Freshwater harmful algal bloom (FHAB) suppression with solar powered circulation (SPC)

Abstract

Freshwater harmful algal blooms (FHABs) incidence is increasing worldwide, presenting risks for human and animal health, aquatic-ecosystem sustainability and economic vitality. Increasing nutrient input to freshwater, increasing temperatures and decreasing flow rates that create quiescent or stagnant waters are primary causes of increasing incidence. Ecological approaches to FHAB control target these causes to reduce FHAB incidence without adversely impacting aquatic ecosystems. Artificial circulating increases water flow, and is reported to suppress FHABs in the literature on habitat disturbance. This report evaluates the efficacy of a new technology, solar powered circulation (SPC), designed to create long-distances circulation of the epilimnion (>200 m) to suppress FHABs. Three nutrient-enriched, source-water reservoirs periodically seeded with cyanobacteria from influent served as case studies. Utility personnel systematically and consistently collected limnological data before and during SPC deployment, thus enabling valid within-site comparisons. SPC units were deployed at densities of approximately 0.15 km 2/unit. Pre- to post-SPC initiation changes in cyanobacterial density indicated that SPC strongly suppressed FHABs through a process that strengthened over time. Densities of green algae increased significantly during SPC at the only site where algaecides were never used, and zooplankton density increased significantly at another site. Diatom densities approximately doubled following SPC initiation, although the increases were not statistically significant. Copper sulfate usage declined by 85% at one site during SPC, whereas applications declined from approximately 12/year to 1–2/year at the other site where algaecides were used. SPC provided an effective approach to FHAB control that was ecologically benign and environmentally sustainable.