Efficacy of a novel, aquatically formulated humic acid product (AC Aqua) for mitigating harmful algae blooms

Objective The purpose of this study was to evaluate the efficacy of the aquatically formulated humic acid product AC Aqua in reducing total phosphorus concentrations and controlling the proliferation of two harmful algal species, golden algae Prymnesium parvum and Microcystis aeruginosa. Methods Three treatments of AC Aqua (prophylactic, low dose, and high dose) and a control were tested in 13.3-L mesocosms with and without sediment (n = 6 per treatment) to simulate lined and earthen pond conditions. Prophylactic treatment (5 mL/m3) was applied 10 d before P. parvum (91 cells/mL) and M. aeruginosa (246 cells/mL) inoculation. Low (5 mL/m3) and high (7.67 mL/m3) doses were applied 6 h after inoculation. Phosphorus concentrations were measured before and after application to assess binding, while algal cell counts were monitored weekly for 7 weeks for each trial. Results Within 6 h, all AC Aqua treatments reduced total phosphorus, with the high-dose and sediment treatments having a significant reduction in phosphorus, compared with control groups, in both trials. Significant reductions in P. parvum cell counts were observed in the prophylactic treatments beginning in week 3 (P = 0.017). By week 7, all treatments had substantially lower cell counts compared with controls, with several treatment groups, particularly those with sediment, falling below 10,000 cells/mL, while control groups remained above 90,000 cells/mL. Significant reductions in M. aeruginosa cell counts were observed in the prophylactic treatments beginning in week 2 (P = 0.014). By week 6, all treatments were below 10,000 cells/mL, while control groups remained above 65,000 cells/mL. Conclusions AC Aqua effectively reduced P. parvum and M. aeruginosa cell densities in eutrophic systems when applied prophylactically. Its lag effect indicates a nutrient management mechanism, supporting its use as a sustainable, preventative strategy.