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Solar circulator to restore dissolved oxygen in a hypoxic ice-covered lake

['Kyle F. Flynn', 'Consulting', 'Helena', 'Montana', 'United States Of America', 'Cdm Smith', 'Inc.', 'Kyle A. Cutting', 'U.S. Fish', 'Wildlife Service']

Date: 2022-06

Abstract Hypoxia is common to shallow ice-covered lakes during the winter season, and restorative actions to prevent impacts to aquatic ecosystems are desired yet untested in remote settings. The use of a solar photovoltaic circulator was investigated for reoxygenation in a shallow hypoxic lake in the northern Rocky Mountains. During the fall of 2019, a solar powered lake circulator (SolarBee SB10000LH; hereinafter circulator) was installed near the center of Upper Red Rock Lake, Montana USA (latitude 44° 36’N) and dissolved oxygen (DO), temperature, turbidity, and changes to ice formation were monitored until ice-out the following spring of 2020 using an array of real-time and data logging sondes. Observations indicate the circulator formed a polynya that lasted until late November, did not increase lake turbidity, and facilitated oxygen exchange through the circulator-created-polynya for at least 3 weeks after an adjacent lake became ice covered. Thereafter, operation of the solar circulator failed from accumulation of snow and ice on the solar panels such that the lake froze completely over during a period of low light in December. From that point on throughout the winter, DO subsequently declined from supersaturation to hypoxia over a 41-day period and remained that way for nearly four months until ice-out in April. Based on this outcome, additional work is required to improve the solar-powered circulator design before attempting comparable applications elsewhere as a means of reducing the severity of hypoxia in shallow-lake systems during winter.

Citation: Flynn KF, Cutting KA, Jaeger ME, Warren JM, Johnson T, Kron D, et al. (2022) Solar circulator to restore dissolved oxygen in a hypoxic ice-covered lake. PLOS Water 1(4): e0000012. https://doi.org/10.1371/journal.pwat.0000012 Editor: Sher Muhammad, ICIMOD: International Centre for Integrated Mountain Development, NEPAL Received: August 27, 2021; Accepted: December 16, 2021; Published: April 21, 2022 This is an open access article, free of all copyright, and may be freely reproduced, distributed, transmitted, modified, built upon, or otherwise used by anyone for any lawful purpose. The work is made available under the Creative Commons CC0 public domain dedication. Data Availability: The final dataset are available on Dryad: https://doi.org/10.5061/dryad.m37pvmd4j. Funding: This worked was funded under Montana, Fish, Wildlife & Parks (FWP) research contract #33815A. KF2 Consulting provided support for this study in the form of a salary for KFF. The Federal and State Fish, Wildlife, and Water Resource departments provided support for this study in the form of salaries for KAC, MEJ, JMW, DK, and CB. CDM Smith provided support for this study in the form of a salary for TJ. The specific roles of these authors are articulated in the ‘author contributions’ section. Competing interests: The authors of this paper have read the journal’s policies and declare the following competing interests: KFF is a paid consultant for KF2 Consulting and CDM Smith. KAC, MEJ, JMW, DK, and CB are paid employees of the Federal and State Fish, Wildlife, and Water Resource departments. TJ is a paid employee of CDM Smith. This does not alter our adherence to PLOS ONE policies on sharing data and materials.

Introduction Dissolved oxygen (DO), or the measure of free oxygen in water, is critical to supporting life in aquatic ecosystems [1]. Poorly oxygenated conditions have been documented in shallow ice-covered boreal and temperate lakes worldwide [2–4]. Characterized by DO concentrations <2 mgO 2 /L, the onset of winter hypoxia typically begins with the physical separation of lake water from the atmosphere, which eliminates transfer of DO [5]. Surface ice and snow act as a barrier that limits air-water exchange [6] thereby reducing large scale convective and wind induced circulation within the water column [7]. Opaque surface ice and/or snow can also reduce light penetration and photosynthetic production of oxygen by macrophytes and algae [8]. At the same time, bacterial respiration, and decomposition of organic material in the bottom sediments and suspended sediment particles in the water column continue to consume DO [9]. If the duration of ice cover lasts several months, oxygen levels continue to decrease until either hypoxia or anoxia is reached [10]. The period of low oxygen oftentimes lasts until near the break-up of lake ice during the spring [10], when oxygen exchange can resume with the atmosphere. Lake depth is a major factor that determines if a lake becomes hypoxic during winter. In contrast to deep lakes, shallow lakes commonly undergo winter hypoxia, are commonly eutrophic, have high hydraulic residence times [11], exhibit high surface area to lake volume ratios [12], and suffer from high winter oxygen depletion rates [10, 12–14]. Additionally, the duration and spatial extent of hypoxia depends on the oxygen storage capacity of the lake prior to ice formation, and duration of ice cover [10, 14]. Factors that increase the probability of hypoxia include indirect effects of air temperature on the duration of ice-cover, snow accumulation changes on light penetration and associated photosynthetic activity by macrophytes and algae [3, 15, 16], percentage of lake volume converted to ice [17], and limited inflow or outflow exchange of water [17]. Hypoxia during winter can result in large die-offs of aquatic organisms including fish that affects species abundance, persistence, and composition [2, 17–22]. Winter is therefore considered a limiting time for many aquatic organisms in lake systems [23, 24]. Shallow lake systems with prolonged ice cover are particularly vulnerable to winterkill [25]. Decomposition and microbial processes in the benthic layer can cause long-term changes in composition of macrobenthos [26], trigger the release of potent greenhouse gases (e.g., methane and nitrous oxide; [27, 28]) or toxic levels of arsenic [29], and/or can cause changes to nutrient dynamic processes (e.g., internal regeneration or changes to nutrient cycling processes [30, 31]). Deleterious effects of winter hypoxia are therefore widespread and well-characterized, making the analysis of winterkill conditions and associated environmental management of such ecosystems a topic of high societal and conservation interest. Many strategies have been considered to remediate hypoxia and avert large-scale winterkill of aquatic organisms [9, 10, 32–34]. Proposed restoration activities that can reverse the negative impacts of hypoxia within a winter include artificial aeration to add dissolved oxygen, snow clearing to promote in-lake photosynthesis, lake level adjustment or depth augmentation, macrophyte control which limits decomposition, and circulation enhancement (Table 1). Nearly all these approaches attempt to delay the winter oxygen depletion rate (hereafter; WODR) or enhance oxygen exchange with the atmosphere during the ice-on period [35]. Several restorative methods tend to create an area of open water, defined as a ‘polynya’, to maintain oxygen exchange during the winter months between the atmosphere and water column. Oxygen transfer through the polynya occurs through surface turbulence, convective flow, and/or the displacement of warmer hypolimnetic water which creates upwelling [36, 37]. Artificial aeration has been the most widely used restorative action [33], which typically involves injection of air under the ice, aeration of a pumped water stream, surface splashers, or molecular oxygen addition [37–40]. Such efforts provide guidance for conventional artificial aeration applications to hypoxic water bodies during winter. PPT PowerPoint slide

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TIFF original image Download: Table 1. Literature methods used to improve winter oxygen conditions in winterkill prone lakes. https://doi.org/10.1371/journal.pwat.0000012.t001 A crucial decision in selecting an aeration approach for ecological restoration is whether electrical power can be supplied to the site [42]. Remote geographic locations may not have access to the electrical power grid which limits the type of mechanical aeration as a means of reducing hypoxia. Ashley [40] describe pros and cons of off-grid power sources used in British Columbia for artificial aeration including: (1) generators, (2) wind power, and (3) photovoltaic (PV) systems (i.e., solar). In review, internal combustion-powered equipment (e.g., compressors or generators) are not appropriate for unattended operation in remote locations due to excessive noise pollution, emissions, refueling, and maintenance [42, 43]. Similarly, windmill aerators, while proven effective in small lakes [33, 44], require considerable up-front cost, a consistent wind source, and constant maintenance to avoid mechanical breakdowns of wind powered compressors or moving parts [33]. As a result, solar-powered circulation systems have been considered due to technological advancements in solar panels and battery storage [40, 45–47]. The use of solar-powered circulation to reduce hypoxia in lake-systems is particularly attractive due to relatively low initial costs, minimal operation and maintenance cost, low mechanical complexity, and limited carbon footprint. Ward et al. [45] document a solar circulator application in Menzies Lake, BC using a 375-watt solar panel and battery storage. That application opened a polynya 6 m across after 10 days of operation at a time when there was 0.6 m of ice depth. In a separate trial, a direct drive PV pump mounted on a raft and tested in Kilpoola Lake, BC ran for seven months through the winter without maintenance [47]. No other studies on solar-powered circulators in ice-covered lakes, or its effectiveness on promoting oxygen exchange, exist in the literature. The purpose of the current study is to document the application of a solar-powered circulator in a remote shallow lake during the winter season as a means of minimizing the duration and spatial extent of hypoxia within a lake system that routinely turns hypoxic for several months during winter. We test the hypothesis that the solar-powered circulator will lead to the formation and maintenance of a polynya, enhance DO exchange near the circulator, and result in a shorter ice-covered period, ultimately reducing the duration of under-ice hypoxia. The current study characterizes for the first time the utility of solar-powered circulation on the spatial and temporal patterns of DO during the winter season in a shallow ice-covered lake system located in a high-elevation and remote landscape.

Discussion Vertical circulation of lakes has been proposed to increase dissolved oxygen for aquatic ecosystem restoration; however, few studies have documented the efficacy of ultra-low head, large flow PV circulation during winter months in remote settings. Ashley [40]) describes this technology at preventing winterkill of aquatic organisms in northern latitudes, and Ward et al. [45] used a Wardun WD 20 Sunstirrer unit to create a 120 m2 polynya after 10 days of trial operation during winter. The batteries in this unit however were soon depleted to between 1/3 to 1/2 full charge following a five-day snowstorm, resulting in a significant reduction in oxygen exchange across the air-water interface [47]. Results from the current study suggest the solar circulator in general, experience similar issues and may not be a feasible alternative for long-term remote unattended operation throughout the winter season. For instance, daily incoming global solar radiation on winter solstice in 2019 was 59 W/m2, which is slightly greater than the motor’s operating power requirement of 40 to 50W. However, during the time of polynya closure in late-November to early-December, two- and five-day average incoming solar radiation was just 19 W/m2 and 34 W/m2 (Fig 4). This demonstrates the energy dilemma for winter circulation. While on clear-sky days there is enough watt-hours of energy to power the motor during the day and nighttime and charge the batteries, the influence of multi-day cloudy, inclement weather on power generation appears to be a limitation. A need exists for advanced power generation or battery storage that can extend period of operation during inclement weather, or when charging is not available (e.g., nighttime). A supplemental generator in the form of a small wind microturbine [70], with enhanced battery storage technology (e.g., lithium-iron-phosphate battery or ultra-low temperature battery technology), could potentially extend the operation of the solar circulator during the winter months. URRL is often windy and although the wind would be insufficient on its own to solely source wind power [57], it may be an excellent companion technology to solar-based power circulation. Another issue is fouling of the solar panels. As noted in Figs 2 and 3, the SolarBee solar panels are in close vertical proximity to the water/ice surface such that coverage from wind drift of snow and ice is a problem at recharging the batteries. Future applications could consider elevated panels, or panels that have wipers to prevent accumulation of blowing or drifting snow. A secondary consideration is whether the circulator decreased the duration of ice-cover, which can shorten the oxygen-exchange time between the water and atmosphere. During our study, the period of ice-cover lasted at approximately 183 days, extending from 27 October, 2019 to the last week of April, 2020. However, the lake did not fully freeze until at least late-November at the circulator, even though the rest of the lake, and an adjacent lake, had become iced over prior to that time. This suggests a localized decrease in ice-cover duration of approximately 30 to 40 days (i.e., ~20% of winter), and that the polynya surrounding the solar circulator created an oxygenated area in the vicinity of the circulator (Fig 6). Many researchers refer to ice-on and ice-off as an event that occurs over a single day [71, 72]. Recent high-frequency data suggests it is a longer process taking upwards of several days to weeks [67, 72]. We confirmed multiple freeze and thaw episodes visually at URRL and note that quantifying a statically based decrease in the period of ice-cover is difficult. The duration of ice cover at URRL was reported to be 143 and 124 days during the winters of 2013–2014 and 2014–2015, respectively [48]. This is considerably shorter than was observed in 2019–2020 (i.e., 183 days). In this regard, the duration of ice cover surrounding the circulator, while decreased by about 4 weeks, was still longer than in previous years and probably did not meaningfully contribute to a reduction in lake ice cover or the number of days of hypoxia experienced around the circulator. Barica and Mathias [10] and Ellis and Stefan [9] indicate that winterkill lakes behave as closed systems, such that the critical time to reach hypoxia is a function of the initial oxygen storage at the time of freezing and the winter DO depletion rate caused by sediment and/or biochemical oxygen demand. In the current study, DO in URRL was approximately 100% of atmospheric saturation at the time of freezing (Fig 4). Furthermore, the water became supersaturated from photosynthetic activity by macrophytes, which has been reported in other shallow productive systems [41]. Dynamic changes in DO brought about by under-ice biological processes add complexity to the use of solar circulators. Another under-ice consideration is suppression of the initial oxygen storage from suspended organic particles and biochemical oxygen demand [73], which could have dramatic consequences on the ability to overwinter fish and may shorten the time to anoxia [10]. In this case, URRL turbidity was high at the time of freezing in the fall; however, macrophyte photosynthetic oxygen production sufficiently overcame the oxygen sink from suspended particles until the turbidity under the ice declined. Snow-clearing has been proposed as a potential remedy to increase macrophyte production [32]. However, a practice such as this is challenging especially in remote locations with considerable amounts of snowfall and wind drifting. As noted in the current and previous studies [3, 5, 6], DO concentrations are heterogeneously distributed in the water column of ice-covered waterbodies and are often depth- and/or location-dependent due to water inputs (i.e., tributaries), sediment-water interactions, under-ice photosynthesis, or other waterbody-morphometric features. Even with artificial circulation in URRL, DO gradients (Figs 6 and 7) and thermostratification (S1 Fig) were noted both in vertical and horizontal directions around the circulator. The lack of horizontal symmetry was particularly surprising, and it is unclear if this was related to ice-cover variability over the entire lake, stream inflows, or some other spatial phenomenon (e.g., convective currents or under ice seiche [74, 75]) that we were unable to ascertain given the available data. Possibly the gradient could have been a transient remnant of localized DO at the circulator center that was depleted over time due to horizontal diffusion and SOD. In closed-basin and shallow-lake systems, the highest oxygen concentrations typically occur directly under the ice and decline with depth [17, 48]. Prior investigators note in some instances fish have been shown to avoid hypoxic conditions by either moving to the ice-water interface, towards highly oxygenated stream inlets, or migrating entirely from the lake [48, 70]. As is the case of our study system, Arctic grayling select areas with deeper and more well-oxygenated waters along with congregating near stream tributaries [48, 50]. In this regard, understanding the contribution of dissolved oxygen sources reoxygenating the water column from either macrophyte or plankton photosynthesis, tributary sources, or groundwater inflow require further investigation [75]. Potential limitations and hazards of solar-circulation devices should be recognized. These include whole lake cooling from increased exposure to the atmosphere [76], resuspension of sediments due to mixing [39], increases in sediment oxygen demand rates [9, 33], and open water conditions that create a hazard to winter lake recreation or wildlife [9]. It is believed negative outcomes such as these were largely minimized in our study based on evidence that only a small polynya (~65 m in diameter) was present and for a limited duration during early winter (limiting thermal loss), turbidity during the circulation period following ice cover as turbidity was <5 NTU (sediment resuspension was not a concern), and the remote location of our study site largely precluded hazards to recreationalists or wildlife. Finally, it is important to recognize that prior experience with solar circulator technology in shallow ice-covered lakes is limited. Solar circulators have been used in deeper lakes during winter months [45, 47], or for reservoir mixing or destratification and aeration of effluents discharged from wastewater treatment facilities during open water conditions [59]. Based on the current study, we are of the opinion that applications in shallow lakes at similar latitudes and extreme winter conditions for ecological restoration purposes will be of limited success without additional technological development. Future research into renewable lake aeration technologies should be undertaken to address the abovementioned limitations before practitioners can remedy hypoxic conditions during the winter using such technology.

Conclusion We found the use of a solar circulator to be insufficient for maintaining an open water polynya or for appreciably improving dissolved oxygen concentrations throughout the entire winter. We found the circulator may have improved DO conditions for upwards of 30–40 days during the initial ice cover but then failed during the subsequent winter months during times of low incident radiation. Despite the limitations of our results, solar-powered circulation methods should not be discarded but rather necessitate additional research and development. A primary benefit of solar circulators is that they only require sunlight as a power supply, there are no direct fossil fuel emissions, and they can generate electricity with no moving parts and little maintenance. However, the use of solar mixers at restoring under-ice hypoxia thereby improving conditions for aquatic organisms (e.g., fish) is limited at this time. Additional research should be pursued to better understand the benefits and limitations in hostile environments where species of high conservation concern reside.

Acknowledgments We thank the staff at Red Rock Lakes National Wildlife Refuge, especially Cortez Rohr and Mike Bryant, along with Lucas Bateman and Tim Gander of Montana Fish, Wildlife & Park for field support.

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