Hypolimnetic Withdrawal and Drawdown

It is well established that vertical water column stability and long water residence times favor cyanobacteria over eukaryotic phytoplankton (Ibelings et al. 2016, Paerl et al. 2016, Mitrovic et al. 2003). 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 decrease water levels or selectively release nutrient-rich bottom waters can be effective management tools that affect nutrient delivery to HCBs (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.

Hypolimnetic withdrawal is the removal of nutrient-rich bottom waters in stratified ponds and lakes to eliminate nutrient supplies that support the growth of cyanobacteria in the epilimnion (surface layer) of the water body. Bormans, Marsálek, and Jancula (2015) have reviewed this strategy and its use in multiple lakes and report variable results. It has been successful in eliminating blooms of Aphanizomenon in Ford Lake, Michigan, while not having any effect on Microcystis or microcystin toxicity (Lehman, McDonald, and Lehman 2009). Further, destabilization led to diatom prevalence (McDonald and Lehman 2013). In Lake Mauensee in Switzerland, Gächter (1976) reported the disappearance of Planktothrix rubescens following withdrawal. This cyanobacterium was also reduced in a Slovenian lake following withdrawal and reduction in external loads (Vrhovšek et al. 1985).

Water level fluctuations (drawdown) may be defined as the lowering of the water level to expose littoral zone habitat and sediments, with the goal to switch the water body from a turbid, algae-dominated system to a clear-water, plant-dominated system (Scheffer et al. 1993). However, the timing of the water level drawdown is critical, because summer-time drawdowns can increase cyanobacteria production given the increased water retention time, increased water temperature, and nutrients (Bakker and Hilt 2016). In shallow lakes, lower water elevations at key times of the year may promote the growth of submerged and emergent macrophytes due to increased light availability and reduce the potential for cyanobacteria development (Coops and Hosper 2002, Scheffer and van Nes 2007). Mechanisms that indirectly affect cyanobacteria development during drawdown include exposure of over-wintering cyanobacteria populations on surficial sediments to winter freezes, disruption and loss of colonizable habitat for benthic cyanobacteria (Turner et al. 2005), uptake of nutrients by macrophytes, excretion of allelopathic substances by macrophytes that may inhibit cyanobacteria growth (Hilt and Gross 2008), or development of macrophyte beds that support invertebrate and fish assemblages (Bakker and Hilt 2016). In contrast, water level drawdown is often used in deeper lakes to reduce aquatic nuisance plants and fish. Due to the timing of drawdown (for example, winter), the strategy generally limits the effectiveness of managing cyanobacteria blooms.

As a control strategy, hypolimnetic withdrawal from stratified systems is most effective in systems where internal nutrient loads are the primary cause of the HCB and external nutrient loads are declining or low. Withdrawal can result in destratification and increases in NO₃ deeper in the water column. Further, there may be total phosphorous concentration thresholds for some species. Bormans, Marsálek, and Jancula (2015) reported that cyanobacteria declined when epilimnion total phosphorus levels were less than 25 µg/L. This might suggest that hypolimnion total phosphorus levels >25 µg/L could be an indicator for selecting use of withdrawal as a strategy to consider in HCB control. In addition, Lehman, McDonald, and Lehman (2009) noted that Aphanizomenon was found when the total nitrogen/total phosphorus ratio approximated 48, while Microcystis was common at ratios approximating 70. Other metrics for assessing whether cyanobacteria (or non-cyanobacteria) could follow hypolimnetic withdrawal; however, successful reductions in cyanobacteria may not always occur (see Table 1 in Bormans, Marsálek, and Jancula 2015, Dunalska et al. 2014).

Withdrawal can be accomplished through pumping sub-thermocline water from depth into downstream areas. A special withdrawal tube—an Olszewski pipe, with openings set at depths below the thermocline—has been used in the past. In lakes or reservoirs with dam outlets at depth, if those outlets are deeper than the thermocline, then opening the outlets following stratification and nutrient accumulation at depth could remove the regenerated nitrogen and phosphorus, thereby limiting access by cyanobacteria populations in the epilimnion.

At Milford Reservoir in Kansas, which has a surface area of over 15,000 acres, the management plan implemented since 2017 incorporates a spring drawdown that exposes a broad shallow area in the upper portion of the water body; this is specifically designed to reduce habitat where cyanobacterial blooms develop (USACE 2019).

Regional rainfall patterns may impact 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 hypolimnetic withdrawal or drawdown will be beneficial for the reservoir. The cost of raw water and limited supplies in many regions of the United States may also be a deciding factor. 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: Hypolimnetic withdrawal

ITEM	RELATIVE COST PER GROWING SEASON
Material	$$
Personal Protective Equipment	$
Equipment	$$
Machinery	$$
Labor	$
O&M Costs	$$
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 hypolimnetic withdrawal 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.

REGULATORY AND POLICY CONSIDERATIONS

Nearly all in-lake prevention or intervention techniques, including hypolimnetic withdrawal 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 hypolimnetic withdrawal or drawdown techniques may include but is not limited to Clean Water Act Sections 401 or 404 permitting, NPDES permitting, drawdown permitting, and 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. Local and state officials should be contacted regarding permitting and use, particularly for potential impacts downstream from nutrient-rich, potentially sulfidic bottom waters.

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