RESUMO
Once established within a water resource, harmful algal blooms (HABs) can occur seasonally with an intense and rapid onset, giving water resource managers limited time to respond to lessen risks. An attractive strategy to decrease human, ecological, and economic risks from HABs is to implement proactive algaecide treatments applied to overwintering cyanobacteria (i.e., akinetes and quiescent vegetative cells) in sediments prior to the formation of a HAB; however, this approach is novel and very limited efficacy data exist. Therefore, the specific objectives of this research were to 1) evaluate copper- and peroxide-based algaecides, applied as single and repeat treatments at the bench scale, to identify effective proactive treatments, and 2) compare correlations between cell density and other response measurements (i.e., in vivo chlorophyll a and phycocyanin concentrations and percent benthic coverage), to identify informative metrics to assess overwintering cyanobacteria responses. Twelve treatment scenarios using copper- and peroxide-based algaecides were applied to sediments containing overwintering cyanobacteria prior to a 14 d incubation under favorable growth conditions. Responses of cyanobacteria in the planktonic (i.e., cell density, in vivo chlorophyll a and phycocyanin concentrations) and benthic (percent coverage) phases after a 14 d incubation were evaluated in treatments and controls. The HAB-forming cyanobacteria present after a 14 d incubation were: Aphanizomenon, Dolichospermum, Microcystis, Nostoc, and Planktonthrix. Successive treatments of copper sulfate (CuSulfate) followed by sodium carbonate peroxyhydrate (PeroxiSolid) (second algaecide applied after 24 h) as well as repeat applications of a single algaecide, PeroxiSolid (second treatment applied after 24 h) resulted in statistically significant (p ≤ 0.05; α = 0.05) declines in cell density relative to untreated controls. Planktonic cyanobacteria responses measured in terms of phycocyanin concentrations were strongly correlated with cyanobacteria density measurements (Pearson's correlation coefficient (r) = 0.89). Chlorophyll a concentrations and percent benthic coverage did not correlate with planktonic cyanobacteria density measurements (r = 0.37 and -0.49, respectively) and therefore, were unreliable metrics for cyanobacterial responses in this study. These data provide initial evidence of the efficacy of algaecides for treating overwintering cells in sediments and contribute to our overarching hypothesis that proactive treatments may delay the onset and intensity of HABs in impacted waterbodies.
RESUMO
Growth of microcystin-producing cyanobacteria in Lake Okeechobee (Florida, USA) and surrounding waters has resulted in adverse health impacts for humans and endangered species, as well as significant economic losses. As these issues worsen, there is growing pressure for efficacious solutions to rapidly mitigate harmful algal blooms (HABs) and protect critical freshwater resources. Applications of USEPA-registered algaecides as management tactics meet many decision-making criteria often required by water resource managers (e.g., effective, scalable, selective), but have not yet been evaluated on a large scale within the Lake Okeechobee waterway. This study was conducted to bolster the peer-reviewed database for available management tactics against microcystin-producing cyanobacteria in waters of this region. Laboratory-scale experiments can be conducted first to minimize uncertainty at larger scales and improve confidence in decision-making. In this study, samples containing microcystin-producing cyanobacteria collected from Lake Okeechobee were exposed to several USEPA-registered algaecides in laboratory toxicity experiments. Responses of target cyanobacteria were measured 3 days after treatment (DAT) in terms of cell density, chlorophyll-a concentrations, and phycocyanin concentrations. Based on responses of the cyanobacteria, minimum effective exposure concentrations were identified for each algaecide. Microcystin release (i.e. proportion of total microcystins in the aqueous phase) was measured and compared 1 DAT among effective exposures. Total microcystin concentrations were measured in effective treatments at 1, 4, and 9 DAT to discern potential for microcystin persistence following exposures to the effective formulations and exposure concentrations. Overall, several formulations including GreenClean Liquid® 5.0, GreenClean Liquid® 5.0 combined with Hydrothol® 191, and the copper-based algaecides evaluated (Algimycin® PWF, Argos, Captain® XTR, Cutrine® Ultra, and SeClear®) achieved significant and similar effects on target cyanobacteria. The chelated copper-based formulations (Algimycin® PWF, Argos, Captain® XTR, and Cutrine® Ultra) resulted in relatively less microcystin release 1 DAT and lesser total microcystin concentrations 4 DAT. At 9 DAT, total microcystin concentrations were significantly lower than in untreated controls in all treatments evaluated. These results provide the necessary comparative performance data for preliminary decision-making and designing additional studies at larger scales. Importantly, the comparative toxicity data and approach provided in this study demonstrate the initial steps for development of site-specific management strategies for Lake Okeechobee and other areas impacted by harmful algal blooms with large spatial and temporal scales.
