The increasing incidence and durations of toxigenic Freshwater Harmful Algal Blooms (FHABs), threaten human and animal health, aquatic-ecosystem sustainability and economies. Successfully confronting the risks of FHABs requires effective and environmentally sustainable methods for bloom prevention. The U.S. Environmental Protection Agency (EPA) has not established FHAB-prevention policy. Relevant EPA and other Agency activities are heavily weighted toward limiting nutrient input to freshwater through soil conservation and watershed management practices. Although nutrient-input reduction is an important long-term goal, combined Federal expenditures of > 1 B/yr for more than a decade, including EPA expenditures of > $200 M/yr through the Nonpoint Source Management Program, have failed to reverse the trends of increasing FHAB incidence and duration. Ecological approaches to within water-body management are needed to achieve near-term and sustainable FHAB suppression. The current study evaluated the ability of solar-powered circulation (SPC) of the epilimnion to suppress FHABs in nutrient enriched water bodies.

A survey of water body managers indicated that municipal personnel at three eutrophic source-water reservoirs collected planktonic-density data sufficient for assessing the efficacy of SPC for FHAB control. Cell density data were collected from Crystal Lake, a 25.5 ha reservoir where algaecides were never used, before SPC in 2005 and during 2006-2008 after two SPC units were deployed. Peak cyanobacterial cell densities prior to SPC exceeded 300,000 cells/mL. Density decreases in 2006-2008 were statistically significant at approximately 85, 93 and 95%, respectively. Green algal densities increased significantly during SPC. Total nitrogen and phosphate influent levels peaked at approximately 18 and 8 mg/l throughout the study period. Personnel at the other two reservoirs, 46.5 ha Thornton Lake and 94.7 ha Lake Palmdale, routinely applied copper sulfate whenever chlorophyll a concentrations exceeded approximately15 ug/L. Cyanobacterial peak density in Thornton Lake exceeded 3,900 units/ml in 2001 prior to the deployment of three SPC units. Peak densities after deployment initially were < 200 cells/ml and declined further through 2009. Densities of green algae and diatoms increased significantly during SPC, presumably due to decreased cyanobacterial densities and algaecide-application declines from 1-2/mth to 1-2/yr. Nitrate and total phosphorus levels occasionally exceeded 10 and 1.3 mg/l throughout the study period. Algaecide usage at Lake Palmdale declined by approximately 85% after SPC deployment in late 2002. Although peak cyanobacterial density in 2003 exceeded the peak of 2,100 cells/mL observed in 2002, peak densities subsequently declined to < 1,000 cells/mL through 2009. Zooplankton densities increased significantly during SPC, indicating a more balanced planktonic assemblage. Peak nitrate levels were approximately 6 mg/l throughout the study period.

These results indicate that SPC suppressed FHABs through an ecological process that strengthened over time, even as algaecide usage declined and nutrient levels remained elevated. This evidence is consistent with the scientific literature indicating that quiescent, stagnant water is a stimulatory factor for FHABs, and the literature on FHAB suppression through habitat disturbance. Nutrient input-reduction programs may eventually reduce FHAB occurrence and risks, but both occurrence and risks have increased along with programmatic expenditures for several decades. The EPA should re-examine its policy of limiting FHAB prevention strategy solely to nutrient-input reduction through watershed management. Effective, environmentally sustainable approaches to within water-body management are needed to reduce FHAB risks in the near term.