In-lake Intervention and Prevention Strategy
Substantial Supporting Data
It is well established that vertical water column stability and long water residence times favor cyanobacteria over eukaryotic phytoplankton (Ibelings et al. 2016, Mitrovic et al. 2003, Paerl et al. 2016). Thus, the disruption of these conditions can, under certain circumstances, reduce nuisance HCBs (Havens et al. 2019, Lehman 2014, McDonald and Lehman 2013). Management strategies that change the hydraulics by flushing (shorter water retention time) can be effective management tools that both affect nutrient delivery to HCBs and disrupt habitat conditions that favor HCB development (calm, warm water) in smaller water bodies (Paerl et al. 2016). The geographic setting of the water body and lake depth will dictate which type of in-lake management strategy is feasible, based on water availability or lack thereof. For example, arid western regions of the United States may have more restrictions than eastern to midwestern regions.
In-lake hydraulics may be defined as the movement of water such as surface waves or internal waves that are influenced by wind mixing, internal currents influenced by tributary inflows or discharge, stratified water layers influenced by density gradients, or concentrations that affect turbulent mixing within the water body (Starosolszky 1974). Disrupting seasonal stratification by changing reservoir hydraulics can promote the development of diatom and green algae rather than cyanobacteria.
Lake and reservoir flushing may be defined as the passthrough of a large volume of water, preferably lower in nutrient concentrations, with sufficient velocity to flush lake water containing cyanobacteria downstream before cyanobacteria populations can regrow in the water body (Ibelings et al. 2016, Mitrovic, Hardwick, and Dorani 2010). Flushing reduces the water retention time (Romo et al. 2012) and disrupts water column stability, thereby minimizing the contact time between cyanobacteria and nutrients while eliminating calm waters that favor growth of buoyant cyanobacteria species (Anderson, Komor, and Ikehata 2014). Reservoir flushing may also be defined as the seasonal release of hypolimnetic water from thermally stratified lakes that are enriched with bioavailable nutrients from internal nutrient loading (Nürnberg 2007). The discharge of water before fall turnover reduces the amount of nutrient-rich hypolimnetic water that mixes with near-surface epilimnetic water and may reduce cyanobacteria blooms that occur post-turnover.
The frequency of flushing flows may also affect the proliferation of benthic cyanobacterial mats (Quiblier et al. 2013). Wood, Wagenhoff, and Young (2014) estimated the specific flushing flows necessary to reduce Phormidium cover below 20% for multiple locations in New Zealand rivers. A study across multiple New Zealand river systems demonstrated accrual of this cyanobacterium also increased with time since the last flushing flow (McAllister et al. 2018). Stanfield (2018) derived river discharge thresholds that, once exceeded, removed attached benthic cyanobacteria in the upper Potomac River in Maryland.
EFFECTIVENESS
- Water body types: Lake/reservoir
- Any surface area
- Depth: Shallow
- Trophic state: Eutrophic
- Mixing strategy: Polymictic
- Water body uses: Recreation, drinking water source
- Requires more planning for water management
- Reservoir releases of 80 MGD (critical flow velocity of 1 foot/second) have been effective in mitigating HCB development via suppression of stratification and cell washout
- Reservoir releases of 800 MGD have been effective in removing an established HCB
- Run-of-river reservoirs are more suitable for managing hydraulics given flow conditions
- Water body types: Lake/reservoir
- Any surface area
- Depth: Shallow
- Trophic state: Eutrophic
- Mixing strategy: Polymictic
- Water body uses: Recreation, drinking water source
- Requires more planning for water management
- Reservoir releases of 80 MGD (critical flow velocity of 1 foot/second) have been effective in mitigating HCB development via suppression of stratification and cell washout
- Reservoir releases of 800 MGD have been effective in removing an established HCB
- Run-of-river reservoirs are more suitable for managing hydraulics given flow conditions
- Releases that induce bottom shear remove over-wintering populations from bottom sediments’
NATURE OF HCB
- Effective on most types of cyanobacteria in the epilimnion
- Microcystis colonies in sheltered inlets or bays may be less affected by flushing
- Large releases of 80 MGD were effective in suppressing Anabaena circinalis
- In stratified lakes, flushing may not affect cyanobacteria in the metalimnion
- Flushing flows may reduce accrual of benthic cyanobacterial mats in rivers
- Delay timing of occurrent for nitrogen-fixing (Aphanizomenon) and non-nitrogen-fixing taxa (Microcystis)
- Change in algal composition favoring diatomsIntervention and prevention strategy
Flushing management strategies have been moderately effective in eutrophic lakes and reservoirs of less than 125 surface acres (Cross et al. 2014, James, Eakin, and Barko 2004, Pawlik-Skowronska and Toporowska 2016), as well as in some larger reservoirs, provided that sufficient flows are available (Qin et al. 2010). Releases of 80 MGD with a critical flow velocity of 1 foot/second have been effective in mitigating HCB development in a large reservoir by suppressing thermal stratification along with cell washout. Reservoir releases of 800 MGD have been effective in removing an established HCB (Lehman 2014).
ADVANTAGES
- Variability in regional rainfall patterns may benefit flushing capability, influence water residence time and stratification, and change cyanobacteria dominance and persistence
- Horizontal flushing by increasing the flowthrough of water can reduce HCB development via reduction in nutrient supply
- Does not require capital or equipment investment
- Weigh the cost of water versus intangible cost of closing water body due to HCBs
- A series of reservoirs may be managed to store and release water for the benefit of flushing a downstream reservoir
- Numerical modeling may indicate that changing reservoir hydraulics or flushing may or may not improve nutrient water quality or HCB conditions
- Short pulses of water spread out over the season may be as effective as one flushing event for planktonic species
LIMITATIONS
- Large volumes of low-nutrient water are needed to flush a reservoir
- Variable costs; can be low to expensive
- Not practical or effective on larger reservoirs
- Drinking water or irrigation reservoirs generally do not have the luxury of water surplus for flushing
- Requires more long-term planning to coordinate flushing events
- Changing reservoir hydraulics may warm the bottom water, affecting cold-water fisheries
- Potential for downstream impacts related to HCBs and cyanotoxins during flushing events
Regional rainfall patterns may benefit flushing capability, influence water residence time, and change cyanobacteria dominance and persistence (Jagtman, Van der Molen, and Vermij 1992, Larsen et al. 2020). Other environmental factors—such as thermal stratification, water temperature, and potential fisheries—should be considered before implementing this strategy (Fulton III and Hendrickson 2011, Nelson et al. 2018). Often, numerical modeling can help evaluate these environmental factors and determine whether changing the reservoir hydraulics or flushing will be beneficial for the reservoir. The cost of raw water and limited supplies in many regions of the United States may also influence the decision to implement this lake management strategy. In these cases, the intangible cost (economics) of closing a water body due to HCBs should also be considered.
COST ANALYSIS
Relative cost per growing season: Hydraulic flushing
| ITEM | RELATIVE COST PER GROWING SEASON |
| Water Availability | $$–$$$ |
| O&M Costs | $–$$$ |
| OVERALL | $$–$$$ |
Financial costs depend on site-specific geographical settings and water availability. For example, if hydroelectric facilities are associated with run-of-the river facilities, the financial tradeoffs of water, electric power, and public perception must be thoroughly vetted before hydraulic, flushing, or drawdown management strategies are implemented. In the arid West, water availability and the cost of water severely limit the feasibility of hydraulic or flushing strategies, although water level drawdown may be more practical in this region.
Nearly all in-lake prevention and intervention techniques, including flushing and water level drawdown, will require some form of permitting or approval at the federal, state, or local level (Holdren, Jones, and Taggart 2001). Because these management strategies have the potential to flush sediment, nutrients, cyanobacteria, cyanotoxins, and other metalloid or hydrocarbon compounds to downstream regulated water bodies (as well as affect streamflow and water availability downstream), the state water quality regulatory office is the most appropriate agency to contact early in the planning phase.
Regulatory planning for hydraulic, flushing, and drawdown techniques may include but is not limited to Clean Water Act Sections 401 or 404 permitting, NPDES permitting, drawdown permitting, or Water Rights Administration permitting. Depending on the scale of the project and the extent of stakeholders, permitting could take months to years, so planning is critical. Depending on the size of the water body, its physical characteristics, and its environmental setting, implementing these techniques as short-term intervention approaches may require extensive planning.
REFERENCES
CASE STUDY EXAMPLES
Various locations: Bakker and Hilt (2016) identified 13 case studies that illustrate key factors to consider when implementing this management tool to reduce or prevent cyanobacteria blooms.
Lake Williston, Maryland, United States: Sellner et al. (2015) employed manually operated wooden dam control in a 67-acre lake to flush overwintering cyanobacteria populations from bottom sediments, thereby reducing potential “seed” populations for summer growth.
Anderson, Michael A., Andy Komor, and Keisuke Ikehata. 2014. “Flow routing with bottom withdrawal to improve water quality in Walnut Canyon Reservoir, California.” Lake and Reservoir Management 30 (2):131-142. doi: 10.1080/10402381.2014.898720.
Bakker, Elisabeth S., and Sabine Hilt. 2016. “Impact of water-level fluctuations on cyanobacterial blooms: options for management.” Aquatic Ecology 50 (3):485-498. doi: 10.1007/s10452-015-9556-x.
Cross, Iain, Suzanne McGowan, T. Needham, and C. Pointer. 2014. “The effects of hydrological extremes on former gravel pit lake ecology: management implications.” Fundamental and Applied Limnology 185. doi: 10.1127/fal/2014/0573.
Fulton III, R. S., and J. C. Hendrickson. 2011. Relationships of Water Flow with Plankton and Water Quality In St. Johns River Water Management District. Palatka, FL.
Havens, Karl E., Gaohua Ji, John R. Beaver, Rolland S. Fulton, and Catherine E. Teacher. 2019. “Dynamics of cyanobacteria blooms are linked to the hydrology of shallow Florida lakes and provide insight into possible impacts of climate change.” Hydrobiologia 829 (1):43-59. doi: 10.1007/s10750-017-3425-7.
Holdren, C. , W. Jones, and J. Taggart. 2001. “Managing Lakes and Reservoirs.” North American Lake Management Society; Terrene Insitute. https://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=20004KKC.txt.
Ibelings, Bastiaan W., Myriam Bormans, Jutta Fastner, and Petra M. Visser. 2016. “CYANOCOST special issue on cyanobacterial blooms: synopsis—a critical review of the management options for their prevention, control and mitigation.” Aquatic Ecology 50 (3):595-605. doi: 10.1007/s10452-016-9596-x.
Jagtman, E., D.T. Van der Molen, and S Vermij. 1992. ” The influence of flushing on nutrient dynamics, composition and densities of algae and transparency in Veluwemeer, The Netherlands.” In Restoration and Recovery of Shallow Eutrophic Lake Ecosystems in The Netherlands. Developments in Hydrobiology, edited by L. Van Liere and R.D. Gulati. Dordrecht: Springer.
James, William F. , Harry L. Eakin, and John W. Barko. 2004. “Limnological Responses to Changes in the Withdrawal Zone of Eau Galle Reservoir, Wisconsin. ERDC WQTN-MS-07.” U. S. Army Engineer Research and Development Center. http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.549.1531&rep=rep1&type=pdf.
Larsen, Megan L., Helen M. Baulch, Sherry L. Schiff, Dana F. Simon, Sébastien Sauvé, and Jason J. Venkiteswaran. 2020. “Extreme rainfall drives early onset cyanobacterial bloom.” bioRxiv:570275. doi: 10.1101/570275.
Lehman, John T. 2014. “Understanding the role of induced mixing for management of nuisance algal blooms in an urbanized reservoir.” Lake and Reservoir Management 30 (1):63-71. doi: 10.1080/10402381.2013.872739.
McAllister, Tara G., Susanna A. Wood, Javier Atalah, and Ian Hawes. 2018. “Spatiotemporal dynamics of Phormidium cover and anatoxin concentrations in eight New Zealand rivers with contrasting nutrient and flow regimes.” The Science of the total environment 612:71-80. doi: 10.1016/j.scitotenv.2017.08.085.
McDonald, Kahli E., and John T. Lehman. 2013. “Dynamics of Aphanizomenon and Microcystis (cyanobacteria) during experimental manipulation of an urban impoundment.” Lake and Reservoir Management 29 (2):103-115. doi: 10.1080/10402381.2013.800172.
Mitrovic, S. M., R. L. Oliver, C. Rees, L. C. Bowling, and R. T. Buckney. 2003. “Critical flow velocities for the growth and dominance of Anabaena circinalis in some turbid freshwater rivers.” Freshwater Biology 48 (1):164-174. doi: 10.1046/j.1365-2427.2003.00957.x.
Mitrovic, Simon M., Lorraine Hardwick, and Forugh Dorani. 2010. “Use of flow management to mitigate cyanobacterial blooms in the Lower Darling River, Australia.” Journal of Plankton Research 33 (2):229-241. doi: 10.1093/plankt/fbq094.
Nelson, Natalie G., Rafael Muñoz-Carpena, Edward J. Phlips, David Kaplan, Peter Sucsy, and John Hendrickson. 2018. “Revealing biotic and abiotic controls of harmful algal blooms in a shallow subtropical lake through statistical machine learning.” Environmental Science & Technology 52 (6):3527-3535. doi: 10.1021/acs.est.7b05884.
Nürnberg, Gertrud K. 2007. “Lake responses to long-term hypolimnetic withdrawal treatments.” Lake and Reservoir Management 23 (4):388-409. doi: 10.1080/07438140709354026.
Paerl, H. W., W. S. Gardner, K. E. Havens, A. R. Joyner, M. J. McCarthy, S. E. Newell, B. Qin, and J. T. Scott. 2016. “Mitigating cyanobacterial harmful algal blooms in aquatic ecosystems impacted by climate change and anthropogenic nutrients.” Harmful Algae 54:213-222. doi: 10.1016/j.hal.2015.09.009.
Pawlik-Skowronska, Barbara, and Magdalena Toporowska. 2016. “How to mitigate cyanobacterial blooms and cyanotoxin production in eutrophic water reservoirs?” Hydrobiologia 778 (1):45-59. doi: 10.1007/s10750-016-2842-3.
Qin, B., G. Zhu, G. Gao, Y. Zhang, W. Li, H. W. Paerl, and W. W. Carmichael. 2010. “A drinking water crisis in Lake Taihu, China: linkage to climatic variability and lake management.” Environ Manage 45 (1):105-12. doi: 10.1007/s00267-009-9393-6.
Quiblier, Catherine, Susanna Wood, Isidora Echenique, Mark Heath, Villeneuve Aurelie, and Jean-François Humbert. 2013. “A review of current knowledge on toxic benthic freshwater cyanobacteria – Ecology, toxin production and risk management.” Water research 47. doi: 10.1016/j.watres.2013.06.042.
Romo, Susana, Juan Soria, Francisca Fernández, Youness Ouahid, and Ángel Barón-Solá. 2012. “Water residence time and the dynamics of toxic cyanobacteria.” Freshwater Biology 58 (3):513-522. doi: 10.1111/j.1365-2427.2012.02734.x.
Sellner, Kevin, Allen Place, Ernest Williams, Yonghui Gao, Elizabeth VanDolah, Michael Paolisso, Holly Bowers, and Shannon Roche. 2015. “Hydraulics and Barley Straw (Hordeum vulgare) as effective Treatment Options for a Cyanotoxin-Impacted Lake.” International Conference on Harmful Algae.
Stanfield, Kevin. 2018. “Developing methods to differentiate species and estimate coverage of benthic autotrophs in the potomac using digital imaging.” M. S. Thesis, Environmental Biology, Hood College.
Starosolszky, Ö. 1974. “Lake hydraulics.” Hydrological Sciences Bulletin 19 (1):99-114. doi: 10.1080/02626667409493874.
Wood, Susie, Annika Wagenhoff, and Roger Young. 2014. “The Effect of River Flow and Nutrients on Phormidium Abundance and Toxin Production in Rivers in the Manawatu-Whanganui Region.” Horizons Regional Council. https://envirolink.govt.nz/assets/Envirolink/Reports/1454-HZC108.pdf.
