. The 10.0 mg L HO treatment selectively suppressed cyanobacteria throughout the experimental period, leading to succession from a cyanobacteria-dominated to a Chlorophyta-dominated community after Day 9. Overall, using hydrogen peroxide is more effective than algal harvesting as a one-time quick curative measure.