In-lake Intervention and Prevention Strategy
Limited/Emerging Supporting Field Data
Many species of cyanobacteria can regulate their buoyancy in the water column through special internal structures known as gas vesicles (Reynolds and Walsby 1975, Walsby et al. 1997). These provide a competitive advantage over other phytoplankton. Cells with gas vesicles accumulate at the surface during the day to use available light for photosynthesis, shading out competing non-cyanobacterial species. Late in the day, accumulated sugars or carbohydrates from daytime photosynthesis overcome the buoyancy from the gas vesicles and cells sink to cooler, nutrient-rich water, which allows them to continue to grow and maintain dominance. Not all cyanobacteria species appear to be capable of producing gas vesicles, and even among similar species, differences in relative abundance and activity of gas vesicles is evident (Brookes, Ganf, and Oliver 2000).
Disrupting the ability of cyanobacteria to maintain their position in the water column is a strategy employed by several cyanobacteria control methods. While some strategies do this by artificially mixing the water column (see artificial circulation and mechanical mixers strategy), others may bind the cells with flocculants to sink them out of the euphotic zone (see clay and surfactant flocculation technique). However, ultrasound generates acoustic cavitation bubbles (Wu, Joyce, and Mason 2012) and, through bubble collapse, targets these specific buoyancy control structures that are unique to cyanobacteria and a few other bacterial groups.
Ultrasound refers to a wide range of applications, so care must be taken to distinguish among technologies. Typically, ultrasonic generators produce a frequency (measured in megahertz, MHz), at a set power intensity (measured in watts per square centimeter), at a set duration (measured in time, typically minutes). High-power ultrasound is used to destroy bacteria and plankton in wastewater treatment (Wu and Mason 2017) and ship ballast (Holm et al. 2008). Ultrasonic technologies intended for cyanobacterial control use high-frequency sound waves to collapse gas vesicles (Rajasekhar et al. 2012).
The technology appears to have been used in the field first in the early 2000s (Lee, Nakano, and Matsumura 2002), though it was conceptualized earlier (Park et al. 2017). This exposure results in gas vesicle collapse but typically not complete lysis or degradation of the cell. Frequencies between 1.7 MHz and 20 kHz are typically used in some modulation (Hao et al. 2004), with reported durations ranging from few-second pulses to pulses of several hours . Effective removal of most HCB species appears to occur within 10 minutes of exposure under laboratory conditions, though there are limited field data to support this observation (Wu, Joyce, and Mason 2012, Park et al. 2017). At high energies, however, ultrasound may also disrupt colonies and even break cell walls, thus inhibiting growth (Lürling and Tolman 2014). Some laboratory testing shows destruction of the cyanotoxin microcystin (Liu et al. 2018, Song et al. 2005), probably by generation of free radicals (Joyce, Wu, and Mason 2010). However, the mechanisms responsible for these effects in laboratory settings may not apply directly to field application. Controlled laboratory conditions are rarely true to field conditions, where rainfall, water quality, water flow, turbulence, and water volume under sonic generators appear to play a vital factor in device performance (Park et al. 2017). Even under ideal conditions, energy transmission falls off quickly with increasing distance (Rajasekhar et al. 2012). Hence, the technique has limited range.
Under field conditions, effectiveness is thought to be dependent on generation of frequencies that match resonant frequencies of the gas vesicles (Rajasekhar et al. 2012). The few results are anecdotal with highly variable results. In a recent review, Lürling and Mucci (2020) conclude that low-frequency ultrasound should be avoided, as it is ineffective; high-frequency treatment is more effective, but it is costly due to energy demand, and its effective range is limited. Review of commercial claims on efficacy is difficult, as manufacturers consider technical specifications as proprietary information, making controlled, independent testing difficult. Studies that include technical details are rare and usually confined to laboratory conditions (Kong et al. 2019). Ultrasonic technologies are also not a short-term improvement technology, with many observed decreases or changes to ecological condition occurring over several weeks (Schneider, Weinrich, and Brezinski 2015, Villanueva et al. 2015). Off-target effects on other aquatic organisms, including zooplankton (Lürling and Tolman 2014), insects, and vertebrates such as fish, are possible, though documentation is limited.
- Water body types: Pond, lake/reservoir
- Depth: Shallow to moderate
- Surface area: Small
- Any trophic state
- Any mixing regime, though mixed systems could result in less contact time
- Any water body use
NATURE OF HCB
- Effective on planktonic, gas-vesicle-containing cyanobacteria
- Toxic or nontoxic HCBs
- Other aquatic algae can be targeted
- Intervention strategy
- Can move generators as needed and adjust frequency and length of exposure to target different species
- Some devices are coupled with real-time sensors to measure effectiveness
- Highly variable results
- Does not appear to remove cyanotoxins
- If frequency causes cell lysis, extracellular cyanotoxin levels could increase
- Does not control nutrients
- Benthic blooms may still occur
- Expensive and proprietary constraints prevent inspection of conditions, frequencies, etc.
- High-power treatments can affect other organisms
- Limited by an effective radius for impact
CASE STUDY EXAMPLES
Reservoir, New Jersey, United States: Schneider, Weinrich, and Brezinski (2015) deployed a system of ultrasonic buoys in a 200-acre reservoir that historically had blooms with taste and odor issues. The reservoir had previously used copper as its primary treatment.
Reservoir 1 is a 200-acre water body with a mean depth of 17 feet. It is fed by a small brook and adjacent reservoir (Reservoir 2).
Four ultrasonic buoys were deployed in May 2014 to reduce total algae abundance and concentrations of taste and odor compounds. While total numbers of algae cells appeared to decline, it should be noted that copper applications were used along with the buoys. Also, technical specifications of the ultrasonic buoys (frequency and intensity) were not reported.
General levels of cyanobacteria increased during the monitoring period; however, a bloom of Aphanizomenon occurred once water from Reservoir 2 was allowed to flow into Reservoir 1 (August 13, 2014). A reduction in the bloom was not noted until September 17, 2015, and may have been due to either the length of exposure or the change in the ultrasound frequency to target Aphanizomenon spp.
Financial costs depend on site-specific geographical and lake morphology factors and water conditions. For example, for a large water body, multiple generators may be required to effectively prevent a bloom. The range and limitation, as well as the service and maintenance of each generator, must be factored into the cost of deploying this technology. As this is a preventive technology that does not address nutrient input, a backup treatment option should be planned for blooms of cyanobacterial species that do not form gas vesicles or are otherwise outside the treatment range of the technology.
Relative cost per growing season: Ultrasound
|ITEM||RELATIVE COST PER GROWING SEASON|
REGULATORY AND POLICY CONSIDERATIONS
Some generators can use solar panels for electricity, while others require shoreline tethering for power. Local permitting for installation and potential impacts to zooplankton and other aquatic life must be considered.
Brookes, Justin D., George G. Ganf, and Roderick L. Oliver. 2000. “Heterogeneity of cyanobacterial gas-vesicle volume and metabolic activity.” Journal of Plankton Research 22 (8):1579-1589. doi: 10.1093/plankt/22.8.1579.
Hao, Hongwei, Minsheng Wu, Yifang Chen, Jiaowen Tang, and Qingyu Wu. 2004. “Cavitation Mechanism in Cyanobacterial Growth Inhibition by Ultrasonic Irradiation.” Colloids and Surfaces B: Biointerfaces 33:151-156. doi: 10.1016/j.colsurfb.2003.09.003.
Holm, E. R., D. M. Stamper, R. A. Brizzolara, L. Barnes, N. Deamer, and J. M. Burkholder. 2008. “Sonication of bacteria, phytoplankton and zooplankton: application to treatment of ballast water.” Mar Pollut Bull 56 (6):1201-8. doi: 10.1016/j.marpolbul.2008.02.007.
Joyce, E. M., X. Wu, and T. J. Mason. 2010. “Effect of ultrasonic frequency and power on algae suspensions.” J Environ Sci Health A Tox Hazard Subst Environ Eng 45 (7):863-6. doi: 10.1080/10934521003709065.
Kong, Y., Y. Peng, Z. Zhang, M. Zhang, Y. Zhou, and Z. Duan. 2019. “Removal of Microcystis aeruginosa by ultrasound: Inactivation mechanism and release of algal organic matter.” Ultrason Sonochem 56:447-457. doi: 10.1016/j.ultsonch.2019.04.017.
Lee, T. J., K. Nakano, and M. Matsumura. 2002. “A novel strategy for cyanobacterial bloom control by ultrasonic irradiation.” Water Sci Technol 46 (6-7):207-15.
Liu, Cheng, Zhen Cao, Siyuan He, Zhehao Sun, and Wei Chen. 2018. “The effects and mechanism of phycocyanin removal from water by high-frequency ultrasound treatment.” Ultrasonics Sonochemistry 41:303-309. doi: 10.1016/j.ultsonch.2017.09.051.
Lürling, M., and Y. Tolman. 2014. “Beating the blues: is there any music in fighting cyanobacteria with ultrasound?” Water Res 66:361-373. doi: 10.1016/j.watres.2014.08.043.
Lürling, Miquel, and Maíra Mucci. 2020. “Mitigating eutrophication nuisance: in-lake measures are becoming inevitable in eutrophic waters in the Netherlands.” Hydrobiologia. doi: 10.1007/s10750-020-04297-9.
Park, J., J. Church, Y. Son, K. T. Kim, and W. H. Lee. 2017. “Recent advances in ultrasonic treatment: Challenges and field applications for controlling harmful algal blooms (HABs).” Ultrason Sonochem 38:326-334. doi: 10.1016/j.ultsonch.2017.03.003.
Rajasekhar, Pradeep, Linhua Fan, Thang Nguyen, and Felicity Roddick. 2012. “A Review of the Use of Sonication to Control Cyanobacterial Blooms.” Water research 46:4319-29. doi: 10.1016/j.watres.2012.05.054.
Reynolds, C. S., and A. E. Walsby. 1975. “Water-Blooms.” Biological Reviews 50 (4):437-481. doi: 10.1111/j.1469-185X.1975.tb01060.x.
Schneider, Orren D., Lauren A. Weinrich, and Scott Brezinski. 2015. “Ultrasonic Treatment of Algae in a New Jersey Reservoir.” Journal – AWWA 107 (10):E533-E542. doi: 10.5942/jawwa.2015.107.0149.
Song, W., T. Teshiba, K. Rein, and K. E. O’Shea. 2005. “Ultrasonically induced degradation and detoxification of microcystin-LR (cyanobacterial toxin).” Environ Sci Technol 39 (16):6300-5. doi: 10.1021/es048350z.
Villanueva, Maria V., Maria C. Luna, Maria I. Gil, and Ana Allende. 2015. “Ultrasound treatments improve the microbiological quality of water reservoirs used for the irrigation of fresh produce.” Food Research International 75:140-147. doi: 10.1016/j.foodres.2015.05.040.
Walsby, Anthony E., Paul K. Hayes, Rolf Boje, and Lucas J. Stal. 1997. “The selective advantage of buoyancy provided by gas vesicles for planktonic cyanobacteria in the Baltic Sea.” New Phytologist 136 (3):407-417. doi: 10.1046/j.1469-8137.1997.00754.x.
Wu, X., E. M. Joyce, and T. J. Mason. 2012. “Evaluation of the mechanisms of the effect of ultrasound on Microcystis aeruginosa at different ultrasonic frequencies.” Water Res 46 (9):2851-8. doi: 10.1016/j.watres.2012.02.019.
Wu, Xiaoge, and Timothy Mason. 2017. “Evaluation of power ultrasonic effects on algae cells at a small pilot scale.” Water 9:470. doi: 10.3390/w9070470.