Oxygen management is one of the most common water quality restoration techniques used worldwide. This is because restoring deep water dissolved oxygen (DO) not only reduces internal nutrient loading from the release of phosphorus and ammonia from sediments but also enhances a lake’s fishery, simultaneously promoting a shift in the algal community to a more desirable one. As DO levels drop below 2 mg/L over the sediments, chemical reactions accelerate, causing iron and phosphorus in surficial sediments to mobilize to the overlying water column and accumulate in the hypolimnion. Sediment ammonification (the release of ammonia, which is preferred by cyanobacteria) is also promoted when DO drops below 5 mg/L (Beutel 2001). The released phosphorus and ammonia, in their reduced forms of nitrogen, can then be accessed by cyanobacteria, triggering a harmful algae bloom (HAB). Moreover, the absence of oxygen in deep waters is ecologically undesirable, as it limits habitat for fish and other aquatic life. Note: a level of >5 mg/L DO 24-7 is the national US EPA’s minimum DO criteria for aquatic life. Hence, adding adequate oxygen to balance these demands is a sound strategy to address these interconnected issues.

Oxygen management strategies should be aimed to achieve desired DO levels, rather than just meeting the minimum. By managing only to the minimum, one can anticipate minimal results. The optimal DO levels for aquatic life (where there’s no growth or production impairment across all life stages) are >8 mg/L for non-salmonid waters and >11 mg/L for salmonid waters (USEPA 1986). For both aesthetics and water quality, the desired DO levels range between 15-25 mg/L right above the sediments. However, it’s essential to ensure that DO levels do not surpass 25 mg/L to prevent oxidative stress (Colt 2006).

Oxygen management falls into two primary strategies: mixing (or destratifying circulation) and non-destratifying oxygenation.

Aeration (mixing via bubble plumes)

Saturation Technology (hypolimnetic oxygenation):

Oxygenation, specifically hypolimnetic oxygenation, directly addresses lake anoxia and its resultant by-products at their source: the sediments. For lakes less than 60 feet deep, side-stream saturation (SSS) is technically the favored method. This technique leverages the pressure created by pumping to amplify the driving force and consequently the oxygen transfer efficiency (OTE). A prevalent form of SSS is oxygen saturation technology (OSTTM), which has seen significant advancements since its first installation in 2014. Now patented, OST is available commercially to all, not just engineering firms. It represents the most technically effective approach for managing oxygen in shallower lakes (those less than 60ft deep) where the OTE from a pure oxygen bubble plume might be absent, thereby posing potential risks to water quality via mixing. Additionally, OST can be adapted to suit site-specific needs and is designed to preserve thermal stratification. This facilitates higher oxygen concentrations to develop over and into the sediments. Such an oxygen blanketing approach offers significant water quality enhancements over mere mixing strategies (Austin et al., 2019; Zhanling et. al., 2022). Comparing the results of oxygenation and circulation projects, traditional aeration systems reached their desired goals 56% of the time, whereas saturation technology boasted a success rate of 90% (Wagner 2015). This stark difference stems from the fact that non-destratifying oxygenation eliminates the risks associated with mixing surface sediments and ensures high oxygen levels are achieved. The bottom line: the higher the oxygen levels above and within the sediment, the better the water quality.

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