Hypolimnetic oxygenation is a relatively new aeration technology that uses pure oxygen gas to increase dissolved oxygen (DO) concentration and improve water quality in the bottom of lakes. A key design parameter for oxygenation systems is sediment oxygen demand (SOD). Oxygenation tends to increase SOD above natural levels since it elevates turbulence and DO at the sediment-water interface. To assist in the pre-design of an oxygenation system for Lake Perris, CA (0.16 km3 , 930 ha), this study evaluated the effects of mixing and DO levels on SOD. SOD was measured in replicate bench-scale chambers containing minimally disturbed sediment-water interface samples from four stations. Average water velocities overlaying sediments were maintained at 0, 1, 2, 4 and 8 cm/s and DO uptake was continuously monitored using luminescent DO probes. Under quiescent conditions SOD was independent of DO and ranged from 0.15 to 0.35 g/m2 /d. Under mixed conditions SOD followed a simple biochemical model composed of both a biological and chemical component. Values of µb, the maximum biological SOD oxidation rate, ranged from 0.1 to 0.2 g/m2 /d. Values of kc, the chemical SOD first-order rate constant, ranged from 0.03 to 0.06 m/d. Organic sediments from deeper stations tended to have elevated biological SOD, while shallow, sandy sediments rich in benthos tended to have elevated chemical SOD. Based on the SOD model parameters developed in this study, a target DO of 7.5 mg/L, and a hypolimnetic surface area of 7.7 million m2 , a design SOD of 5,500 kg/d was recommended. 

Ammonia is an important compound in freshwater ecosystems. It can stimulate phytoplankton growth, exhibit toxicity to aquatic biota, and exert an oxygen demand in surface waters. In productive lakes, ammonia commonly accumulates in bottom waters in conjunction with the onset of anoxic conditions, thereby degrading lake water quality. Sediment cores from 12 lakes and reservoirs were incubated in experimental chambers to examine how dissolved oxygen levels impact sediment release of ammonia. Sediments from all sites released ammonia under anoxic conditions. Release rates for oligo/mesotrophic, meso/eutrophic, and eutrophic/hypereutrophic sites were <5, 5–10, and >15 mg-N m-2 day-1, respectively. Under oxic conditions, ammonia release was negligible in sediments from oligotrophic and mesotrophic sites, and generally lower or reversed in sediments from eutrophic sites. Ammonia release under anoxic conditions is the result of ammonia build up in sediments due to a loss of biological nitrification and a decrease in ammonia assimilation by anaerobic microorganisms. Lake oxygenation, the dissolution of pure oxygen gas into surface waters, can improve water quality in the bottom of thermally stratified lakes, and potentially in shallow lakes, by eliminating sediment ammonia release through the maintenance of a well-oxygenated sediment–water interface.

Summertime hypolimnetic anoxia can occur in productive drinking water reservoirs as a result of the decay of phytoplankton. Anoxic conditions promote ecological processes that degrade water quality through the release of problem-causing compounds from anoxic sediments including phosphates, ammonia, sulfides, methyl- mercury, iron and manganese. Hypolimnetic aeration systems are commonly installed in reservoirs to prevent hypolimnetic anoxia, but these systems have been historically undersized due to an underestimation of the magnitude of oxygen demand in the hypolimnion. To gain insight into the sizing of hypolimnetic aeration systems, this study evaluated the effects of water current and DO concentration near the sediment-water interface on sediment oxygen demand (SOD) in nine California drinking water reservoirs of various size (5-220 million m3) and trophic status (mean annual chlorophyll a of 0.5-11 µg/L). SOD measured under quiescent conditions in 1.8 L experimental chambers ranged from O.1-0.8 g/m2·d. Currents near the sediment-water interface of 3-8 cm/s induced a two to four-fold increase in SOD, and resulted in a shift from first-order to zero-order DO uptake by sediment with respect to DO concentration in overlaying water. Results support the diffusive boundary layer model for SOD, with increased DO concentration and currents resulting in a larger SOD since there is a greater diffusional driving force across a smaller diffusive boundary layer. The study also evaluated the effects of trophic status and morphometry on hypolimnetic anoxia at the nine study sites. A number of significant correlations were discovered between factor quantifying hypolimnetic anoxia (areal and mass based hypolimnetic Oxygen demand, SOD) and those quantifying morphometry (mean depth of the hypolimnion, volume of the hypolimnion) and trophic status (mean annual chlorophyll a).These results suggest that both increased size of the hypolimnion and higher productivity lead to higher oxygen demand within the hypolimnion. In addition, shallower reservoirs had a larger fraction of their total oxygen demand exerted in the sediments versus the water column. As a result, increased mixing at the sediment-water interface after start-up of aeration systems, and the resulting stimulation of SOD, will be particularly important in productive reservoirs of moderate depth (mean depth of 10-15 m). Aeration systems should be designed to enhance SOD by maintaining high oxygen concentrations and by inducing currents at the sediment-water interface. This will increase the depth of penetration of DO into sediment and promote beneficial aerobic biogeochemical reactions in surface sediments. Aeration systems that utilize pure-oxygen with horizontal discharge of highly oxygenated water across the sediment surface, rather than the traditional air-lift aeration system, will be more successful in satisfying SOD and improving hypolimnetic water quality.

Walker Lake is a monomictic, nitrogen-limited, terminal lake located in western Nevada. It is one of only eight large (Area>100 km2, Zmean > 15 m) saline lakes of moderate salinity (3–20 g/L) worldwide, and one of the few to support an endemic trout fishery (Oncorhynchus clarki henshawi). As a result of anthropogenic desiccation, between 1882 and 1996 the lake’s volume has dropped from 11.1 to 2.7 km3 and salinity has increased from 2.6 to 12–13 g/L. This study, conducted between 1992 and 1998, examined the effects of desiccation on the limnology of the lake. Increases in salinity over the past two decades caused the extinction of two zooplankton species, Ceriodaphnia quadrangula and Acanthocyclops vernalis. Recent increases in salinity have not negatively affected the lake’s dominant phytoplankton species, the filamentous blue-green algae Nodularia spumigena. In 1994 high salinity levels (14–15 g/L) caused a decrease in tui chub minnow populations, the main source of food for Lahontan cutthroat trout, and a subsequent decrease in the health of stocked trout. Lake shrinkage has resulted in hypolimnetic anoxia and hypolimnetic accumulation of ammonia (800–2000 µg-N/L) and sulfide (15 mg /L) to levels toxic to trout. Internal loading of ammonia via hypolimnetic entrainment during summer wind mixing (170 Mg-N during a single event), vertical diffusion (225–500 Mg-N/yr), and fall destratification (540–740 Mg-N year-1) exceeds external nitrogen loading (<25 Mg-N/yr). Increasing salinity in combination with factors related to hypolimnetic anoxia have stressed trout populations and caused a decline in trout size and longevity. If desiccation continues unabated, the lake will be too saline (>15–16 g/L) to support trout and chub fisheries in 20 years, and in 50–60 years the lake will reach hydrologic equilibrium at a volume of 1.0 km3 and a salinity of 34 g/L.

Walker Lake (area = 140 km2, Zmean = 19.3 m) is a large, terminal lake in western Nevada. As a result of anthropogenic desiccation, the lake has decreased in volume by 75% since the 1880s. The hypolimnion of the lake, now too small to meet the oxygen demand exerted by decaying matter, rapidly goes anoxic after thermal stratification. Field and laboratory studies were conducted to examine the feasibility of using oxygenation to avoid hypolimnetic anoxia and subsequent accumulation of ammonia in the hypolimnion, and to estimate the required DO capacity of an oxygenation system for the lake. The accumulation of inorganic nitrogen in water overlaying sediment was measured in laboratory chambers under various DO levels. Rates of ammonia accumulation ranged from 16.8 to 23.5 mg-N m-2 d-1 in chambers with 0, 2.5 and 4.8 mg L-1 DO, and ammonia release was not significantly different between treatments. Beggiatoa sp. on the sediment surface of the moderately aerated chambers (2.5 and 4.8 mg L-1 DO) indicated that oxygen penetration into sediment was minimal. In contrast, ammonia accumulation was reversed in chambers with 10 mg L-1 DO, where oxygen penetration into sediment stimulated nitrification and denitrification. Ammonia accumulation in anoxic chambers (18.1 and 20.6 mg-N m-2 d-1) was similar to ammonia accumulation in the hypolimnion from July through September of 1998 (16.5 mg-N m-2 d-1). Areal hypolimnetic oxygen demand averaged 1.2 g O2 m-2 d-1 for 1994–1996 and 1998. Sediment oxygen demand (SOD) determined in experimental chambers averaged approximately 0.14 g O2 m-2 d-1. Continuous water currents at the sedimentwater interface of 5–6 cm s-1 resulted in a substantial increase in SOD (0.38 g O2 m-2 d-1). The recommended oxygen delivery capacity of an oxygenation system, taking into account increased SOD due to mixing in the hypolimnion after system start-up, is 215 Mg d-1. Experimental results suggest that the system should maintain high levels of DO at the sediment-water interface (\~10 mg L-1) to insure adequate oxygen penetration into the sediments, and a subsequent inhibition of ammonia accumulation in the hypolimnion of the lake.

Hypolimnetic aeration is an increasingly common management technique that aerates the hypolimnion while preserving thermal stratification. While most hypolimnetic aeration systems use air as an oxygen source, use of pure oxygen is growing. Potential benefits of hypolimnetic oxygenation include maintenance of an oxygenated source of cool water to meet consumer and environmental needs, decreases in internal nutrient loading, inhibition of sediment release of problematic reduced compounds, and maintenance of summertime habitat for cold-water fish, zooplankton and zoobenthos. A number of short-term experimental oxygenation systems were operated in the 1970s, but large- scale systems were not implemented until the 1980s. Deep oxygen injection systems have been operating in Lakes Sempach, Baldegg, and Hallwil, Switzerland, since the early 1980s to ameliorate cultural eutrophication. Deep oxygen injection has also been used to increase DO in hydroelectric releases from a number of large reservoirs in the southern USA A comprehensive study of physical, chemical and biological impacts of deep oxygen injection was performed at Amisk Lake, Alberta, from 1988-1993. In the early 1990s, downflow oxygen bubble contact chambers (Speece Cones) were installed in Newman Lake, Washington, and Camanche Reservoir, California, mainly to improve the quality of cold- water fishery habitat. Compared to hypolimnetic aeration, oxygenation results in higher hypolimnetic dissolved oxygen levels, lower levels of induced oxygen demand, and maintenance of more stable thermal stratification. Operational experiences over the past two decades confirm that hypolimnetic oxygenation is a successful management strategy with numerous water quality benefits.